Localized sources of far infrared radiation (approximately 50 microns) have been detected during a high altitude balloon flight with a 40 cm telescope and silicon detectors. The flight system is described and preliminary results are presented. A large area of the sky has been scanned for localized sources of far infrared radiation, using a balloon-borne system that was sensitive to wavelengths beyond about 55 microns. Two Molectron silicon bolometers were used, with a Newtonian telescope having a 40 cm primary. The telescope was driven in azimuth at a fixed elevation; this mode of scanning was carried out for the duration of each of two balloon flights. The flight system is described

Friedlander, M. W.

Goebel, J. H.

Joseph, R. D.

English

NASA Technical Reports Server

N75 12750
*•
Paper 2.10
SKY SURVEY AT FAR INFRARED WAVELENGTHS
USING A BALLOON-BORNE TELESCOPE
M. W. Friedlander
J. Goebel
R. D. Joseph*
Department of Physics
Washington University, St. Louis, Missouri
(*Now at Imperial College, London)
ABSTRACT
Localised sources of far infrared radiation (x>50y) have
been detected during a h i g h altitude balloon flight with a 40
cm telescope and silicon detectors. The flight system is de-
scribed and preliminary results are presented.
I. INTRODUCTION
A large area of the sky has been scanned for localised
sources of far infrared radiation, using a balloon-borne system
that was sensitive to wavelengths beyond about 55 microns. Two
Molectron silicon bolometers were used, with a Newtonian tele-
scope having a 40 cm primary. The telescope was driven in azi-
muth at a fixed elevation; this mode of scanning was carried out
for the duration of each of two balloon flights from the
N.C.A.R. Balloon Facility in Palestine, Texas, in September
1973. The flight system is described in this paper, and pre-
liminary scientific data w i l l be presented at the meeting.
II. FLIGHT SYSTEM
a) Optics.
The primary mirror had a diameter of 40 cm and a focal
length of 100 cm; the flat secondary was used as the chopper and
was rocked at about 8 Hz. The dewar with the detectors was lo-
cated at the one side of the telescope, with focus in the en-
trance plane of two brass light-pipes. The field of view is de-
fined by the ratio of the focal length of the primary and the
front aperture of each light pipe, and is a rectangle of 0.65°
in azimuth and 1.0° in elevation. The amplitude of the chopping
motion was adjusted so that the two fields of view that were to
be compared were separated by 1.0°
Each light-pipe was of square cross-section, starting with
1.8 cm side and tapering.over a length of 10 cm to a square exit
aperture with 0.5 cm sides, leading to a spherical cavity of
0.95 cm diameter, in which the silicon chip was suspended. The
l i g h t - p i p e assemblies were made of brass; the spherical housing
and a mounting plate were made of oxygen-free copper, and mount-
ed to the underside of the l i q u i d - h e l i u m container. The two
light-pipes were arranged with the one beneath the other. The
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silicon detector elements were each 5 x 5 x 0.4 mm, manufactured
by Molectron Corporation.
Wavelength selection was set by the filters: 2 mm crystal
quartz on the dewar (0.5 mm of clear polyethylene was used for
one flight); 1.8 mm of teflon on the aperture in the radiation
shield within the dewar; and 2 mm of crystal quartz plus 6 mil
black polyethylene at the entrance to the light-pipes. The
overall spectral response of the system was compared to a Gola.y
cell at the University of Missouri -Rol la , using a Fourier-trans-
form spectrometer. The response is fairly flat from 60 microns
to the longest wavelength checked, 600 microns; the response
falls to half by about 55 microns and is not measurable below 50
microns. A preliminary estimate yields 10% as the overall
transmission of the filter combination at 100 microns and 20%
for A>200p.
b) Orientation
The telescope was
driven continuously in
neous direction of the
a magnetometer and sky
used as
Polaris
locked at 26.5° elevation, and then
azimuth during the flight. The instanta-
telescope was obtained through the use of
cameras. A Schonstedt magnetometer was
a sensor to serve two cameras to continuously follow
These cameras were operated with open shutters and
continuously-moving film, so that Polaris was recorded as a
trace. (Time markers were supplied by an Accutron watch.) The
azimuthal difference between the cameras and the telescope was
monitored with a 9-bit shaft encoder, and corrections were later
included based on the actual position of the Polaris trace, the
local time, and the balloon drift in longitude. The moon was
detected by the bolometers several times in each flight, and
these sightings provide a calibration of the co-ordinate meas-
urement. The accuracy of other source positions is then about
0.4° in azimuth and +_ 0.5° in elevation; the corresponding un-
certainties in R.A. and declination vary somewhat in different
parts of the sky but generally do not exceed + 0.5°.
c) Signal Processing
The preamplifiers were located in a shielded box immediate-
ly adjacent to the dewar. The output of the preamps was both
telemetered directly and also processed further on board after
which it was recorded on-board and telemetered. On-board, lock-
in amplifiers were used in conjunction with a reference signal
provided by the chopper, and each detector output was recorded
in high-gain and low-gain channels. The on-board recording was
