The design, calibration, and testing of a photometer to be used in an airborne telescope is described. A description of the cryogenics of the photometer is given, and photographs and blueprints of the photometer are included. The photometer is designed with a focal plane beam switching system so that the airplane telescope can be used in a normal optical mode at the bent Cassegrain focus and with the photometer operating in the pressurized cabin of the airplane. The concept was to produce a system which could be used in almost the same manner as ground based infrared photometers and dewars of the O'Brien Observatory at the University of Minnesota

Ney, E. P.

English

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UNIVERSITY OF MINNESOTA
FINAL REPORT - NSR24-005-238
E.P.NEY
(1ASA-CF-145E1C)	 [FHCTCME7EF LEWAR SYSTEM
FCr NASA C141 AIRECENF TEIESCCFE (KUIFEE
FLYING CESEFYATCRY) 1 Final report
(Min nescta Univ.)	 :7 p HC $4.00	 CFCL 03A
N76-12896
Unclas
G3/89 01470
FINAL REPORT
Introduct i on.
In 1972,tlie University of Minnesota proposed to build a
photometer dewar system for the NASA 0141 airborne telescope, now
known as the Ki_uper flying Observatory. One motivation for the
unsolicited proposal which led t.o this c(-ntract was the br_lief
of the principal investigator (F.P. Noy) that the 0141 might become
1	
operational as a flying telescope without adequate instrumentation
for making infrared observations of the stars and the planets.
In order to minimize the requirements in the aircraft the photo-
meter was to be designed with a focal plane beam switching system
!	 so that the airplane telescope could be used in a normal optical
i
mode at the bent Cassegrain focus and with the photometer operated
in the pressurized cabin of the airplane. The photometer was to
be equipped with a flip mirror so that the astronomer could inspect
the object being studies' nd verify that the system was tracking
properly. The concept was to produce a system which could be used
in almost the same manner as the ground based infrared photometers
and dewars in use at the O'Brien Observatory of the Space Science
Canter of the University of Minnesota. The initial instrument was
to have multiple interference filters matched to the atmospheric
windows between 1.6 microns and 20 microns allowing direct comparison
of the airplane telescope with ground based telescopes utilizing
these same windows in the atmosphere but with the around based
telescopes suffering from atmospheric extinction. The original
goals were threefold:
•	 1). To make an operating infrared system without requiring a
wobbling secondary on the telescope.
_2_
2) To use this system in flight and to compare observations of
standard stars with those made at O'Brien Observatory in order to
'	 evaluate the operation of the aircrafL observatory and to measureI
I
the transmission of the atmospheric windows widely used i n ground
based infrared observations.
3) To evaluate the airplane environment as an observatory and
in particular Lo find out how serious airplane vibration and the
flow of air past the open telescope might be in introducing noise
in the obser.va__ons.
Instrumentation.
Figures 1 and 2 are drawings of the photometer dewar system,
and Figures 3, 4 and 5 are photographs of the finished equipment.
The dewar is a liquid nitrogen jacketed, liquid helium con-
taining dewar holding one quarter liter of nitrogen and one
quarter liter of liquid helium. It is smaller than ie uptimum
ground based instrument which holds one half liter each of the two
cryogens. however, the ground baScd system holds pumped liquid
helium for three days and requires the addition of liquid nitrogen
every twelve to eighteen hours. Since an airplane flight only
lasts for eight hours the smaller dcaar was chosen for economy of
size and weight. The airplane dewar will hold pumped helium in
excess of twelve hours and requires nitrogen addition every six
hours. The nitrogen addition is 3 simple procedure involving
pouring the cryogen into the dewar through a funnel. Ten filters
1--3-
are mounted inside the dewar and are cooled to 80°K. Nine of
these filters operate at wavelengths of 1.2, 1.6, 2.2, 3.5, 4.8,
8.5, 10.6, 12.5 and 1.8 microns. Fach of , these has a bandpass AX
such that QI 	 10. The tenth filter (nicknamed Al.i_ce) is a wide
band filter extending from 0.3 inicrons to about 3 microns. It
passes all the short wavelength radiation but eliminatos the long
wave thermal radiation. It is possible to detect hot sources with
Alice at about 15 times greater sensitivity than the other short
wavelength filters. Actually, Alice is a piece of microscope
slide glass cooled to nitrogen temperature - hence, the nickname
