ADA @1981?

'An Atlas of the Solar Ultraviolet Spectrum
Between 2226 and 2992 Angstroms

 

 

R. Tousey, E. F. Milone, J. D. Purcell,
W. Palm Schneider, and S. G. Tilford

Rocket Spectroscopy Branch
Space Sciences Division
2%
E
53»

October 19 74

 

 

NAVAL RESEARCH LABORATORY
Washington, D.C.

Approved for public release; distribution unlimited.

PREFACE

This atlas was made possible by the invention of the echelle diffraction grating by Harrison
of the Massachusetts Institute of Technology in 1949, followed soon after by the production of
echelles jointly with Bausch and Lomb, Inc. under David Richardson. The idea of making use
of an echelle grating in a rocket-home spectrograph to map the solar spectrum at high resolution
from 2000 to 3000 A occurred to R. Tousey and F.S. Johnson in about 1952, only six years
after the first spectra in this wavelength range were obtained by the Naval Research Laboratory.
Work on the project began at once, and in 1961 and 1964 excellent echelle spectra were ob-

tained. With the publication of this atlas, and the line list, the original goal has been achieved.

The unusual format of the echelle spectra required the development of special procedures
to derive accurate values of wavelength and intensity; the large amount of information dictated

processing by computer. However, personal judgment was required at every step.

Extraction of the information contained in all the exposures from both flights would prob-
ably increase, slightly, the accuracy of the atlas. However, we feel reasonably comfortable with

the results and believe that little would be gained by striving for absolute perfection.

Many individuals have been involved in this program over the years. The paper in the ap-
pendix makes proper acknowledgments for the experimental work ending with the flights of
1961 and 1964. The preparation of the intensity atlas was commenced by L.E. Giddings in
1966. His work was taken over in 1967 by E.F. Milone, then a staff member at Gettysburg
College and now at the University of Calgary. Others to whom we are indebted are Eugene
McClurken and LB. Fox, each of whom spent a summer on the project; Rocket Spectroscopy
Branch personnel D.L. Garrett and J.-D.F. Bartoe, who participated in the early reduction
stages; and R. Hedler, among other Upper Air Physics Branch personnel, who helped with the
formats of the final data tapes.

R. Tousey

ii

 

 

 

CONTENTS

PREFACE .......................................................... ii
ABSTRACT .......................................................... iv
INTRODUCTION ..................................................... 1
DESCRIPTION OF THE ATLAS ........................................ 1
PROCEDURES FOR REDUCTION TO INTENSITIES ........................ 1
CRITIQUE OF THE RESULTS .......................................... 4
REFERENCES ...................................................... 4
APPENDIX - An Echelle Spectrograph for Middle Ultraviolet Solar Spectroscopy

from Rockets ............................................ 5
ATLAS ............................................................ 9

iii

 

 

ABSTRACT

The photochemistry of the earth’s upper atmosphere is controlled fundamentally by the
incident radiation from the sun, especially the ultraviolet that is absorbed at levels above the
tropopause. This atlas presents the incident solar irradiance with the high spectral resolution
necessary to calculate the reaction produced when this energy is absorbed by atmospheric mo-
lecular gases. Many of these molecules have absorption bands that consist of very narrow lines.
Solar irradiance in units of ,uW cm“2 A—1 are presented in the wavelength range of 2226 to
2992 A. The curves were determined from high-resolution rocket echelle spectrograms photo-
graphed above the earth’s atmosphere. Each panel of the atlas covers 14 A, with some overlap

from panel to panel. The spectral resolution is approximately 0.03 A.

iv

 

AN ATLAS
OF THE SOLAR ULTRAVIOLET SPECTRUM
BETWEEN 2226 AND 2992 ANGSTROMS

INTRODUCTION

This atlas presents the sun’s absolute spectral irradiance
over the range 2226 to 2992 A with approximately 0.03-A
spectral resolution. The spectra on which it is based were
photographed on Eastman Kodak IV-O film on August 29,
1961 at altitudes of more than 350,000 ft, where the atmos-
pheric attenuation is close to zero. Twelve exposures of dura-
tion from 2 to 84 sec were made using the echelle grating spec-
trograph described by Purcell, Garrett, and Tousey [1,2] in the
appendix. The appendix describes the instrument in detail
and gives the instrumental profile. The instrument had the
unique advantages of packing a spectrum covering nearly
2000 A with 30-mA resolution onto an area of film approxi-
mately 2.5 cm2 and a size and shape that allowed it to fit into
the instrument section of an Aerobee rocket, a volume of
about 1 ft3.

The echelle spectrograph had several disadvantages. Most
serious was the high intensity of background stray light,
characteristic of the earliest echelle gratings. To minimize
stray light, a double-monochromator predisperser designed by
J. D. Purcell was used. A knife edge in this device was ad-
justed to reject direct sunlight of wavelengths greater than
3300 A. Nevertheless, all exposures made during the 1961
flight were exposed to some residual scattered light. This be-
came increasingly important at the shorter wavelengths,
where the solar spectrum becomes weaker, and finally
swamped the spectrum at wavelengths less than about 2280 A
at order 107. On a subsequent flight in 1964 the predisperser
was adjusted to reject wavelengths greater than 2500 A (see
appendix.) As a result, spectra from that flight had a back-
ground that was freer from stray light. However, because of
not quite perfect adjustment, the spectral resolution was
slightly lower than for the 1961 results.

A second disadvantage of the echelle lay in the fact that the
two-dimensional array of spectra contained distortions of sev-
eral types. As a result, an extremely complicated procedure
was required to measure accurately densities and wavelengths
with the microdensitometer and to determine correct values
of intensity by photographic photometry. Each spectral order

was slightly curved and the tilt of the lines changed along each

order. The direction of dispersion of the residual stray light
was not quite parallel to that of the echelle, as explained in
the appendix; this caused the accurate measurement of the
background by microphotometry to be impossible. The trans-
mittance of the instrument varied from the center to each
edge of each order and also of course from order to order;
moreover, the wavelength dispersion varied from order to

Manuscript submitted June 21, 1974.

“Ha. y.. k
swflggsflv . ~ 3600

m .> ' .‘ 3400

 

 

 

 

 

 

 

2» we anal) an “hem ~

WAVELENGTH (ANGSTROMS)

2 200

 

Xma-

 

4—10 ANGSTROMS»

Fig. 1 — The middle ultraviolet spectrum of the sun. This com-
posite is from 12 echellograms on 35-min, Kodak IV-O film taken
from an Aerobee rocket on August 29, 1961. Portions of the
best films have been edited and assembled here. These data have
been reduced to produce the intensity atlas.

order and changed slightly across each order. Finally the
separation between orders and height of each order decreased
with increasing wavelengths. These properties can be seen in
Fig. 1, a composite of prints made from several exposures,
and in Fig. 10 of the appendix.

Because of these characteristics, it was necessary to calibrate
and standardize the echelle spectra by reference to existing
well-calibrated solar spectra of lower spectral resolution.

DESCRIPTION OF THE ATLAS

The atlas curves present on a linear scale the solar irradiance
above earth’s atmosphere at 1 AU. in units of uW cm'2 A1.
The wavelength range extends from 2226 A at the beginning
of order 109 to 2992 A at the end of order 82. Each sheet
covers 14 A , providing some overlap from sheet to sheet. The
atlas is arranged so that the beginning or the end of an order
coincides with an edge of a sheet. The edges of orders are
indicated by heavy broken horizontal lines, which show ap-
proximately those regions where the intensity values and wave-
lengths are somewhat less accurate.

Each sheet of the atlas is based almost exclusively on the
particular exposure from the 1961 spectra that was judged
best for that order, considered from the points of view of
density, contrast, freedom from stray light, and emulsion
defects. Other exposures were used to check the procedures
employed for conversion to intensities and to fill in where
defects such as pinholes were present. These intervals are
indicated by broken sections of the intensity curves. Because

of the widespread interest in the Mg II H and K lines, an addi-
tional sheet is included spanning the region 2794 to 2800 A
containing intensities from exposure 9 of the 1961 flight film.

