A liquid helium cooled infrared telescope (IRT) was to be flown in July 1985 on Spacelab 2. The instrument is designed to measure both diffuse and discrete infrared astronomical sources, including the zodiacal light, galactic, and extragalactic components, as well as to evaluate the induced Orbiter environment. The focal plane contains ten photoconductive detectors covering six broad bands from 2 to 120 microns. Each detector has a 0.5 by 1.0 deg field of view optimized for detection of extended sources of IR radiation. Except for the 2 micron detector, the system noise is limited by the sky background noise. The measurements planned for the IRT use the 1 meter base of the Plasma Diagnostic Package (PDP), an already existing SL 2 experiment, as the glow generating surface. The measurements are repeated changing the position of the PDP, the attitude of the Orbiter, and the ram direction in an effort to remove both the thermal component of the PDP emission and the cosmic background radiation

Fazio, G. G.

Koch, D. G.

English

NASA Technical Reports Server

INFRARED MEASUREMENTS OF SPACECRAFT GLOW 
PLANNED FOR SPACELAB 2 
Giovanni G. Fazio and David G. Koch 
Smithsonian Astrophysical Observatory 
Abstract. A liquid helium cooled infrared telescope (IRT) will 
be flown in July 1985 on Spacelab 2. The instrument was designed to 
measure both diffuse and discrete infrared astronomical sources, 
including the zodiacal light, galactic,and extragalactic components, 
as well as to evaluate the induced Orbiter environment. The focal 
plane contains ten photoconductive detectors covering six broad 
bands from 2 to 120 microns. Each detector has a 0.6 by 1.0 deg. 
field-of-view optimized for detection of extended sources of IR 
radiation. Except for the 2-micron detector, the system noise is 
limited by the sky background noise. 
This paper describes the measurements planned for the IRT using 
the 1 -meter base of the Plasma DiagnosticsPackage (PDP) , an al- 
ready existing SL 2 experiment, as the glow generating surface. The 
measurements will be repeated changing the position of the PDP, the 
attitude of the Orbiter, and the ram direction in an effort to re- 
move both the thermal component of the PDP emission and the cosmic 
background radiation. 
Introduction 
The primary intent of these observations is to obtain measure- 
ments of the glow phenomenon in the infrared produced by the base of 
the Plasma Diagnostics P a c k a g e  (PDP). The IRT is capable of obtaining 
data in six broad passbands: 2.0-3.0, 4.5-9.5, 6.1-7.1, 8.5-14.5, 
18-30, and 70-120 microns. The two longest wavelength passbands, 
18-30 and 70-120, include three detectors each. See Koch, et al. 
[I9821 for a complete description of the instrument. 
c19811 contains a detailed description of the IR detector perform- 
ance. Additional data that will be of use in interpreting the re- 
sults will come from the in situ measurements by the PDP of the 
particles and fields associated with the ram gases. Specifically, 
data from the mass spectrometer will be used. Further data that 
will be of use are measurements available in the visible from the 
image-intensified camera [Mende et al., 19851. The equatorial 
altitude for this mission will be 383 km and the inclination 49.5 
deg . 
Young, et al. 
243 
https://ntrs.nasa.gov/search.jsp?R=19860003795 2020-03-20T15:58:07+00:00Z
Method of Observation 
In an attempt to determine a dependence on material two observa- 
tional methods will be used. One will be to see if there is any 
glow from the inside of the gold-coated mylar sunshade of the IRT. 
The other will be to look for the glow from the base of the PDP, 
which is covered with beta cloth and aluminum screen as the PDP is 
positioned near the beam of the IRT. The Orbiter Remote Manipulator 
System (RMS) will be used to position the PDP so that the flat base 
of the PDP will be in the Orbiter Y-Z plane. In both of the methods 
of observation described below the telescope will be fixed (stowed) 
and viewing along the Orbiter -Z axis (see Figure 1). 
During the glow measurements, the field-of-view of the telescope 
