High resolution solar spectrometer for CV-990 aircraft infrared observation

Cruikshank, D. P.

De Wiess, F. A.

Kuiper, G. P.

English

NASA Technical Reports Server

No. 126 A HIGH-RESOLUTION SOLAR SPECTROMETER FOR AIR-BORNE 
INFRARED OBSERVATIONS 
by D. P. CRUIKSHANK, F. A. DE WIESS, AND G. P. KUIPER 
November 22, 1968 
ABSTRACT 
The construction and operation of a 4-meter Czerny-Turner scanning spectrometer in the NASA Convair 990 Jet air- 
craft are described. The design of the spectrometer and its mounting support permit high resolution in spite of aircraft vibra- 
tions. Maximum resolution achieved was about 60,000, close to the theoretical limit of the optics. 
1. Objectives 
his paper describes the construction and opera- 
observations of the solar spectrum in the region 0.8- 
5.0 p from the NASA Convair 990 aircraft at alti- 
tudes of 39,000 to 42,000 ft ( 12-13 km). 
Ground-based astronomical spectroscopy in the 
infrared is severely hampered by strong telluric ab- 
sorptions, with H,O the most prominent absorber. 
In a discussion of the importance of infrared studies 
at conventional aircraft altitudes, Kuiper, et aZ(1968) 
pointed out that the scale height of water vapor in 
the atmosphere is about 1.6 km while that of the 
remainder of the air is about 8.0 km. Thus, at alti- 
tude 12 km the water vapor concentration is reduced 
by a factor of about lo3 while the ambient air pres- 
sure is reduced only by a factor of 5. 
Because of the strength and complexity of the 
absorption bands between 0.8 and 5.0 p, a drastic 
reduction of the H 2 0  concentration alone is insuffi- 
T tion of a 4-meter Czerny-Turner spectrometer for 
cient for a survey of solar lines in these regions. 
Resolutions up to 0.1 cm-' are desirable to reach the 
fainter solar lines, given the various broadening 
effects present and the need to resolve blends as 
much as possible. 
2. Instrumental Requirements 
The restrictions placed on airborne spectral ob- 
servations are severe. Aircraft dimensions restrict 
the size and weight of the spectrometer. On a given 
flight the observing time is limited by fuel load and 
flight path, demanding rapid operations. A com- 
promise in resolution and recording speed must be 
reached on the basis of feasible spectrometer dimen- 
sions. 
Sunlight must be introduced into the spectrom- 
eter with a telescope limited in aperture by avail- 
able windows and the strength of window materials. 
Effective infrared transmitters are generally thin 
crystaline substances obtainable with clear apertures 
of 6 inches or less. Because of limb darkening, the 
233 
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234 CRUIKSHANK, D E  WIESS A N D  KUIPER 
guiding accuracy of the aircraft-telescope combina- 
tion must be sufficient to keep the sun’s image nearly 
centered on the spectrometer entrance slit during the 
observing period. 
Aircraft vibrations require the spectrometer com- 
ponents to be rigid. The amplitude and frequency of 
vibrations in the aircraft depend on spectrometer 
location and orientation, which must be con3fdered 
in the spectrometer design. For example, vibrations 
of frequencies perceptible to the touch are often of 
50 times greater amplitude in the aft sections of an 
airplane than those in forward positions. 
Finally, the equipment and its mounting to the 
aircraft must be capable to withstand emergency 
landing conditions with accelerations up to 9 G. This 
requirement constrains the spectrometer design and 
the shock and vibration mountings permitted. 
3. The 4-Meter Spectrometer 
Scanning spectrometers of the Ebert and Czerny- 
Turner types have been used successfully at the 
Lunar and Planetary Laboratory for several years 
(see Kuiper, et al, 1964), both in the laboratory 
and at the telescope. Several medium-large high- 
quality diffraction gratings have been used inter- 
changeably between spectrographs and spectrom- 
eters. These gratings are plane, 128 x 154 mm in 
size, and are readily adaptable to different instru- 
ments. 
In experiments with the Laboratory’s 0.9-m 
spectrometer (“B”) , the maximum resolution h /  Ah 