As an independent overall check on the curve derived from
the 1961 spectra, a comparison was made with several orders
of the 1964 spectra, reduced and normalized in the same way.
The accuracy of the values of irradiance is difficult to estimate.
Except near the ends of orders and in regions where the curve
is shown as broken, the absolute values of irradiance are be-
lieved correct within i20%.

The Fraunhofer line profiles in the atlas curves apply to the
radiation emitted by a central region of the solar disk of ap-
proximately 10 arcmin in diameter. The mean intensity level
of the curve, however, is normalized to give the irradiance
from the entire sun. Therefore, conversion to solar radiance,
without taking limb darkening into consideration, will result
in absolute values that apply to a mean over the sun’s disk.

PROCEDURES FOR REDUCTION TO INTENSITIES

A straightforward reduction of the spectra to intensities by
means of H and D curves was not possible because the
scattered-light background was not uniformly distributed over
the film but varied in a complicated way with wavelength and
order. The basic procedure employed was that described in
the appendix, which proposed treating the stray light as a
parameter because a meaningful value of stray light back-
ground could not be determined directly from the echelle ex-
posures. This parameter and the contrast of the H and D curve
were to be varied until the intensity curves reduced from the
echelle spectrum agreed with the intensity curve obtained
from reference spectra that had approximately the same instru-
mental resolution but were free from stray light. The pro-
cedure was tested by Tousey et a1 (see the appendix), and
satisfactory agreement was obtained between the echelle and
the G'ottingen atlas [3] in the region 3330 to 3338 A , spectra
that had approximately the same instrumental resolution.

To apply this procedure over the wavelength range 2226 to
2992 A, however, required making comparisons with spectra
having much lower resolution than the echelle. This was done
by Gaussian smoothing the echelle spectra to match the com-
parison spectra. The description of the detailed procedure
follows.

First, the echelle spectra were measured with a Jarrell-Ash
microphotometer adjusted to read out the central strip along
each order. The slit was moved transversely during scanning
to keep it centered on the region of maximum density of the
slightly curved order. The microphotometer output in trans-
mittance was recorded on magnetic tape, converted to densi-
ties using a CDC 3800 computer, and stored on cards.

In the next step, the difficult task, the density data were
converted to intensities and normalized against the reference
spectra using a CDC 3100 computer. The reference spectra
used were the rocket spectra of Wilson, Tousey, Purcell,
Johnson, and Moore [4] covering the 2990 to 2635 A region
and of Malitson, Purcell, Tousey, and Moore [5] covering the

region 2635 to 2085 A. These spectra had been normalized

to apply to the radiation from the entire sun by comparison
with spectra of still lower resolution that were exposed to the
radiation from the entire sun. Numerical values of the refer-
ence spectra intensities had been derived from the published
curves and were available in digital form from Brinkman,
Green, and Barth [6] . The resolution of these reference spec-
tra was approximately 0.6 A. Therefore the echelle data were
subjected to an averaging and Gaussian smoothing process that
deresolved them to approximately 0.6 A resolution.

The comparison of the deresolved echelle spectra and the
Naval Research Laboratory (NRL) reference spectra was
carried out in short sections of each order because of the
characteristics of the echelle, previously discussed in the In-
troduction. An approximation to the background density was
obtained from the strips of film above and below each order.
The difference between densities of the deresolved spectra and
the approximate background densities was fitted section by
section to the reference spectra to achieve the best overall fit.
For each section the fit was achieved using the following
equation:

logI=A(D —D0) +3 + Cx

where

x is the distance from the center of the order

D is the density of the spectrum

D0 is the mean of the two background densities adjoining
the spectrum at each position

I is the irradiance of the reference spectrum.

The parameter A is the reciprocal of the effective contrast;
parameter C adjusts for nonuniform instrument efficiency
across an order, and parameter B serves to make the correction
from order to order and to adjust to the absolute level of the
reference spectrum. The need for these adjustments is appar-
ent from Fig. 9 of the appendix. In this process a correction
to the value of D0 is automatically introduced so that the
optimum fit at the centers of Fraunhofer lines was obtained.
The various wavelength ranges and orders involved in the fit-
ting process are listed in Table 1.

The final stage in the reduction was to apply this equation,
using the values of fitting parameters determined as described,
to the respective sections of the unsmoothed echelle density
data. A discussion of the general operation, the original den-
sitometer system, and some of the computer programs in-
volved has been published by Ditzel and Giddings [7 , 8]. The
curves were plotted by the computer; however, corrections
by hand were required, to take care of artifacts.

Table 1 also includes data concerned with the determina-
tion of wavelengths. The wavelength scale was computed
separately for each order using wavelength values from the
echelle line list (published separately) and a least-squares third-
order-polynomia] fitting procedure over the 28mm of each
spectral order. The mean standard error e and the number
of lines n used for making each fit are tabulated. The wave-
length accuracies achieved were approximately 0.005 A. How-
ever, wavelengths can be read from the curves to no better
than 0.02 A.

Table 1
Principal Sections of the 1961 Echelle Spectra Used in Preparation of the Atlas

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

. Mean . Fitting Section Limits _ Mean . Fitting Section Limits
Exposure Exqposure Altltude Order Standard Lines Used - ' . Exposure Exposure Altitude Order Standard Lines Used . . ‘
Number 1me Range Number Error Per Flt Beginning Ending Number T1me Range Number Error Per Flt Beginning Endmg
(S) (km) (6) (n) Wavelength Wavelength (5) (km) (6) (n) Wavelength Wavelength
(A) (A) (A) (A)
2 8 132-139 109 $00078 36 2226.0 2228.9 2 8 132-139 102 i0.0038 19 2376.0 2380.4
2228.9 2235.0 2380.4 2382.3
2235.0 2241.1 2382.3 2388.0
2241.1 2244.1 2388.0 2397.1
2244.1 2252.0 2397.1 2402.2
2402.2 2404.6
108 0.0039 14 2249.0 2253.2 2404.6 2406.0
2253.2 2258.1
2258.1 2264.8 101 0.0043 43 2399.0 2405.3
2264.8 2268.7 2405.3 2414.0
2268.7 2273.0 2414.0 2422.0
2422.0 2428.2
107 0.0042 23 2269.0 2275.6 2428.2 2430.0
2275.6 2277.6
2277.6 2279.7 100 0.0046 41 2423.0 2431.3
2279.7 2282.9 2431.3 2439.1
2282.9 2285.6 2439.1 2447.9
2285.6 2291.3 2447.9 2453.0
2291.3 2294.0
99 0.0037 7 2447.0 2462.7
106 0.0039 28 2286.0 2288.2 2462.7 2473.5
2288.2 2289.7 2473.5 2480.0
2289.7 2299.7
2299.7 2302.5 98 0.0037 21 2472.0 2477.4
2302.5 2305.9 2477.4 2491.5
2305.9 2310.6 2491.5 2505.0
2310.6 2313.0
97 0.0047 32 2497.0 2504.1
105 0.0063 28 2312.0 2317.9 2504.1 2510.5
2317.9 2322.9 2510.5 2524.7
2322.9 2328.6 2524.7 2529.0
2328.6 2337.0
96 0.0043 53 2524.0 2529.5
104 0.0018 21 2333.0 2338.0 2529.5 2549.2
2338.0 2344.4 2549.2 2557.0
2344.4 2347.6
2347.6 2350.2 4 4 141-144 95 0.0043 62 2550.0 2580.0
2350.2 2353.2 2580.0 2583.0
2353.2 2355.1
2355.1 2360.0 94 0.0036 31 2577.0 2584.0
2584.0 2605.3
103 0.0039 29 2352.0 2360.6 2605.3 2611.0
2360.6 2365.1
2365.1 2374.8 93 0.0044 36 2605.0 2617.1
2374.8 2376.8 2617.1 2629.1
2376.8 2382.0 2629.1 2634.4
2634.4 2639.0
92 0.0031 42 2633.0 2650.1
2650.1 2656.4
2656.4 2668.0

 

 

 

 

 

 

 

 

 

(Table Continues)

 

 