"through the looking glass".
The detector substrate contains a gallium deed germanium
bolometer constructed by J. Stoddart. It has a thermal conductance
( G) of about .3 micr.owatts/ oK at the operating temperature of
1.1°K. Its responsivity is about a million volts per watt and
its D.C. electrical noise equivalent power (NEP) about 2_x10-14watts.
The photometer contains a translating mirror beam switcher
is'	 of the sort first developed by Fred Gillett at U.C.S.D. The
chopper is meehai,ically moved by a cam at 10 Hz. Alternate gearing
allows chopping at lower frequencies.
The dewar is of the general design developed at Minnesota.
The nitrogen dewar is supported and filled through two fiberglass
necks and the helium dewar is supported by a single inner fiber-
glass neck and by stainless steel "rigidizers" at the opposite end.
fNo super insulation is utilized. Tne helium dewar is packe3 with
4P
-4-
"chore girl" copper to minimize "sloshing noise" whon the dewar
contains superfluid helium below the lambda point. Attention is
paid to attempting to minimize microphonics as much as possible.
The dewar weighs 5 pounds and the photometer weighs 10.5 pounds.
Observational Experience.
The system characteristics were determined in the laboratory
with the "Maasometer" - a device for measuring the responsivity
at all wavelengths with a small source operating at 550'K. This
source is viewed through 10 Meters of room air by an eight-inch
telescope to which the airplane system is attached.
After the characteristics had been determined the airplane
dewar and photometer were used at the O'Brioi, Observatory on the
30-inch telescope to observe standard stars and establish the
ground based sensitivity. When the resistance of a bolometer is
i
monitored it becomes an absolute instrument and actual fluxes
may be measured to several percent.
Two observing sessions were scheduled on the 0141 telescope.
The-e will be referred to as the A and B series. The A series
took place near the fall equinox, August 12-23, 1974. This series
has been described in detail. The system was used on the ground
at Ames and in three flights. Table 1 shows the comparison of the
signals obtained with those at O'Brien. The airplane telescope
has the expected aperture advantage (36" versus 30") and in addition
p roduces appreciably larger signals in the poorer atmospheric
windows. Note especially 5 microns, 18 microns and to a lesser
1	 extent 3.5 microns.
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Since %.,At.e)_ v,ar,ar is the primary obscuring agent, the 5
microns of pr.rcipitable water vapor above the aircraft is a great
advantage compared to a minimum of 500 microns at the hitjilest
driest mountain sites. (Mt. Lemmon and Mauna Kea).
Tn. the A series we dial have some problems with microphonics
which kept us from reaching the ultimate sensitivity of the system.
However, in spite of the microphonic noise there were strong
indications of the presence of sky noise worse at longer wavelengths
and probably introduced by the turbulen t- boundary layer flow over
the telescope cavity.
Although the A series was primarily an engineering sequence,
a rather important scientific result was obtained. A very exciting
object dubbed the "Egg nebula" had just been discovered and was
being studied by a number of scientists at many observatories.
This object (which is the most optically polarized object in the
sky) is believed to be an excellent candidate for a pre-planetary
system (like the sun and its planets). It was crucial to deter-
mine the nature of the infrared radiation at waveleng ths longer
than 18 microns, and an additional filter operati.ig at 29 microns
was used in the airplane to compare the egg wit 14 the Galilean
satellites Jupiter III and Jupiter IV. The capability of the
airplane system in making this measurement was a non negligible
input in determining the physical characteristics of this very
interesting astronomical object.
Attachments I and II describe the "Egg nebula" alias IVI	 Zw4cky 67 and indicate the role the airplane observations played.
The B series took place about the spring equinox, March 20,
1.974.
During the interval between fall and spring a series of
laboratory tests had been made to reduce the microphonics in the
system. A modified doorbell banger was used to pound +=he dewar
j	 and produce levels of vibration comparable to those e;.countered
in flight. These experiments led to a considerable improvement.
In addition, improvements had been made in the aircraft servo
system and the net result was that in the one flight that was
made the micr3phonics were essentially negligible.
The B flight confirmed the presence of sky noise at the
	