achieved was 7000 at 2.1 p (corresponding to 3 A). 
In order to obtain resolutions better than 1 A with 
existing gratings and detectors, an instrument of 4 
meters focal length was designed, near the maximum 
length that could be carried through the doors of the 
NASA CV-990. With the maximum beam diameter 
of 5 inches, governed by the dimensions of the grat- 
ings, the optical system operates at F/31, sufficient 
for sun and laboratory sources at x < 10 p. 
The spectrometer installation in the CV-990 has 
been illustrated in LPL Comm. No. 123, Figs. l a  
and lb,  which includes the 12-inch feeder telescope 
and heliostat, units that were available from the 
planetary IR program (smaller units would of course 
have sufficed for the sun). The grating is supported 
on a table provided with a six-speed motor drive. 
At the focus of the 6-inch camera (shown with the 
collimator in Fig. 1 ; focal length 392 cm), the detec- 
tor serves as the analyzing slit. Lead-sulfide and lead- 
selenide detectors are available in a variety of dimen- 
sions. The focal plane end of the spectrometer 
(mounted in the laboratory) is shown in Figs. 2 and 
3. Highest spectral resolution is obtained with detec- 
tors of 0.10 or 0.05 mm width and 1.5 and 2.0 mm 
length, acquired from Eastman Kodak (PbS Ektron) 
and Santa Barbara Research Corp. (PbS and PbSe). 
They are factory-mounted in small bakelite Amphe- 
no1 plugs to which Microdot low-noise coaxial cables 
are soldered. The detectors are operated in dry-ice 
temperature (-78°C). The preamplifier and ampli- 
fier are external to the spectrometer. 
The amplifier and preamplifier were designed by 
Dr. H. L. Johnson and constructed under his super- 
vision. The design is similar to that pubIished by 
Kuiper, et al ( 1962) , except that the new preampli- 
fier uses solid-state components. A Sanborn recorder 
(Hewlet-Packard Model 7700) with response time 
of sec. is used for rapid data acquisition. 
Because of the great cost of aircraft operations, 
high reliability of the spectrometer was essential. 
For this reason, a modular design was adopted. The 
least reliable elements in the system are the chopper 
motor and the grating drive motor. A slit-chopper 
module was constructed in duplicate, each unit hav- 
ing alignment screws permitting optical adjustment 
before flight. A 900 rpm Gaylord-Rives (No. 
B-3274) synchronous motor and a four-blade chop- 
per give a chopping frequency of 60 Hz. The assem- 
bly permits synchronization of lamp and photocell 
to the amplifier system at other chopping frequen- 
cies, but normally they were used at the 60 Hz line- 
frequency available in the aircraft, which is suitable 
to a cooled detector. 
The grating drive module is seen in Figs. 3 and 
4. An Insco muIti-speed motor drives a single thread 
worm and a 360-tooth gear upon which the grating 
rests. Adjustment screws are provided for the optical 
alignment of the module. A double conical bearing 
carries the rotation axis of the worm gear and grat- 
ing, and it was found that this is the component most 
susceptible to vibration in flight. The optical lever 
from grating to camera mirror to detector being 8 
meters, small rotations of the grating surface cause 
considerable displacements at the detector. This 
module could probably be improved for flight opera- 
tions by the use of two well-separated bearings. The 
grating drive will need some improvement for future 
laboratory use. As noted in LPL Comm. 124, a peri- 
odic error in the dispersion, while not seriously inter- 
fering with operations, required more detailed wave- 
length calibration subsequently than would have 
AIR-BORNE SOLAR SPECTROMETER 235 
Fig. 1 
ror cells, controlled by adjustment screws shown in LPL Comm. 123, Fig. la. 
Top view of camera and collimator mirrors, with cover plate removed. Note mounting of mir- 
e slit (with chopper in front), 
screw seen on top for focusing 
detector; small crank at bottom, of grating position. 
236 CRUIKSHANK, D E  WIESS A N D  K U I P E R  
0 
1 a 
0 
d 
E 
AIR-BORNE SOLAR SPECTROMETER 237 
TABLE I 