Table 1 (Continued)
Principal Sections of the 1961 Echelle Spectra Used in Preparation of the Atlas

 

 

Mean

Fitting Section Limits

 

 

Mean

Fitting Section Limits

 

 

 

 

Exposure Altitude Lines Used Exposure Altitude Lines Used
E35322? Time Range N235; Stggiird Per Fit Beginning Ending E13322: Time Range N01113:); Stggiird Per Fit Beginning Ending
(5) (km) (6) (n) Wavelength Wavelength (5) (km) (e) (n) Wavelength Wavelength
(A) (A) (A) (A)
9 2 176-175 91 10.0043 42 2662.0 2666.8 2826.4 2829.7
2666.8 2671.6 2829.7 2836.9
2671.6 2676.1 2836.9 2841.3
2676.1 2679.3 2841.3 2849.0
2679.3 2690.7 2849.0 2854.0
3233; 32333 85 0.0035 53 2850.0 2851.5
' ' 2851.5 2853.6
90 0.0039 31 2691.0 2698.8 28535 ”591
2698.8 2704.3 2859.1 2868.9
2704-3 2707.6 2868.9 2872.2
2707.6 2712.1 2872.2 2879.3
27121 2715.0 2879.3 2883.5
2883.5 2888.0
2715.0 2720.6
272% 27260 12 13 124-110 84 40.0041 60 2884.0 2886.4
2886.4 2890.9
89 0.0047 35 2722.0 2726.1 2890.9 2894.0
2726.1 2744.9 2894.0 2901.3
2744.9 2753.3 2901.3 2906.5
2753.3 2755.1 2906.5 2911.4
2755.1 2757.0 2911.4 2913.6
1913.6 2919.7
88 0.0040 39 2753.0 2756.6 2919.7 2922.0
2756.6 2790.0
83 0.0044 60 2918.0 2923.0
87 0.0049 20 2794.0 2803.5 2923.0 2933.2
2803.5 2808.0 2933.2 2936.8
2936.8 2947.3
12 13 124-110 87 0.0050 38 2785.0 2792.6 2947.3 2950.0
2792.6 2803.2 2950.0 2953.5
2803.2 2811.3 2953.5 2958.0
2811.3 2821.0
82 0.0039 99 2954.0 2966.9
86 0.0035 53 2817.0 2819.7 2966.9 2981.6
2819.7 2826.4 2981.6 2992.0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CRITIQUE OF THE RESULTS

The procedure as it applies to the determination of intensi-
ties over a given section assumes that the H and D curve, ad-
justed for stray-light density, contrast, and speed change, is
linear; that is, the shoulder and toe are disregarded. Errors
arising from this assumption were minimized by choosing for
each order, to the greatest extent possible, the exposures
whose density fell within the linear part of the H and D curve.
However, deep Fraunhofer lines and regions of unusually high
intensity are somewhat less accurate.

The consistency of the results can be assessed by comparing
spectral intensity plots derived from different exposures.
Comparisons between exposures 12 and 9 at order 87 spanning
the range 2784 to 2823 A , and between exposures 9 and 4 at
order 91 for the range 2662 to 2699 A show substantial agree-
ment across each order with the exception of regions near the

edges Where the focus and the background light change rapidly.

This is illustrated by Fig. 2 in which a 5—A portion of the latter
range is reproduced. Curve 4 is displaced upward by 1.5 units
from 9, which is taken from the atlas. It can be seen that the
agreement is good, although some information was lost in ex-
posure 4 because of its greater density. Another comparison
can be made from the atlas itself, which includes the curve
from exposure 9 from 27 94—2808 A, a shorter exposure than
the principal atlas curve which was from exposure 12 in this
region.

It is of interest to compare the absolute irradiance values
of this atlas with results obtained by others. The atlas pro-
duced by McAllister [9] covers this region with approxi-
mately 0.3-A resolution; however, it was normalized to the
same reference spectra used in the calibration procedure for
the echelle spectra.

Completely independent data were obtained by Kacalov
and Jakovleva [10] . Their spectra were obtained with a
rocket-borne concave grating spectrograph. The results cover
3100 to 2700 A and 2700 to 2570 A with resolution values
of 0.15 and 0.3 A, respectively. For a detailed discussion of
these results see Tousey [2] . Regions from their curves
selected at random were compared with the corresponding
portions of the present atlas, which were deresolved to the
same spectral resolution. The comparisons are shown in Fig. 3
through 7. The results agree as well as one could expect.
Some of the differences probably result from the deresolution
process.

A rather critical test is the comparison of the range 2790
to 2810 A that includes the Mg 11 H and K lines. Figures 6 and
7 compare the curves of Kacalov and Jakovleva over the K and
H lines, respectively, with the 1961 spectra deresolved to the
same resolution. For Mg II the NRL curve for K1 was signifi-
cantly lower, but for H1 it is nearly coincident. The agree-
ment for H2, 3 and K2, 3 is satisfactory. To examine this
further, the echelle spectrum from the 1964 flight was cali-
brated and deresolved exactly as was done for the spectrum
from the 1961 Flight. The curves over the Mg II K line, as
compared in Fig. 8, are in good agreement.

Intensity, l

 
   
 

 

 

 

 

 

 

 

 

6 I
l
l
l ;, 4—91
_. I II
I II
I II A
| ll ‘
.' II l. .'
4 Ii 5 -' l I'
I , I l
l [1' ,'
" . if” [\I‘ ,
llll
I
I' l
2— II I I
I - I/\ A
I l
‘I' f I
9—9l
l ..... . l.. .I ....... I... I. 11..

 

 

2692 Wavelength (A) 2695
Fig. 2 — Intensities from the 9lst order of the 1961 echelle in
the 2691 to 2696 A region from two different exposures.
Exposure 4 is offset above exposure 9 by 1.5 units of irradiance
(,uW cm‘2 A“1). Exposure 4 has a much higher density in this
region, causing some loss of information. Exposure 9 was used
for the atlas.

0.5 ——NRL
-—- K&J

 

 

2500 2510 2520
Wavelength (A)

Fig. 3 — A comparison between the 97th order of the
echelle atlas (solid line) and the Kaéalov and Jakovleva
[11] atlas, (dashed line), in the 2495 to 2520 A re—
gion. The echelle atlas segment has been smoothed with
a Gaussian function to approximate the resolution of the
Kacalov and Jakovleva curve. The vertical axis is in
units of the logarithm of solar irradiance (MW cm—2

A*1).

l.0

 

‘2723\’ 2726 v 2729
Wavelength (A)

Fig. 4 —- A comparison between a smoothed portion
of the 89th order of the echelle atlas (solid line) and
the Kac’alov and Jakovleva [11] curve (dashed line),
in the 2723 to 2730 A region. (Units as in Fig. 3.)

———NRL
———K8.J

2735'
Wavelength (A)

   

Fig. 5 - A comparison between a smoothed portion of the 89th
order of the echelle atlas (solid line) and the Kaéalov and Jakov-
leva [11] curve (dashed line), in the 2730 to 2740 A region.
(Units as in Fig. 3.)

0.5F —NR|_

log I

o.o~ "’\

 

 

l l l l l I l l

I
2795 2799
Wavelength (A)

-0. I
5 2791

Fig. 6 — A comparison between a smoothed portion of exposure
12 at the 87th order of the echelle atlas (solid line) and the Ka-
calov and Jakovleva [11] curve (dashed line) in the 2790 to
2800 A region. (Units as in Fig. 3.)

 

0‘5F ___NRL
—~-K&J
logI
0.0
.05 I I I I l l I I

 

 

J
2805 2809
Wavelength (A)

Fig. 7 —- A comparison between a smoothed portion of exposure
12 at the 97th order of the echelle atlas (solid line) and a panel
of the Kaéalov and Jakovleva [11] atlas (dashed line), in the
2800 to 2810 A region. (Units as in Fig. 3.)