S	 long wavelengths and showed that this sky noise is the limiting
Ieffect in determining sensitivity l.ong,.vard of 5 microns, in the
mode of operat.ion which we chose (chopping at 10 Iiz). It is
alleged by other observers that the sky noise is less at higher
	
r	chopping frequencies but a synchronous demodulator noise run
	
-W	 which we made did not indicate a large improvement at higher
frequencies. There must be some improvement, however, because
ground based sky noise seems to have a 1/f characteristic-
It was hoped in the B series to extend the observations
to some interesting infrared stars, and to observe at wavelengths
outside the atmospheric windows. These observations required
the use of a wobbling secondary with adjustable throw and accurate
offset guiding by the airplane telescope system. Neither of these
8-
capab i 1. i t i es existed in a wo.-}.able form in the airplane as of
the date of the March series. The success of the first flight
in proving the limitations and verifying the transmission of the
windows first measured in 1':ugust	 was such that another
engineering flight was not required. Alternate scientific programs
utilizing the airplane systems "a:, is" were not judged by the
1	 principal investigator to be of ,ufficient scientific value to
justify further flights.
It is hoped that further dovelopment of chopping secondaries
and offset guiding will make such observations possible in the
future.
Summa.
A). The Minnesota dewar photometer system allowed realization
!	 of the three goals listed in the introduction. To reiterate:
1) The system produced did allow the evaluation of an
operating infrared system without a wobbling secondary on the
telescope.
2) The system was used to study the transmission in the
atmospheric windows and showed the very great advantage of obser-
ving with only 5 microns of precipitable water as opposed to
500 microns at the best ground based sites.
3) The airplane environment was evaluated and it was shown
that in the chosen mode (chopping at 10 Hz) the sky noise in the
aircraft produced the limit on sensitivity at wavelengths longer
than 5 microns.
iw	 ..9-
An unexpected scientific bonus was obtained in measurements
of 1-he "Egg nebula" at wave len(lths longer than 20 microns.
Recommendations.
_'ut_ure scientific usefulness of the airplane will depend on
the following:
1) Installation of a completely reliable fully adjustable
wobbling secondary for which the throw and frequency are adjustable
from she control panels.
2) Offset guiding capability which will make it pos,il,le
to nod the telescope utilizing stars as dim as 12 th for offset
guiding.
4F	 311 Attention to the problem of the boundary layer and the4
apparent limitation it imposes on signal to noise in the wave-
length range from 5 to 50 microns.
4) Attempts to make the system more like a ground based
telescope where one has accurate pointing and knowledge of right
ascension and declination to the order of 10 arc seconds. The
inertial navigation system accurately gives the position of the
aircraft but no system presently measures the position of the
telescope with respect to the aircraft allowing the calculation
of right ascension and declination and image rotation which the
astronomer needs and is used to.
It is believed by the principal investigator that further
development along these lines can make the Kuiper airborn
observatory an important national facility with unique capabilities
14
10-
to complement the largest and the most sophisticated ground based
telescopes.
Tt is important as a goal, however, to try to improve the
reliability and ease of operation so that astronomers may come
with difficult observing programs and some hope of succeeding
y	 with them rather than having to compromise with less interesting
"} pack-up programs" inverted to try to "work around" the short-
comings of the airplane system in its present form.
DR AWINGS-	 871 D1- PHOTOMETER SECTIONAL VIEWS ( NOT FILMED)
I	 871 E1-. PHOTOMFTER EXTERIOR VIEWS (NOT FILMED)
REPRINTS:
STUDIES OF THE INFRARED SOURCE CRL 2688
F P NE Y	K M MERRILL,	 E E BECKLIN G NEUGEPA II ER AND C G WYNN-WILLIAM
REPRINT FROM
	
THE ASTROPHYSICAL JOI 1 RNAL 198-L129-L134 June 15
SKY AND TELESCOPE
	
VOL 49 NO 1 JANUARY 1975
SKYLAB PUBLISHING CORP 	 1974
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Photometer dewar system for NASA C141 airborne telescope (Kuiper Flying Observatory)

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