must encompass regions of low infrared brightness. Such regions 
include the ecliptic pole or the galactic pole and must avoid re- 
gions near or crossing the galactic plane. (The preferred orbit 
would have the plane of the orbit pass near the ecliptic pole.) 
While the data are being taken the field-of-view, hence the Orbiter, 
must be held inertially on the sky. In addition, the data must be 
taken on the nighttime side of the orbit in order to observe with 
the lowest possible background and to permit optical photography of 
the glow. The velocity vector should be kept approximately normal 
to the base of the PDP. Data cannot be taken while near or passing 
through the South Atlantic Anomaly (SAA). Water dumps, operation of 
the flash evaporators, experiment purges,and other sources of efflu- 
ent must be inhibited prior to and during data taking. 
Since the thermal radiation from the PDP is substantial it will 
be important to have the thermal conditioning prior to and during 
the observing runs as nearly identical as possible. 
Due to the uncertainties in both the absolute position of the PDP 
when manipulated by the RMS and the alignment of the telescope with 
respect to the Orbiter, it will be necessary to make the glow meas- 
urements by gradually cycling the PDP into and out of the field-of- 
view of the telescope. This will also provide a means of establish- 
ing the baseline intensity without the PDP. The intensity will be 
correlated with the RMS positional information. This cycling with 
the RMS will be repeated for each of four separate inertial posi- 
tions on the sky during the 36 minutes of Earth shadow during two 
consecutive orbits. On the first orbit the PDP base will be pointed 
into the wake and on the second orbit the PDP base will be pointed 
into the ram, a l l  other conditions remaining the same. During the 
first three inertial positions, the PDP will be moved in closer and 
closer to the telescope beam at about 15 cm above the sunshade aper- 
ture. During the fourth inertial position the PDP will be raised 
5.7 meters above the aperture where the 1-meter PDP diameter will 
still fill the 15-cm telescope beam, but the thermal effects from 
the PDP will fill a much smaller portion of the telescope side 
244 
lobes. This alternate position is shown in Figure 2. Before each 
RMS cycle, the cold shutter at the focal plane will be actuated to 
provide an absolute flux level. 
base is into the ram, the mass spectrometer on the PDP will be 
pointed into the ram to provide in situ measurements of the plasma. 
On the second orbit when the PDP 
Data Deconvolution 
To be able to distinguish the thermal radiation of the PDP from 
the glow, as well as to measure the intensity scale height of the 
glow, the method of observation has to provide many differential 
experiments. These include moving the source (PDP) in and out of 
the telescope beam, repeating the cycles at different distances 
from the beam, and repeating at a different height above the aper- 
ture. Before and after each cycle the cold shutter is used to pro- 
vide an absolute flux level. All of this is done first with the PDP 
base in the wake where there should not be any glow and only the 
infrared of the environment and sky is measured. Then the entire 
process is repeated on the next nighttime pass with the base into 
the ram where any difference in signal should be due to the glow. 
Any slight variations of the sky background due to variations in the 
inertial position can be corrected, since the prime objective of the 
IRT is to measure the large-scale structure of the sky. In addition 
to the many signal modulation and differentiation effects, the glow 
should be distinguishable spectrally from the zodiacal and the PDP 
thermal radiation,both of which are approximately 300 K blackbodies. 
Finally, any glow off of the inner surface of the gold-coated 
mylar sunshade should be detectable and separable from other signals 
since during each cycle of the RMS, the telescope is held inertially 
on the sky, but during the 4 to 9 minutes of inertial hold, the 