RUNNING TIME ( S E C O N D S / ~ ~ ~  A) AND DISPERSION (ciq/lOOA) 
FOR AVAKABLE MOTOR AND CHART SPEEDS AND 
1200 LINES/MM GRATING AT A 1.2 p 
PAPER SPEED (MM/SEC) 0.5 2.5 10. 50. 
REDUCTION OF TIME NEEDED FOR 
MOTORSPEED ~OOAINTERVAL DISPERSION 
(SEC) (LENGTH 100 A INTERVAL IN CM) 
1 30 1.5 7.5 30 150 
2 60 3 .O 15.0 60 300 
5 150 7.5 37.5 150 750 
10 300 15. 75. 300 1500 
20 600 30. 150. 600 3000 
50 1500 75. 375. 1500 7500 
Multiply all Table entries by 0.86 for 1200 l/mm grating at A 1.0 p ;  by 0.43 for 600 l/mm grating at A = 2.0 p ;  and by 0.215 for 300 l/mm grating at 
X = 4.0 p .  (These cases refer to grating angles at 37"; for other angles a, multiply with the further factor sec a/sec 37" or cos 37' /cos~.  
been necessary otherwise. However, the versatility 
of the drive motor, with its six gear shifts, was an 
asset in a program where observing time was at a 
premium. Table 1, prepared by Mr. L. A. Bijl, lists 
the available options of motor and chart speeds for 
a particular grating and grating angle. As indicated 
in the footnote of Table 1, the numbers are readily 
converted to other wavelength settings with the same 
grating or other gratings, 
The cold box is shown in Fig. 2. The detector in 
its Bakelite mount is held in a brass cylinder that for 
focusing can be moved axially against a spring, using 
a metric micrometer screw with 1.5 cm travel. Fo- 
cusing is accomplished in two stages: (a) the spec- 
trum of an emission line source (fluorescent lamp) 
is formed on a translucent screen in a dummy detec- 
tor assembly. Visual inspection gives the approxi- 
mate focus; (b) repeated scans with various microm- 
eter screw settings through the spectrum of a nearby 
HaO vapor band refine the focus. The adjustment 
appears fairly insensitive to changes of +- 1 mm 
around the optimum focus, as expected at F/30. 
Usually step (a) is adequate. 
The capacity of the cold box is 2 liters, sufficient 
to keep the detector cold for 8 hours. Thermal insula- 
tion is provided by a mantle of finely divided silica 
(Santocell. ) A small electrically-heated sapphire 
window prevents condensation of water on the detec- 
tor surface. 
During the CV-990 flights the plane's interior is 
flushed with heated, compressed outside air. The 
moisture content of the cabin atmosphere is thus 
potentially quite low. However, the total water vapor 
introduced by the 20-25 scientists and other person- 
nel aboard into the 16-m spectrometer path was 
found to be far greater than in the entire atmospheric 
column from aircraft to sun. A simple solution was 
achieved at no cost by continuously flushing the spec- 
trometer interior with fresh air taken from the plane's 
airconditioning system. We confirmed on several oc- 
casions that this continuous flushing was essential 
to keep the instrumental H 2 0  vapor concentration 
within tolerable limits. For example, when an incan- 
descent source was used for calibration in flight, the 
lines in the H,O band at 1.13 p were invisible when 
the spectrometer was flushed, but quite strong when 
the ambient cabin air was admitted. During the solar 
observations, the entire heliostat and telescope area 
were enclosed with plastic sheets and also flushed 
with outside air. The total optical path inside the 
aircraft is about 22 meters, of which less than one 
meter is through the ambient cabin air. 
4. Vibration Problems 
The total length of the instrumentation, consist- 
ing of heliostat, telescope, and spectrometer, is 6.3 
meters. Over this distance flight-induced vibratory 
deformations of the fuselage vary in amplitude by 
values far greater than the relative displacements 
permissible for the optical components of the sys- 
tem. An analysis indicated that the weight of a single 
structure, to be suspended floating in the fuselage 
and of sufficient vibratory rigidity, would be a mul- 
tiple of the flyable value for the airplane. 
238 CRUIKSHANK, DE 
It was therefore decided (a) to divide the system 
into two units, one heliostat plus telescope, the other 
the spectrometer; (b) to provide each unit with rigid- 
ity sufficient to maintain alignment of its optical com- 