intended as a back-up system, and consisted of the continuous
photographing of meters that displayed the outputs. In parallel,
these outputs were telemetered via the NCAR FM/FM system.
During the first flight, a major DC offset was observed
which was attributed to differences in emissivity of the two
fields that were viewed during the successive halves of the
chopping cycle. Before the second flight, all parts of the
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152 . /
telescope that could be 'seen' by the light-pipes were covered
with aluminum foil; these included the areas behind the primary
mirror and on the inside of the telescope tube behind the sec-
ondary and directly opposite the entrance of the light-pipes.
The result was a noticeable reduction in offset for the second
flight. The offset was still large, but some celestial sources
could still be seen on both flight records.
Electronically, the saturation effects of the large DC off-
set occur in the lockin amplifier, and these could be obviated
after the flight. A new lockin amplifier was constructed; the
inputs for this were the directly telemetered preamp output and
the reference signal (both of which had been recorded). Far
larger signals could not be handled, although the lunar signals
still completely saturated the system.
Two further electronic filters were introduced and proved
to be of major use in cleaning up the signals. These comprise
our 'notch' filter: one pass-band is centered at the chopping
frequency of 8.3 Hz and the other at 3.1 Hz. At the lower fre-
quency, persistent noise had been detected, and was attributed
to beating from the chopping of the secondary mirror.
The post-flight record shows an average noise level record-
ed at 60 nv/HzV2. Extensive pre-flight calibration of the de-
tector had yielded a value of 50 Kv/watt responsitiyity. With
20% as the filter transmission, this leads to a minimum detect-
able signal of -06 x 10-12 watt at the entrance to the light-
pipe, or an incident intensity ^ 10~23 W/m2.Hz.
It should be pointed out that the minimum detectable sig-
nal could be considerably reduced if an oriented system were
used. This would permit a much smaller field of view with con-
sequent smaller apertures in the dewar and radiation shield.
d) Scanning Mode
It was decided to adopt the unusual arrangement of the two
detectors to allow a check on the sighting of celestial objects.
If an infrared source is detected in the east it may first be
seen by the lower detector. Four minutes later, when the gondo-
la has returned to the same azimuth, the source will have risen
in the sky and should be observed in the upper detector. For a
source first seen in the west, the order of sighting in the two
detectors w i l l be reversed. Without repeated sighting of an ob-
ject, there w i l l usually be a residual suspicion that one might
instead have noted an instrumental glitch, and the two-detector
system is designed to obviate this problem. It might, of
course, happen that a source is first viewed in the upper detec-
tor but this should happen only for those objects that are al-
ready too high to be seen with the lower detector, and one might
then expect to observe such objects twice more (one with each
detector) later in the flight.
The elevation angle of the telescope is adjustable on the
ground, and was set at 26.5° for the September flights, to give
us the possibility of sweeping through the galactic center.
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153
The rate of scanning was chosen at 4 minutes per revolu-
tion, and the drive system was provided by NCAR.
e) Balloon flights.
Both flights were successfully carried out from N.C.A.R.-
Palestine, in September 1973. Data for the second flight (Sept.
20-21) is being analysed first because of the reduced offset
problem. Both flights were at float altitudes of about 97,000
ft, for about 9 hours.
III. RESULTS
Only preliminary results were available at the time of the
meeting. The upper detector record shows a total of 82 events
that at first sight appear to be possible signals, while the
lower detector show 78 such events. With only a preliminary
analysis so far available, detailed listings of possible sources
and their locations are not being included with the present
paper. Even at this early stage, it can be seen that the moon
was detected four times and that these observations provide a
calibration of the co-ordinate measurements. Several other
sources were seen more than once. Small corrections in co-
ordinates still need to be included.
Acknowledgements
As always, we are grateful to the crew at N.C.A.R. Pales-
tine for their flight operations. Thanks are also due to
A. Gorman, M. Jones, J. Borgwald and L. Lissak for assistance
at various stages of the preflight and postflight work. This
program has been supported by N.A.S.A. under grant
NGR-26-008-069.
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DISCUSSION SUMMARY - PAPER 2.10
The discussion was concerned with the identification and intensities of
the sources observed in the Washington University investigation.
It was reported that the stronger sources observed had intensities of
10,000 to 20,000 flux units. Some sources observed were within one or
two degrees of known pulsars but it will be necessary to process the photo-
graphs from the onboard star camera before identification is completed.
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Sky survey at far infrared wavelengths using a balloon-borne telescope

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