The characteristics of a telescope system which was developed for high altitude balloon astronomy are discussed. A drawing of the optical system of the telescope is provided. A sample of the signals recorded during one of the flights is included. The correlation between the infrared flux and the radio continuum flux is analyzed. A far infrared map of the radio and infrared peaks of selected stars is developed. The spectrum of the planet Saturn is plotted to show intensity as compared with wavenumber

Jennings, R. E.

English

NASA Technical Reports Server

N75 1273s
PAPER 1.3
ASTRONOMICAL OBSERVATIONS WITH THE UNIVERSITY COLLEGE
LONDON BALLOON BORNE TELESCOPE
R. E. Jennings
University College London
ABSTRACT
Since 1970 the U.C.L. balloon borne telescope had made fourteen flights
in different parts of the world. Some of the features of the detection
system used are described together with examples of the different types of
astronomical observations that have been made.
INTRODUCTION
The telescope system was designed and built by the engineering group
in the physics department at University College. The timing was most oppor-
tune, coming as it did just when the early observations by F.J. Low from air-
craft and W.F. Hoffmann from balloons were showing the importance of making
observations in the far infrared ('v-lOOum) where many objects, principally HII
regions, were radiating most of their power, amounting to as much as 105 to
10° times the energy from the sun. The great advantage of the systenrwas its
ability to point, by means of a star tracker, to better than a minute of arc
enabling the identifications of sources to be made with little chance of
ambiguity.
All our photometric observations have used the same broad band of wave-
lengths, from 40 to 350um. The short wavelength 'cut on1 is determined by the
quartz Fabry lens and black polythene filter mounted on the base of the cryo-
stat together with the rigidex entrance window (see Fig.2). The long wave-
length limit is determined by diffraction effects. The detector used is a 1mm
square gallium doped germanium bolometer (Infra-red Laboratories Inc.). The
system's relative response is known from laboratory measurements in a vacuum
tank using a Michelson interferometer and this is put on aa absolute basis by
observing a suitable planet during flight. The aperture stop most frequently
used is 5 arc min«
FEATURES OF THE DETECTION SYSTEM
It is hoped that a brief mention of one or two particular features will be
of interest to other experimenters,
(i) Chopping System
Two types of chop have been employed, both oscillating the secondary mirror,,
The mirror is supported by a phosphor bronze disc with a rod some 20 cm in
length firmly attached to the back of the secondary mirror mounting. The type
of chop is determined by the movement given to the further end of this rod.
A 'circular chop' has been used for a number of flights and the phase
sensitive detection (P.S.D.) performed parallel to and perpendicular to the
direction of scan. The resulting infra-red signals - INFRARED A and INFRARED B
respectively - are shown in Fig.l, which shows the signals recorded while cali-
brating on Saturn.
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https://ntrs.nasa.gov/search.jsp?R=19750004666 2020-03-23T00:43:48+00:00Z
Figure 1. Signals Recorded while Calibrating System.
The star tracker is locked on to Saturn and the main telescope is being
commanded to make a small raster scan about the planet. Whilst a chart record
is sufficient during the flight, data on tape is used for the final analysis.
For detailed mapping of regions we prefer to use a linear chop in which the
secondary is rocked from side to side (by solenoids operating on the top end of
the bar). The resulting movement, as determined by a laser shining onto a
mirror attached to the back of the secondary mirror, is a fair approximation to
a square wave.
(ii) Importance of Phasing in Flight
We phase sensitively detect at the balloon end and find that it is important
to be able to adjust the phase of the reference sugrial to the P.S.D. system
during flight, by repeated observations of a bright object (planet). This
applies particularly to the linear chop, where the optimum phase has been found
to be as much as 30° different to what was thought to be the optimum on the
ground - an awkward adjustment to make without special equipment.
(iii)"Red Button"
To offset residual mismatch of telescope radiations between the two parts of
the chop, we make use of a simple sample and hold circuit immediately prior to
the B.C. amplifiers. By command, it samples the B.C. signal immediately after
the P.S.D. and applies it to the other side of the operational amplifier in the
first D.C. stage.
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It is called a 'Red Button' because we heard that a ground based observa-
tory made use of such a device of that name - only to find later that they had
never heard of it I
(iv) Visual Maps
The beam out of the telescope is separated into its infra-red and visual
components by means of a dichroic mirror as shown in Fig.2.
Figure 2. Dichroic mirror reflecting the infra-red beam up into
the cryostat and passing the visual beam, which is then
reflected into the photomultiplier.
Thus in effect an infra-red map is recorded over which can be laid the
corresponding visual map. By superimposing the visual signal on the X-Y plotter,
which is used to indicate the telescope position relative to the guide star, a
visual map as shown in Fig.3 can be produced. In practice it is far more accurate
1.3-3
19
to use the digitised coordinates off tape when use is being made of the visual
signals in the analysis. In practice better use can be made of the observing
time by only scanning limited regions.
.t\ :-v.^ j£r3*<J*Sfy0hg^
N^ f^^ -^^ j^ j^S^ lLy,---,---. -.- :
•• ••••--^ v^ v^v^ -*!^ 3C^ :;?.v,-^ -.. ::.-.-.
Visual star map of *5* scan around guidt star a Cygnus
25th Sept. 1971 Flight 631 P
Figure 3. Visual record of stars around guide star.
OBSERVATIONS
Rather than attempting to present a comprehensive account of our observ-
ations to date, examples of the different types of measurement which have been
made with the U.C.L. system will be given.
Much of our observing time has been spent in determining the far infra-