0.5 _ '9“ Fig. 8 — A comparison of the

"' '9“ 87th order of exposure 12 of
the August 1961 flight and a
similarly treated portion from
exposure 1 of the 1964 flight
film. (Units as in Fig. 3.) Both
spectra have been Gaussian
smoothed to the same resolution
as the Kacalov and Jakovleva
spectra. The y axis scale is the
same as the one used in Figs.
4 and 5.

log I

 

—0.5

‘2794 2796
Wavelength (A)

Additional results over the region of the Mg II H and K
lines have been reported by Lemaire [11, 12] and Bates et a1.
[13] . Their spectra were of very high resolution and were
also stigmatic. They showed that the profiles of these lines
vary greatly with activity and also with position relative to
the limb.

Since the compilation of this atlas, new measurements of
the solar irradiance have been made by Broadfoot [14] using
an Ebert-Fastie spectrometer flown in an Aerobee—150 rocket.
The spectral resolution was 3 A half width. According to
Broadfoot, the intensities of the N RL reference spectra that
had 0.6 A resolution were in excellent agreement with his
own results over the entire range covered by this atlas, with
the exception of the regions 2500 to 2600 A and 2850 to
3000 A. There the values of Broadfoot were lower than those
of NRL by as much as 25%. No reason for this difference is
apparent, and these independent results agree to well Within
the stated accuracies. Therefore, correction to match Broad-
foot’s results is regarded as optional.

There is, of course, the question whether the sun’s spectrum
is affected by solar activity and the solar cycle in the spectral
range covered by this atlas. Here the radiation from the center
of the disk is almost entirely photospheric. Exceptions are
the Mg II emission cores and possibly a few emission lines
such as Si II at 2334.50 and 2350.28 A. Although the solar
constant has not been shown to vary with solar activity, there
remains the possibility that the contribution from the short
wavelength end of the photospheric spectrum may vary
slightly with the solar cycle.

REFERENCES

1. J.D. Purcell, D.L. Garrett, and R. Tousey in Space Research 111
(W. Priester, editor), North-Holland Publishing 00., Amsterdam,
1963, p. 781-786.

2. R. Tousey, Space Sci. Rev. 2, 3 (1963).

3. G. Bruckner, Photometric Atlas of the Near Ultraviolet Solar
Spectrum, Vanderhoeck and Reprecht, Gottingen, 1960.

4. N.L. Wilson, R. Tousey, J.D. Purcell, F.S. Johnson and CE. Moore,
Astrophys. J. 119, 590 (1954).

5. H.H. Malitson, J.D. Purcell, R. Tousey, and C.E. Moore, Astrophys.
J. 132, 746 (1960).

6. RT. Brinkman, A.E.S. Green, and C.A. Barth, JPL Technical Re-
port 32-951, Jet Propulsion Laboratory, Pasadena, Calif., 1966.

7. E.F. Ditzel and LE. Giddings, Jr., Appl. Opt. 6, 2085 (1967).

8. ER Ditzel and LE. Giddings, Jr., “A Computer Method for the
Automatic Reduction of Spectroscopic Data,” NRL Report 6632,
Jan. 1968. AD 665281.

9. H.C. McAllister, A Preliminary Photometric Atlas of the Solar
Ultraviolet Spectrum from 1800 to 2965 Angstroms, University
of Colorado Printing Services, Boulder, 1960.

10. V.P. Kachalov and A.V. Jakovleva, Izv. Krymskyi Astrofiz. Obs.,
Akad. Nauk. SSSR 27, 5 (1962) (NRL Translation 923).

11. P. Lemaire, Astrophys. Ltrs. 3, 43-48 (1969).

12. P. Lemaire, in Ultraviolet Spectra and Related Ground-Based
Observations, (L. Houziaux and HE. Butler, editors), Reidel,
Dordrecht, Holland, 1970, p. 332.

13. B. Bates, D.J. Bradley, C.D. McKeith, N.E. McKeith, W.M.
Burton, H.J.B. Paxton, D.B. Shenton, and R. Wilson in Ultra-
violet Stellar Spectra and Related Ground—Based Observations
(L. Houziaux and HE. Butler, editors), Reidel, Dordrecht,
Holland, 1970, p. 274.

14. A.L. Broadfoot, Astrophys. J. 173, 681 (1972).

rinted from APPLIED OPTICS, Vol. 6, pjage 365. March 1967
e

Re
Copyright 1967 by the Bptical Society of America and reprint

y permission of the copyright owner

An Echelle Spectrograph for Middle Ultraviolet Solar

Spectroscopy from Rockets

R. Tousey, J. D. Purcell, and D. L. Garrett

An echelle grating spectrograph is ideal for use in a rocket when high resolution is required because it
occupies, a minimum of space. The instrument described covers the range 4000—2000 A with a resolution
of 0.03 A. It was designed to fit into the solar biaxial pointing-control section of an Aerobee-150 rocket.
The characteristics of the spectrograph are illustrated with laboratory spectra of iron and carbon arc
sources and with solar spectra obtained during rocket flights in 1961 and 1964. Problems encountered
in analyzing the spectra are discussed. The most difficult design problem was the elimination of stray
light when used with the sun. Of the several methods investigated, the most effective was a predis-
persing system in the form of a zero—dispersion double monochromator. This was made compact by

folding the beam four times.

I. Introduction

High resolution spectroscopy from rockets requires
the design of extremely specialized equipment, since it is
hardly possible to fly a 21—ft (6.4 In) concave grating
spectrograph in the solar—pointed section of an Aerobee
rocket with length but 3 ft (0.9 m) and cross section
6 in. X 8 in. (15 cm X 20 cm). Even now, after twenty
years of research with rockets, this is the largest space
available in this country for solar instrumentation that
must be recovered after flight. The design of a high
resolution spectrograph, covering the wavelength range
4000 A to 2000 A, approximately, that was compact
enough to fit into this space was made possible by the de-
velopment of the echelle diffraction grating by Harri—
son.1 If produced by a conventional spectrograph, this
spectrum would have been 5 ft (2.3 m) in length, nearly
twice as long as the instrument space. When cut into
sections and stacked by means of the echelle and prism
combination, the entire spectrum covered an area only
1 in. (2.5 cm) on a side. Thus, the echelle system was
ideally suited for high resolution rocket spectroscopy
over a wide wavelength range.

The original design of the echelle spectrograph was
undertaken in 1952 by Tousey and Johnson, and was
completed by Purcell. The first solar spectra were ob—
tained in 1956 by Purcell et al.2 Greatly improved
spectra were photographed on 29 August 1961 and 12
November 1964 by Purcell et al.3 Theoresolution in the
1961 spectra was about 100,000, or 0.03 A; this was five or

The authors are with the E. O. Hulburt Center for Space
Research, Naval Research Laboratory, Washington, D.C.
20390.

Received 28 October 1966.

more times less than the theoretical resolving power and
was a limit set by aberrations in the lens system and by
the slit width.

The resolution attained with the echelle spectro—
graph was more than an order of magnitude greater
than other experimenters have obtained in this wave-
length range; Kacalovo and Jakovleva4 covered the
range 3100 A to 2470 A using a 1-m, 600 lines/mm
grating spectrograph, flown in a rpcket in the USSR;
they obtained a resolution of 0.3 A in certain regions.
With the same instrument in a later flight, Kacalov et
al.5 attained 0.15 A resolution from 3100 A to 2700 A
using the second order. Behring et (1U, employing the
grating spectrograph described by Miller et al.7, in which
a 600 lines/ mm, 50—cm radius grating was illuminated at
495° incidence, photographed the spectrum from 3000
A to 1800 A with a resolution of approximately 0.5 A.
The excellent solar atlas of McAllister8 was compiled
from the latter spectra.