ponents under maximum amplitude of their har- 
monic response to fuselage vibrations; and (c) to 
control this response by suspending each unit sepa- 
rately by vibration absorbers. 
The penalty for decoupling the two units com- 
prising the optical path, namely, a displacement of 
the ray bundle at the interface under different oscilla- . 
tory reactions of the two units, was made acceptable 
by locating the plane of separation close to the slit 
of the spectrometer, where the light bundle is nar- 
rowest, and because of the fact that the solar image 
at this point is 50 times wider than the effective slit 
length (100 mm vs. 2 mm). 
The first unit containing heliostat, telescope and 
some of the electronic equipment and consisting of 
a boxframe of crossbraced aluminum angles, was 
attached to several points of the fuselage by vibra- 
tion absorbers. To provide this unit with the required 
rigidity posed no serious problem as its length-to- 
diameter is low (1.85) and the masses of its compo- 
nents could be distributed quite evenly in its volume. 
More of a problem was posed by the spectrom- 
eter unit, as its length-to-diameter ratio is high (9.5) 
and its main masses - the optical components - 
are located at its extreme ends. These components 
and, therefore, the end faces of the unit, have to re- 
main parallel within 1.3 seconds arc; or, stated dif- 
ferently, the unit has to be of sufficient rigidity to 
prevent it from responding to flight-induced vibra- 
tion with transversal vibrations with amplitudes of 
more than .05 mm at distances from the node up to 
2 m. 
The design developed to meet these constraints 
provides a rectangular tube fabricated of aluminum 
angles, crossbraced and covered with corrugated 
aluminum sheeting. Both ends are formed into boxes 
of aluminum plate, attached to bulkheads, providing 
the supports for the respective optical components. 
Eight tensioned steel cables attached to both ends 
of the tube and tilted 10” away from the tube axis 
toward the center, where they are supported by an 
external frame, serve to load the tube in compres- 
WIESS A N D  KUIPER 
sion. A total cable tension of 2455 Ib ( 1  120 kg) was 
chosen for a calculated safe column strength of the 
spectrometer tube of 3890 lbs (1770 kg). The weight 
of the complete assembly came to 189.3 lb (86 kg), 
and its length to 13 ft 6% in. (4.13 meters), not 
counting the projecting screws and small entrance 
tube. 
The unit was supported with vibration absorbers 
in two transversal planes, each located at the respec- 
tive center of gravity of the instrument end boxes, 
Two three-dimensional trusses providing these sus- 
pension points were in turn attached with vibration 
mounts to the fuselage in such manners that their 
main axis of absorption is at right angles to those 
of the mounts supporting the instrument tube. 
A stress analysis of the whole system indicated 
a safety factor of 4.9 for its weakest structural mem- 
ber under crash loading of 9g as specified by flight- 
safety regulations. 
5. Operation of the Spectrometer 
The 4-meter spectrometer was used on the CV- 
990 for a series of solar observations from July 2 
to August 12, 1968, as described in LPL Comm. 
123. In flight, the highest resolution obtained with 
a 1200 line/mm grating blazed for 1.0 p, and a 
detector 0.10 mm wide, was about 0.2 A. Refer- 
ence is made to three preceding Communications, 
especially No. 124, for sample reproductions of solar 
spectra. 
Acknowledgments. The construction and testing 
of the airborne solar spectrometer was sponsored 
by NASA through Grant NsG 161-61 and the Uni- 
versity of Arizona Institutional Grant NGR- 
03-002-09 1. 
REFERENCES 
Kuiper, G. P., Goransen, R., Binder, A., and John- 
son, H. L., 1962, “An Infrared Spectrometer,” 
Comm. LPL 1, p. 119-127. 
Kuiper, G. P., Forbes, F. F., and Johnson, H. L. 
1968, “A Program of Astronomical Infrared 
Spectroscopy from Aircraft, Comm. LPL, 6, p. 
155-170. 


A high-resolution solar spectrometer for air-borne infrared observations, number 126

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