orbital motion will result in the velocity vector varying from 
slightly into the sunshade aperture to slightly out of the sunshade. 
During normal IR mapping the telescope aperture will always be 
pitched slightly away from the velocity vector so that the inside of 
the sunshade is never exposed to the ram. 
Expected Results 
An estimate of the expected infrared flux can be made based on 
the following assumptions: 
1. The glow is due to atomic oxygen impacting the surface at 
2. The atomic oxygen flux at 400 km altitude is 1.5 x 
3. The spectral shape of the emitted radiation will resemble 
4. 
7.7 km/sec. 
1014/cm1/sec. 
that of highly excited OH. 
The decay distance from the PDP base produced by oxygen 
impact is 1 meter or more. 
In the paper by Banks 
ble photon emission was estimated to be 2 x l o 7  photons/cm3/sec at 
the 280 km altitude for STS-3. This was observed in about a 
et al. [1983] the volume density of the visi- 
245 
0.1 micron band so t h a t  t h e  s p e c t r a l  f l u x  is 2 x lo8  photons/cm3/ 
sec/micron a t  about 0.65 microns. I f  t h e  emi t t ing  spe- 
cies is OH we can use Langhoff's [1983] calculated spectrum to get 
volume densities of 1010/cm3/sec/micron at 1.3 to 2.2 microns at 
280 km and 10g/cm3/sec/micron at 4.5 to 6.5 microns beyond which it 
drops rapidly with increasing wavelength. The IRT will look through 
a 100-cm-thick volume of gas that is radiating into 4u sr. There- 
fore, the telescope should see a flux of 8 x 1010pho tons / cm2/  
s e c / s r / m i c r o n  a t  1 . 3  a n d  2 . 2  m i c r o n s  a n d  IZeD= 16 x l o 9  
photons/cm2/sec/sr from 4.5 to 6.5 microns. From ground-based meas- 
urements of STS-9 (240 km altitude) by Witteborn et al. [ 19851 a 
flux of 5 x photons/cm2/sec/sr/micron at 1.6 microns was ob- 
served and the observed flux at 2.3 microns was attributed entirely 
to the diffuse reflection of the Earth thermal emissions. This 
would imply a factor of aboutsktimes more emission at 1.6 microns 
than expected from using Langhoff's method. 
7 x l o 9  photons/cm2/sec/sr which is comparable to 12~0. 
will be at 400 km altitude, the oxygen incidence will be ten times 
lower, resulting in an equivalent decrease in the volume densities 
of photon emission. Consequently the irradiance at 400 km from "O- 
glow" would be 1400 = 1.6 x l o 9  photons/cm2/sec/sr. This is only 
20% of the zodiacal emission expected in the ecliptic, 90 deg. from 
the Sun. This irradiance would be about 60% of the zodiacal emis- 
sion (away from ecliptic plane) over most of the hemisphere away 
from the Sun. Distinguishing the 0-glow from the zodiacal emission 
should be easy, since the 0-glow is not expected to contribute to 
the observed flux at wavelengths beyond 15 microns. Thus the 0-glow 
should change the ratio of the radiation in the 4.5-9.5 micron band 
to that in the 18-30 micron band. 
The zodiacal emission in the 4.5-8.5 micron band is about 
Since SL 2 
Conclusions 
Although the plan to make the glow measurements have evolved only 
recently (after the final mission plan for SL 2 was formed) , the 
opportunity to perform these IR measurements with existing hardware 
in so near a term has led us to dedicate two of the 18 revs.planned 
for IR celestial observations to making these measurements. It 
appears that in using the observing plan outlined above, the glow 
should be detectable in the near IR for the given geometry. Other- 
wise, the observations will put severe upper limits on the flux, 
limiting it to well below the zodiacal flux for the geometry de- 
scribed. 
Acknowledgments. Funding f o r  t h e  I R T  is provided by NASA Grant 
NAS8-32845. The ca l cu la t ions  f o r  t h e  expected f l u x  w e r e  performed by 
F. Witteborn, NASA/AmeS Research Center. 
246 
References 
Banks, P. M., W i  
K o c g  D., G. G. F 
10, 118-121, 
A. Dalgarno, 
i n  
t i o n ,  SPIE 280, pp. 12-19, 198 
_I_ 
247 
248 
5 k 5 0 %I a, 5 0 E: -4 k 5 a d 0 -4 +J -4 m 0 a pt pt a 
249 


Infrared measurements of spacecraft glow planned for Spacelab 2

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