II. The Echelle Spectrograph

The completed echelle spectrograph is shown in Fig.
1; it is installed in a biaxial pointing control of the type
produced by the Ball Brothers Research Corporation
of Boulder, Colorado. In operation, the entire nose
cone of the rocket is raised through a distance of more
than three feet by an electromechanical drive, thus un-
covering the spectrograph and the mechanism for point-
ing it at the sun. Stabilization against roll of the rocket
body is accomplished by rotating the entire nose section
with an electrically driven servo system sensing the
sun with phototubes. The rotation is made possible
by a large ring bearing, located at the last break in the
rocket body, that may be seen just above the top of the

March 1967 / Vol. 6, No.3 / APPLIED OPTICS 365

 

Fig. 1. The echelle spectrograph, mounted in the biaxial pointing
control of the Ball Brothers Research Corporation (BBRC), in
the forward section of an Aerobee-150 rocket. The nose cone is
lifted to expose the instrument, as in flight operation. M. W.
Frank of BBRC, left, and J. D. Purcell, right, are about to test
its operation in the New Mexico sunlight by spinning it on the
motor—driven turntable and testing its ability to lock on the sun.

turntable on which it is mounted for simulating roll of
the rocket when testing the unit. The instrument itself
is carried on a transverse bearing that allows it to be
pointed at the sun regardless of the yaw and pitch of the
rocket, within certain limits. This also is accomplished
with a servo system—and solar sensors. After com-
pletion of the exposures, the instrument is once again
lined up with the rocket, and the nose cone is brought
back into its original position to cover the instrument
and to protect it during impact. Recovery is accom—
plished by means of a parachute.

The echelle spectrograph itself, With the cover re—
moved, is shown in Fig. 2. Dimensions of the case are
13.3 cm X 15 cm X 80 cm. The cylindrical tube on the
front end contains the predispersing system used to
eliminate stray light. Figure 3 shows the predisperser
removed from the tube. The optics are mounted on a
short bar whose underside is machined to a curvature
that just matches the inside of the cylindrical tube.

This instrument was flown and recovered successfully
in 1961 and flown again in 1964. On the latter occasion
the parachute failed to operate, and the instrument was
demolished. The film, however, was recovered. On
the original flight in 1956 the instrument was not
mounted in a biaxial pointing control but was installed
rigidly in the nose section of an Aerobee—150 with Optic
axis parallel to the longitudinal axis of the rocket; light

366 APPLIED OPTICS / Vol. 6, No. 3 / March 1967

 

from the sun was directed into the instrument by means
of a servo operated coelostat mirror. The spectra
photographed on this flight, although extending to
about 2700 A and producing new data on the contours
of the Mg II doublet near 2800 A, fell far short of what
the instrument could have produced because the rocket
failed to ascend all the way through the ozone layer.

The optical diagram of the echelle spectrograph is
shown in Fig. 4 in plan and elevation views. Basically,
the spectrograph is a Littrow system With a prism
crossed with the echelle grating in order to separate the
different orders of diffraction produced by the echelle.
Light from the sun enters the slit S, is collimated by
lens L, passes through prism P, is reflected and dis—
persed in one plane by the echelle, passes once again
through the prism which introduces the cross disper-
sion, and is brought to focus by lens L on the film,
located at position F.

The design of the lens presented a difficult problem,
since it was required to coverothe wavelength range
from 3000 A at least to 2000 A and produce a sharp
image. It was necessary to minimize the longitudinal
chromatic aberration, and also the zonal spherical and
spherochromatism. The lens was a triplet of 400 mm
focal length and 65 mm diam, designed and constructed
by the Farrand Optical Company.9 The negative
center component was of crystal quartz, the outer posi—

 

Fig. 2. The echelle spectrograph with the cover removed.
Sunlight enters from the right through the predisperser system
inside the cylinder. The echelle, prism to produce the cross-
dispersion, and Littrow lens are seen at the left. Film from the
supply container at the upper right is carried to the right around
a roller, across the curved plate that determines the focal surface,
around another roller, then to the left and into a cassette.

 

Fig. 3. The optics of the zero-dispersion double-monochromatot
predisperser, removed from the cylindrical housing. At the
extreme left end is the final lens that images the sun on the slir.

 

 

 

 

 

Fig. 4. The optical system of the echelle spectrograph in plan and elevation views. Lens L is an achromat of quartz and LiF, prism
P is of fluorite, and field flattener L' is of quartz.

tive components of lithium fluoride; the quartz com—
ponent was mated to one outer component with glycerin
and was air-spaced from the other. In practice, the
lens was found not to be sufficiently corrected for use at
full aperture f/ 6, and in flight it was stopped down to
f/ 10, for which the circle of confusion was about 30 It
in diameter. The lens was constructed and meticu~
lously adjusted and tested by Nolan and Barcus.

In order to make use of 35 mm roll film it was neces—
sary that the focal surface be kept straight in one
direction. In the other direction the film was made to
conform to a curve that optimized the focus. In the
direction of the echelle dispersion, there was so little
wavelength span across each order that the changing
chromatic aberration was not a problem, and the focal
surface was almost flat. However, to make it exactly
flat it was necessary to introduce a field corrector lens
L’ of crystal quartz. A hole was cut in this lens to
permit passages of the beam from the entrance slit to
the main lens. In the direction of the prism dispersion
there was considerable chromatic aberration because of
the great range in wavelength. This was corrected
by feeding the film over a plate cut to the proper curve,
established empirically, to correct for the residual
longitudinal chromatic aberration, which was quite
severe, as shown by the curve of Fig. 5.

The echelle gratings used in the instrument were
ruled by Harrison at the Massachusetts Institute of
Technology and were replicated by Bausch & Lomb.
The ruling parameters were 73 grooves/ mm and
step ratio two to one. An angle of incidence of
63.25° was required to place the blaze on the center of
the optical system. The echelle itself, of dimensions 68
mm X 145 mm X 25 mm, was mounted in a cast alum-
inum cell of the proper shape to hold it at the desired
angle. It was held firmly in position, by screws located
such that they did not cause distortion but were de-
signed to prevent it from shifting as a result of accelera—
tions experienced during launch.

Various spectra obtained with the instrument are
shown in Figs. 7—11. The many orders produced by the
echelle were separated by the 30° calcium fluoride

prism. The linear dispersion of the system, propor—
tional of course to the order pumber, yaried from 2.5
A/ mm for orgler 60 near 4000 A to 1.25 A/ mm for order
120 at 2000 A. Orders overlapped from end to end by
an amount that increased the shorter the wavelength;
for example, betweeno orders 80 and 81 the overlap
was approximately 04 A, while orders 105 and 106 over—
lapped by about 8 A. The overlap was of considerable
assistance in establishing the wavelength scale. In the
cross direction, using a slit length of 0.25 mm, orders
were just separated at 4000 A and were separated by
more than the slit length at the short wavelength end,
owing to the increasing cross dispersion produced by
the prism with decreasing wavelength.

Focusing and adjustment of the instrument were
tedious, and the empirical determination of the proper
curve for the film guide was especially so. Final focus-
ing was always carried out in vacuum, since the instru—
ment was evacuated in flight. When making adjust-
ments in air, allowance had to be made for the change in
refraction produced by air at the various interfaces;
this caused a marked change in focus and a wavelength
displacement of nearly 1 A.

In the 1961 flight, Eastman Kodak Type IV—O, UV

l4

 

A o 000
:2 8 3 32°
5 B 8 '0 mm:

,c
~IO~ N V l ' 'V
o v
(I)
>7 8
<8~ m
g V
9 /'
«6w :9 g
o . x
8 g l <
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Q I—
O
2-— %
l l I I l l l l I l

 

 

 

2 4 6 8 IO 12 I4 I6 I8 20 22
DISTANCE TRANSVERSE TO AXIS (mm)

Fig. 5. The shape of the focal surface required to correct for the
longitudinal chromatic aberration after two passes through the
lens.

March 1967 / Vol.6, No. 3 / APPLIED OPTICS 367

FEFIV PRISMS

 

PLANE MIRROR / K2555: LENS PLANE MIRROR
SPECTRUM

Fig. 6. The optical system of the zero-dispersion double—mono—

chromator employed as a predisperser. Stray light was rejected

by introducing the knife edge into the intermediate focal position

in order to intercept all radiation of wavelengths longer than the
desired limiting value.