red flux of HII regions known or expected to be bright in this wavelength
region. Data from considerably more than fifty objects has now been obtained
many of which have not previously been measured in the far infra-red. A
sample of the results is shown in-Fig.4. (overleaf).
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100
8
o
I
ro
S 10
3.
O
SOURCE No IDENTIFICATION
13 2
4
7
8
9
10
11 (a)
11 (bl
12
13
15
16
17
18
• 12
NGC 2021
W3 G133 7.
OR 15
H 8
G 5-9 -04
G 353 4 -0-4
NGC 6357 G3S3 1.0 7
NGC 6357 G 351 6-1 3
G 351.6-1 3 KE52
G 351 6 -02
RCW 126
RCW 122
RCW 117
1C 4628 G 3454.1.4
10 100
RADIO CONTINUUM FLUX {Flux Units)
Figure 4. ^lot of far infra-red flux against radio-continuum flux.
In general there is a close correlation between the infra-red flux and
the radio continuum flux but there are a number of exceptions. At the present
time there is considerable interest in explaining exactly what is happening
in these complex regions. The far infra-red flux gives a good measure of the
energy output of the exciting star(s) but the mechanism by which the radiating
dust is heated is not fully understood and is the subject of a number of
theoretical studies.
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The half power beam width of 3.5 arc min unfortunately means that sizes can
only be determined to ^  arc minutes and better angular resolution is required
to decide whether or not a significant fraction of the radiation is being
emitted from outside the HII region. Whilst agreement with the radio position
was in all cases within 3 arc minutes it is not yet possible to say whether
there is any real displacement between the centres of the infra-red and radio
sources.
A second type of measurement which gives far more information is to closely
scan a region so that a contour map in the infra-red can be constructed and
compared with the corresponding radio map. A number of such maps have now been
completed, in particular of NGC 6357 and NGC 6334, the latter being shown in
Fig.5.
-35° 351
6 (1950)
-35°40'
-35°45'
-35° 50'
-35°55'
•36°00' - I
17h 18mOOs 30s 17h 17m 00s 30s
a (1950)
Figure 5. Far infra-red map of NGC 6334. Contour units are
2.3 x 10~10 wm~2
1.3-6
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The three crosses marked indicate the position of the three radio peaks
and, in view of the positional accuracies of the infra-red peaks and the radio
peaks no special significance is attached to their relative displacement.
However, no convincing correlation between them all can be obtained by adjust-
ing their position and it appears that the brightest infra-red component
corresponds to the OH maser source.whose position is marked with a square,while
the other two infra-red peaks correspond to the radio sources, the central
infra-red peak being double but unresolved in our beam. Confidence that this
is a correct interpretation is strengthened by the immediate correlation of the
radio and infra-red peaks in the case of NGC 6357 (not shown). NGC 6334
obviously requires further study but at the present time the observations could
be interpretated in terms of a massive protostar to explain the northern infra-
red peak.
Further details of both the photometric measurements and the infra-red maps
are given in a paper by Emerson, Jennings and Moorwood 1973, which also has the
relevant list of references.
Finally, we have made some preliminary spectral measurements, using a
Michelson interferometer. For various reasons progress has not been as rapid
as hoped but low resolution spectra have been obtained of two objects, Saturn
and W51. The spectrum of Saturn is shown in Fig.6.
'N
X
<N*
'|2.0
1.0
LLJ
-100K
100 150 200
WAVENUMBER (cm'1)
250
Figure 6. Spectrum of Saturn.
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Saturn was not the prime object and the interferograms were obtained in
nine minutes. The spectrum was calibrated photometrically with respect to
Jupiter. The brightness temperature is 100K at about 100 cm~^ and it is of
interest to note that the brightness temperature tends to increase with wave-
length in this region as found by Armstrong, Harper and Low 1972, The 100K
curve shown corresponds to the colour temperature which gives the best fit«
The angular resolutioh was insufficient to separate the contribution from the
disc and rings.
It is intended to continue to develop this type of instrument for spectral
measurements of continuous sources and also, hopefully, to detect line emission
in the far infra-red.
ACKNOWLEDGEMENTS
Professor Sir Harrie Massey initiated this programme of observations and
has always taken a keen and active interest in its progress. The stabilised
platform and telescope were designed and built by Dr. W.A. Towlson and Mr.T.E.
Venis, together with the late Mr. Tomlinson; this group has played a major part
in the overall project, keeping the system in working order and assisting on
campaigns. The members of the infra-red group who have made the observations
and analysed the data include Dr. A.P.M. Moorwood, together with Mr. J.P.
Emerson and Dr. I. Furniss. Mr. J.A. Alvarez developed the Michelson system,
operated it during flight and reduced the data. Valuable technical assistance
before and during the flights was willingly provided by Mr. R.W. Catch, Mr. F.
Want and Mr. A.H. Watts.
We wish to thank the N.S.F. and the personnel at the NCAR base for flying
the payload for us on many occasions. This program is supported in part by a
grant from the Science Research Council.
REFERENCES
Emerson, J.P., Jennings, R.E. and Moorwood, A.P.M., 1973. Far infra-red
observations of HII regions from balloon altitudes. Ap.J., 184:401-414.
Armstrong, K.R., D.A. Harper, Jr. and Low, F.J., 1972. Far infra-red brightness
temperatures of the planets. Ap.J., (Letters), 178:L89-L92.
1.3-8
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er
DISCUSSION SUMMARY - PAPER 1.3
The telescope was 15 inches in diameter. A beam size of five arcminutes
was used and the detector NEP was 10~13 watts/Hz1/2 with a one-second integra-
tion time. Observations of Saturn using this system and an interferometer
yielded a signal-to-noise ratio of about two.
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Astronomical observations with the University College London balloon borne telescope

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