‘—-l0A—-
IIIIIIIIIII
‘I - I>I"|- <3: I,' -3200

 

ORDER NUMBER

I00-

I05- -2300
I I I I I I I I I I I
~——Io ——.
Fig. 7. The spectrum of a low—pressure rf—excited iron arc, as
photographed by the echelle, flown in 1961.

sensitive film 35 mm wide was used, since extreme
resolving power was the most important consideration.
With a slit width of 15 y, there was sufficient intensity
in the solar spectrum to obtain ample exposure with this
emulsion during a rocket flight. It was necessary to
coat the film base with a rim—jet—black backing to avoid
static-electrical fogging when transporting the film in
vacuum. For the 1964 flight, Type III—O UV film was
used, because IV—O, regrettably, was no longer avail-
able. This resulted in somewhat more noise associated
with granularity, and the slightly greater speed was not
needed. Twelve exposures were made on each flight by
means of a programmer, driven by an electric motor.
They ranged from 2 sec to 84 sec in 1961 and 2 sec to
96 sec in 1964. When transporting the film, a shutter
blocked the slit to avoid any possibility of fogging.

The most difficult problem associated with an echelle
spectrograph, particularly one of the Littrow type, is the
stray light. Since the intensity of the solar spectrum
is of the order of 100 times greater in the visible than at
2000 A, the short wavelength limit which it was hoped
to reach, and since the depths and profiles of the strong
Fraunhofer lines were of particular interest, it was
necessary to reject the unwanted part of the solar
spectrum before it entered the slit. The first method
was the low pass optical filter devised by 13mm-
berg“), and by Regener”, consisting of a pair of quartz
prisms separated along a diagonal with a liquid having
a dispersion relative to quartz such that wavelengths
longer than the desired cutoff were totally reflected out
of the prism, while wavelengths shorter than the cutoff

368 APPLIED OPTICS / Vol. 6, No. 3 / March 1967

were transmitted with only a small reflection loss.
Although this prism was found satisfactory by Regener
for removing stray light from a stratosphere balloon-
borne spectrogrgph designed to record the solar spec-
trum to 2500 A and below, it did not prove to be
sufficiently effective for the echelle spectrograph.
Furthermore the cutoff wavelength varied rapidly with
temperature, resulting in a diflicult problem of control.
The prism recently designed by Crosswhite12 oflers
promise of satisfactory low pass characteristics, but in
the form tested, where only four internal reflections per
prism were used, the stray light transmittance was not
reduced sufficiently.

The actual device employed for eliminating stray
light was a small zero—dispersion double—monochroma—
tor of quartz, located in front of the slit, with an
optical system designed by Purcell (Fig. 6). The entire
device fitted into the cylindrical tube shown in Fig. 2,
4.45 cm in diameter and 20 cm long. Two Féry prisms
were used. The first prism dispersed the beam and
focused it into a spectrum of solar images in the inter-
mediate position. The second Féry prism recombined
the spectrum and collimated the beam. With the knife
edge removed, a white light image of the sun was
produced on the slit by lens L. The two plane mirrors
simply folded the system into the compact form re—
quired for use in the rocket. The intermediate lens
served to image the first Féry prism on the second.
When the knife edge was introduced in the intermediate
focal plane, the portion of the spectrum to the red of its
position was cut out; the reconstituted solar image
then contained all the wavelengths to the blue of the
limit determined by the knife edge. Also mounted on
the same base plate, but not shown in the figures, was a
simple boresighting device. This comprised a small
lens, a plane mirror reflecting the converging beam of
sunlight back toward the sun and focusing a solar im-

 

AIIIIIIIIIIIIIIIIII

LOG INTENSITY

 

 

 

I I I I I I L ,I I I I r . * I I I
- I00 0 I00 200
Ai— mIlIi ANGSTROMS

 

Fig. 8. The profile of Fe II 2739.545 A recorded with the echelle
flown in 1961; for comparison there is also shown the profile
of the 6.65-m concave grating instrument used by Briickner to
produce the Gcttingen Solar Atlas, measured with Kr I 4274.97
A, and scaled to 2700 A. Solid curve: echelle Fe II (63) )x 2739.
545 A. Dotted curve: Gottingen Kr 4273.97 A, scaled to 2740 A.

 

1o
I111111111l

ORDER

 

 

 

 

 

105~ _ , (“Wei-films“ ~2300
' _ a . ¥’” ' 1‘ ,
* ’ ME)
110- ‘ .. - A... ...___..., , 72200
I I I I I I l l I I I
10A )\__..

Fig. 9. The spectrum of a carbon arc crater, photographed with

the echelle spectrograph flown in 1961. Ghosts and stray light

associated with the 2478 A line of C I are clearly seen, though not

of great intensity. At long and short wavelengths the bands of
CN from the arc stream are prominent.

age on a ground-glass screen and reticle, located on the
front where it could be easily viewed. This was aligned
with the predisperser and was used to align the entire
instrument with the solar sensors that controlled the
pointing during flight.

When flown in 1961, the knife edge was set to block
off the spectrum longward of 3300 A. Although it
might appear that the cutoff was gradual, because the
intermediate spectrum consisted of images of the entire
sun, this was not the case because the spectrograph
entrance slit and the knife edge position for one par-
ticular wavelength were at conjugate foci. In spite of
the double dispersion, there was sufficient light leakage
through the predisperser to secure a well-exposed spec—
trum all the way to 4000 A. General, residual stray
light limited the range that was covered; at the short
wavelength end below order number 110 at 2200 A
almost nothing could be detected above the background
fog level. In the 1964 experiment an echelle with im-
proved stray light characteristics was available, and the
predisperser knife edge was set to eliminate the spec—
trum longward of 2500 A. As a result the spectrum was
extended to 2090 A before it became swamped by stray
light. To go beyond this limit is especially difficult
because of the sudden decrease in intensity of the solar
spectrum by a factor of about five, from 2100 A to 2090 A.

The echelle grating was re—aluminized before flight
to maximize its reflectance. In order to accomplish
this properly it was necessary to evaporate the alum-
inum in a direction as near as possible to the normal
to the echelle step—surfaces, the condition which
produces maximum reflectance of the coating. But
normal incidence on the steps would have required
63° incidence on the echelle, and the result would have
been a change in the thickness of the coating from one
end to the other because of the difference in distance
to the aluminization source. This would probably have

introduced phase changes and loss of resolving power.
The compromise arrived at was 35° to the echelle
normal and about 23° to the step—surface normal.

Although the absolute transmittance of the instru-
ment was not measured, the speed was high at A >
2300 A. At shorter wavelengths the transmittance
decreased rapidly because it was not possible to obtain
lithium fluoride that was completely free from absorp-
tion near 2000 A at the time when the lens was con—
structed. The over—all transmittance of the system
relative to 3000 A was estimated at 5% and 1% at
2100 A and 2000 A, respectively, not including the
grating efficiency.

The performance of the spectrograph in the labora-
tory can be judged by reference to Fig. 7, the spectrum
produced by an rf—excited low—pressure iron are of the
type described by Corliss et al.13,having narrow lines
free from pressure broadening The resolution was
determined to be 0.030 A by studying the profiles of
several of these 1ron lines. For example, in Fig 8, the
profile of Fe II 2739. 545 A 1s reproduced. Introduced
for comparison is) the profile of Kr 4274. 97 A, scaled
linearly to 2740 A, as recorded by the 6. 65 m concave
grating spectrograph employed by Briicknerl4 to pro-
duce the Gettingen Atlas of the Solar Spectrum. The
width at half-maximum of the instrumental profiles of
both spectrographs was 0.030 A. The principal differ—
ence is that the echelle did not produce a really clean
spectrum. The profiles are alike down to about 10% of
peak intensity; below this level the line shape of the
echelle spectrograph soon becomes broadened and the
stray light level never falls below about 1% of the peak
intensity.

The stray light characteristics and speed anomalies are
best shown by Fig. 9, the spectrum of a carbon arc
crater in air. The carbon arc radiation is quite similar
to that of the sun in distribution, following a graybody

 

 

 

 

 

 

 

WAVE L ENGTH (ANGSTROMS)

  

1 1 1 1 1 [ 1 1 1 l
«10 ANGSTROM$—> X—"

 

Fig. 10. The solar spectrum, photographed with the echelle
spectrograph from an Aerobee—150 rocket on 29 August 1961.
The reproduction is a composite of prints of several exposures.

March 1967 / Vol. 6, No. 3 / APPLIED OPTICS 369

‘———-101‘i———~
I1111l111|l

   

   

ORDER NUMBER
0
A (A)

lilllrulrrl
IOA

 

 

Fig. 11. The short wavelength end of the solar spectrum,

photographed with the echelle spectrograph on 21 November

1964. This is the extreme limit reached with a resolution of the

order of 0.05 A, and marks the end of the region where the solar

spectrum originates from the photosphere and is characterized
by Fraunhofer lines.

curve of 3800°K through this region. Conspicuous In
two orders 1s the carbon line at 2478 A. Below this
wavelength the spectrum gradually becomes compli-
cated and is largely produced by band emission from
cyanogen in the arc stream. The long wavelength
cyanogen bands are also seen, in spite of the predis-
perser cutofi at 3200 A, which shows clearly at order
number 75.

Since long wavelength stray light was largely removed
by the predisperser, the residual stray light, except at
the short wavelength end of the spectrum, was so-called
nearby straylight, which consists of ghosts of various
sorts. They can be seen best by examining the series of
images of the 2478-A line. Since the ghosts have the
same wavelength as the parent line, they are deviated
when passing through the prism by exactly the same
amount as the line. Therefore, they lie along a straight
line that is slightly inclined to the direction of the
echelle dispersion, since each echelle order is deviated
increasingly with decreasing wavelength from right to
left as a result of the increasing cross dispersion of the
prism. In addition to a number of clearly defined
ghosts, there is nearby stray light consisting of many
less distinct ghosts, some close and some far from the
parent line, together with general “grass” which extends
all the way across between the images in different orders.
As a result, it is extremely difficult to make an exact
correction for stray light by examining the density be—
tween orders, since the stray light is associated in a
complicated way with the intensity in the spectrum.

Generally, the nearby stray light increases rapidly in
intensity with decreasing wavelength, since it follows
the rule for ghosts—proportionality with the square of
the order. There is no cure for this defect other than to
produce an echelle which is more perfect. In the
echelle spectrograph of Detwiler and Purcell15 working
in order number 201 at the Lyman-alpha line of hydro-
gen, 1216 A, stray light from this source was so strong
that the efficiency of the echelle was extremely low be-
cause most of the light went into the spectrum alongside
the line and between different orders of the line rather

370 APPLIED OPTICS / Vol. 6, No. 3 / March 1967

than into the line itself; at Lyman-alpha a rough esti-
mate indicated that as much as 90% of the line was lost
between orders.

The transmittance of the optical system was quite
nonuniform along each order, from center to edge, as
can be seen in Fig. 9. There were two reasons; first
the echelle was adjusted so that the blaze came at the
center of each order, and the blaze was rather sharp in
character; second, at the shorter wavelengths where the
transmittance of the optical materials was low, the in—
creased path for the skew rays through the optical
system that reached the edges of the field resulted in a
further reduction of transmittance.

The solar spectrum photographed on 29 August 1961
is shown in Fig. 10, a print already published in many
journals and on the cover of APPLIED OPTICS for
November 1962. This reproduction is a composite of
prints of different exposures since no single exposure
could cover the entire wavelength range because of the
great decrease in intensity of the solar spectrum to short
wavelengths.

The spectrum obtained on 12 November 1964, al-
though having slightly less resolution than the 1961
spectrum, reached shorter wavelengths before merging
into the stray light. A section of one of the 60—sec
exposures is reproduced in Fig. 11.

Examination of Figs. 10 and 11 shows how the gen-
eral, nonfocused stray light, coupled with the decreased
speed of the instrument and reduced intensity of the
solar spectrum, determined the short wavelength limit
that could be reached. Still shorter wavelengths could
have been recorded if optical materials of greater trans-
parency at short wavelengths had been available, and
if the echelle efficiency could have been increased by
better aluminization and further reduction of stray
light. Adjustment of the predisperser to reject even
more of the spectrum would also have reduced the back-
ground still further, but the instrument was demolished
before these changes could be made.

To illustrate the quality of the 1961 solar spectra, a

C11 (63)
F! I (l8l)
F: U (4|?)

-4 LI] ?
Fl I (95)

.>

 

 

Cr 1 (63)

F. I (46)

Cr II (6)
F. I (51

    

——. v n (13)
——Mn 1
... Fe H46)

Fe II (63)
Cr II l6)
%
<4 / S III (l6)
‘/
1.1
*3 Fe II (260)

”,7

4 Fe 11(62)

L.

 

NW

 

 

<

   

 

 

l
2739 2740 2741 2142 2743
WAVELENGYH (ANGSTROMS)

Fig. 12. A short section at the center of order number 89 of the
1961 echelle, greatly magnified, together with microphotometer
traces of two exposures.

 

Table I. Lines in Echelle Spectrum

3000—2091 A

 

 

Lines Lines Percent
Intensity listed identified identified
1 1943 457 24
2 1516 679 45
3 1275 783 61
4 1 104 836 76
5 7 12 614 86
6 393 367 93
7 139 132 95
8 50 47 94
9 14 14 100
Total 7146 3929 55

 

short section near the center of order number 89 is re-
produced in Fig. 12, greatly magnified. Included also
are microphotometer traces of two different exposures.
The finest structure is obvious granularity. Most of the
identifications are positive, but the coincidence of a
line with Li I, 2741. 204 A, is considered to be accidental,
since Li has an extremely low abundance in the sun.
The identification of SIII is in some doubt, because
Fraunhofer lines of no other doubly ionized ions have
been recorded. However, most of the lines of S III that
might be expected were present. The many extremely
intense, broad lines of FeI and Fe II, such as Fe II,
2739. 545 A, form the most characteristic features of the
solar spectrum in the middle uv.

The reduction of these spectra to obtain precise
wavelengths was performed empirically since the
changing magnification in the lens system with wave-
length and the distortion across the field made the pre
cise application of the grating equation impossible. It
was a simple matter, however, to identify many ex-
cellent lines of Fe I and Fe II for use as standards. This
was done for each order. The difference between air
and vacuum wavelengths had to be taken into account,
of course, since the actual solar exposures were made in
vacuum. After selection of the standards, a cubic
equation was used to fit the measured line positions and
the dispersion curve. Overlap regions between ad-
jacent orders were carefully checked. In the case of
sharp, well—defined lines, wavelengths were determined
to an accuracy of 0. 01 A. For faint or diffuse lines the
error was approximately 0.020 A to 0.025 A One diffi-
culty in making the measurements with a microphoto-
meter or comparator arose from the fact that each order
not only slants slightly relative to the normal to the
slit, but also is slightly curved because of the action
of the prism and the field corrector. Weak Rowland
ghosts, though present, generally fell between orders,
as discussed earlier, and did not interfere.

The analysis of the solar spectra to determine wave-
lengths has been completed, and as many of the lines as
possible have been identified. Table I lists the number
of lines recorded, grouped by intensities on a visual
scale of 1 to 9, corresponding roughly to the scale —3 to
25 used in the recent revision of the Rowland Tables

prepared by Moore et al. 16 Comparison with the latter
line list over the region 3028 A to 3069 A resulted in
excellent agreement in wavelength, intensity, and
identification. The average wavelength deviation for
the best lines was 0 003 A, and 0.014 A for poor lines.
The faintest lines of intensity 1, which are close to
the noise level of the spectra, were found to be the least
certain, of course. Thirty—four of the sixty-four faint
lines in this wavelength range were coincident with
lines in the list of Moore et al., whereas fourteen of their
fifty-seven weak lines were coincident with ours. In a
similar comparison with traces made by Migeotte17 on
the J ungfraujoch, fifty—two of the same sixty—four weak
echelle lines were coincident, while forty—four of the
fifty—seven lines appearing in Migeotte’s spectrum were
coincident. These differences are, in part, attributable
to the fact that the background continuum, even on the
shortest echelle exposure, was greatly over-exposed;
therefore, the weak lines in the solar spectrum were
difficult to detect.

It is expected that the 11v solar line list, derived from
the 1961 and 1964 echelle spectra, will be published by
the Government Printing Office in 1967. It will be of
the same general form as the Preliminary Rowland
Tables of St. John et al.18 and the new revision by Moore
et al.19 With the aid of the Ultraviolet Multiplet Ta—
bles by Moore, and other material, about half of the
observed lines were identified—but no more. This
shows that a need exists for much additional research
in laboratory high resolution spectroscopy, especially
for the abundant solar elements.

Until a much larger fraction of the lines are identified,
it will be hazaldous to place confidence in the identifica—
tion of some particularly interesting lines, for example,
Hg I, 2536 519 A. Mercury has not been detected in
the sun; there is a line at 2536. 50 A in the echelle
spectrum which is faint and within one standard
deviation of the mercury line; but in view of the large
probability of a chance coincidence, this cannot be
identified as the mercury line.

The reduction of the echelle spectrum to intensities

 

.. massaging

Fig. 13. A portion of order number 73 of the 1961 echelle spec—
trum, centered three-fourths of the way to the right and en-
larged about seven and one-half times. The heavy curve is a
linear plot of intensity, as reduced from the echelle spectrum,
and the light curve is the solar intensity from the G6ttingen
Solar Atlas, recorded with a 6.4-m concave grating. Upper
curve: Géttingen Atlas. Lower curve: NRL rocket.

 

March 1967 / Vol. 6, No. 3 / APPLIED OPTICS 371

is difficult because of the problem of correcting for the
stray light background. An attempt was made over a
short region, 3330 A to 3338 A, which 1s covered both by
the Géttingen Atlas and the echelle spectrum This is
reproduced in Fig. 13. Line intensities range from
some of those listed as —3 on the Rowland scale, which
are among the faintest listed by St. John et al.13, to lines
of intensity 8. The echelle film was calibrated by con-
structing an H and D curve from exposures made with a
carbon are. The difficulty in application of this curve,
however, was making the proper correction for stray
light. By employing the background density as a vari—
able parameter, it was possible to obtain a nearly perfect
match with the G6ttingen trace, as can be seen from
Fig. 13. However, it was not possible to relate the
stray light correction so determined to any obvious posi-
tion in the echelle spectrum. Therefore, there was no
straightforward way to obtain the stray light correction
below the limit reached by the Gfittingen Atlas (2965
A) by examination of the spectra.

The procedure being followed to reduce the echelle
spectrum to intensities from 3000 A to 2100 A, where
high resolution spectra that are free from stray light are
not available, is to compare the echelle spectrum with
the low resolution spectra of Wilson et (11”, and of
Malitson et (11.21 These spectra have some twenty
times less resolution, but they are fairly free from stray
light. The echelle spectrum is recorded with a micro-
photometer in digital form on magnetic tape so that the
densities can be transformed with a computer. The
microphotometer trace will then be subjected to an
averaging process in order to reduce the resolution to the
value of the earlier spectra; then the stray light correc-
tion parameter in the H and D curve will be adjusted
so that the average echelle spectrum matches precisely
the earlier spectra. After this is done, the detailed
microphotometer trace of the echelle spectrum will be
reduced to intensities using the adjusted H and D curves.

It would be wishful thinking, of course, to hope that
the echelle spectrum could ever be corrected to show the
cores of the deepest Fraunhofer lines: once they are
filled in by stray light, this information is lost. To ob-
tain high dispersion spectra with no stray light at all it
will be necessary to employ a large grating spectrograph
of perhaps 2 m radius of curvature, equipped with an ex—
cellent predispersing system. An instrument of this
type is being constructed for flight in the Astronomical
Telescope Mount Mission of the Apollo Manned Space
Flight Program. It will be mounted in such a way that
it can be pointed with precision at any position on the
sun or in the chromosphere. One of the astronauts will
make the exposures and bring back the film.

Many individuals have made significant contribu—
tions to this program. To the Farrand Optical Com-
pany we are indebted for the expert design and fabrica-
tion of the lenses and prism. Aircraft Armaments,
Incorporated, constructed the coelostat used successfully
for the 1956 flight, and the Ball Brothers Research

372 APPLIED OPTICS / Vol. 6, No. 3 / March 1967

Corporation produced the biaxial pointing control
employed in 1961 and 1964. G. A. Vanasse and later
D. J. Michels and A. Boggess III were involved in
setting up the instrument. The special, technique for
aluminization of the echelles was developed by D. W.
Angel. Reduction of the spectra was commenced by
H. P. Erickson and carried to completion by L. Giddings
and P. A. Simmons. For the echelle gratings we are
particularly indebted to the late David Richardson of
Bausch & Lomb, Incorporated.

References

1.
2.

{/3

G. R. Harrison, J. Opt. Soc. Am. 39, 522 (1949).

J. D. Purcell, A. Boggess III, and R. Tousey, I.G.Y.
Rocket Report Series, No. 1, p. 198, Natl. Acad. Sci-Natl.
Res. Council, Washington, D.C. (1958).

J. D. Purcell, D. L. Garrett, and R. Tousey, in Space
Research III, W. Priester, Ed. (North-Holland Publishing
00., Amsterdam, 1963), pp. 781—786.

4. V. P. Kaéalov and A. V. Jakovleva, Izvestia Krymskyi
Astrofiz. Obs., Akad. Nauk, SSSR 27, 5 (1962).
5. V. P. Kacalov, N. A. Pavlenko, and A. V. Jakovleva,

Izvestia Akad. Nauk, SSSR, Ser. Geofiz. No. 9, 1099
(1958); Bull. Acad. Sci., USSR, Geophys. Ser. No. 9,
636, Izvestia Akad. Nauk, SSSR, Geophys. Ser. No. 8,
1177 (1959); Bull. Acad. Sci. USSR, Geophys. Ser. No. 8,
844 (1959).

W. E. Behring, H. C. McAllister, and W. A. Rense, Astro-
phys. J. 127, 676 (1958).

S. C. Miller, Jr., R. Mercure, and W. A. Rense, Astrophys.
J. 124, 580 (1956).

8. H. C. McAllister, “A Preliminary Photometric Atlas of the
Solar Ultraviolet Spectrum from 1800 to 2965 Angstroms”,
Univ. of Colorado Printing Services, Boulder, Colorado
(1960).

9. S. Rosin, J. Opt. Soc. Am. 42, 451 (1952).

10.
11.

12.
13.

14.

16.

17.

18.

19.

20.

21.

E. M. Brumberg, Compt. Rend. URSS 2, 467 (1935).

V. H. Regener, Quart. J. Roy. Meteorol. 800., Suppl. to Vol.
62, 9 (1936).

H. M. Crosswhite, private communication.

C. H. Corliss, W. R. Bozman, and F. O. Westfall, J. Opt.
Soc. Am. 43, 398 (1953).

G. Bruckner, Photometrischer Atlas des Nahen Ultravioletten
Sonnenspektrums, 2988 141—3629 .4 (Vandenhoeck and
Ruprecht, Gottingen, 1960).

C. R. Detwiler and J. D. Purcell, J. Opt. Soc. Am. 52, 597
(1962).

Charlotte E. Moore, M. G. J. Minnaert, J. Houtgast,
“The Solar Spectrum 2935 A—8770 A”, NBS Monograph
No. 61 (1966), U. S. Govt. Printing Oflice, Washington, D. C.
M. Migeotte, private communication.

C. E. St. John, C. E. Moore, L. M. Ware, E. F. Adams,
and H. D. Babcock, “Revision of Rowland’s Preliminary
Table of Solar Spectrum Wavelengths with an Extension
to the Present Limit of the Infra-red”, Carnegie Institution,
Washington, D.C. (1928).

C. E. Moore, “An Ultraviolet Multiplet Table”, Natl.
Bur. Std. Circ. No. 488, US. Govt. Printing Office, Washing—
ton, D.C. (1950).

N. L. Wilson, R. Tousey, J. D. Purcell, F. S. Johnson, and
C. E. Moore, Astrophys. J. 119, 590 (1954).

H. H. Malitson, J. D. Purcell, R. Tousey, and C. E. Moore,
Astrophys. J. 132. 746 (1960).

 

 

 

 

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