A lamellar grating has been developed to be used for very far infrared observations on the airborne observatory. The design characteristics and performance during laboratory testing and initial observations of Jupiter at wavelengths between 50 and 500 microns are presented

Duthie, J. G.

Pipher, J. L.

Savedoff, M. P.

English

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https://ntrs.nasa.gov/search.jsp?R=19760020048 2020-03-22T14:00:05+00:00Z
(NASA-CE-148192) A SPHERICAL LAMELLAF
	 N76-27136
GRATING INTEFFEROHEIEF FOE AIREORNE
ASTRCNOMICIIL CESEEVITICNS OF FAR ?NFRIRED
CBJECTS (Rochester Univ.) 17 p HC $3.50
	 Unclas
CSCL 03A G3/89 44462
A SPHERICAL LAMELLAR GRATING INTERFEROMETER FOR AIRBORNE
ASTRONOMICAL OBSERVATIONS OF FAR INFRARED OBJECTS
J.L. Pipher, M.P. Savedoff and J.G. Duthie
University of Rochester
Department of Physics and Astronomy
Rochester, New York 14627
rr NFL► 	 -
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Abstract
A lamellar grating has been developed to be used for very far infr-red
observations on the Gerard P. Kuiper Airborne Observatory. The design
characteristics and performance during laboratory testing and initial
observations of Jupiter at wavelengths between 50 and 500u are presented.
I	 f
I. Introduction
We have developed a lamellar grating interferometer for use on the
Gerard P. Kuiper Airborne Observatory. This Observatory consists of a
91cm., f/13.5 Cassegrain focus telescope mounted in a C-141 aircraft
dedicated to infrared astronomy and operated by the National Aeronautics
and Space AdminibL 3tion. The aircraft operates at altitudes of 40,000
feet and above, where residual precipitable water vapor is
	 typically
below 20u and frequently below 10u. The observatory is thus ideal for
studying extremely far infrared astronomical objects at wavelengths in
excess of 50P. In this wavelength range most observations utilizing sen-
sitive detectors such as gallium doped germanium bolometers will be de-
tector noise limited and so that Fellgett's advantage of
	 Fourier
transform spectroscopy can be fully realized. In fact other noise sources
associated with the atmosphere-aircraft-telescope system dominated, and for
this case Fellgett's advantage is also operative. 	 Our interferometer
was designed specifically to study this part of the very far infrared
spectrum. The instrument, which utilizes spherical lamellar facets (such
as those described by Hansen and Strong 1) has been used to obtain spectra
of several astronomical bodies. In this paper we present a description
of the instrument and discuss its response to a laboratory far infrared
source and to Jupiter as viewed from the Airborne Observatory.
II. Instrument Design
In the spectral range of interest the advantage of the lamellar
grating interferometer (which employs wavefront division) over the Michelson
interferometer (which employs amplitude division) has been discussed by
2several authors including Richards and Milward 3 . The lack of really
-2-
suitable materials for use as beam splitters over a wide spectral range and the
further disadvantage of losing half the available signal from what is
usually rather a weak source dictates against the single detector Michelson
interferometer, and the ideal double beam Michelson interferometer exhibits
F times the noise.
Strong and Vanasse4 have discussed in some detail the theory of
lamellar grating interferometers. In these devices modulation results
from relative motion (along the optical axis) of two sets of parallel,
intermeshing facets forming a grating surface. Each set of facets give
rise to a diffracted beam and furthermore these beams interfere as a result
of the relative displacement of two sets of reflecting surfaces. The
zeroth order of the diffraction pattern is modulated as cos t irvA where
A is the optical path difference between the sets of facets. Ideally, all
other even order diffraction patterns are absent and the odd orders are modulated
as E n2 ffvA. To optimize the modulation we exclude all but the zeroth
order by limiting the exit and entrance aperture sizes. The maximum wave
number vmax that can be accepted will depend on the focal length
of the collimating optics,f, the exit aperture size, w, and the distance
between the centers of alternate facets, a , and is given by
v 
max 
=f/aw.
Further constraints restrict the choices of w and a. The exit aperture
should be large enough to optimize throughput and in particular to exceed the
diffraction limit of the telescope at the longest wavelengths, yet block first
order spectra. The grating spacing also has to be large enough to ensure that cavity
effects do not occur. These effects result when wavelengths approaching
-3-
the widths of the facets, a, are involved. In this case propogation
of the radiation to the rear set of facets involves wave guide modes with
resultant phase shifts which limit the instrumental resolution for unpolar-
ized radiation. To be consistent with these considerations the detection
system included a filter to reject v > 200cm 1.
The interferogram (I(A)) of a source, obtained by stepping the lamellar
grating system, leads to an output of the form
m
I(A) - fo S(v)[1 + cos(2nvA)] dv
= 
1(0) 
+ fo EM cos(2nvA)dv
where S(v) is the spectrum of the source. Hence the desired spectrum is
obtained by computing the cosine Fourier Transform of [I(A) - 1(0) ] namely
S 	 - 4 f^ [I(A) - 1 (0) 1 cos (2nVA)dA and is calculated by the Cooley-Tukey
Fast Fourier transform algorithms . Standard usage of this algorithm in-
corporates discrete sampling up to a maximum optical path difference,
Amax and suitable apodization.
A further advantage occurs if the lamella are machined to a spherical
surface rather than plane. In this case, as discussed by Hansen and
Strong, the grating is self-collimating and the need for entrance and
exit collimating mirrors is eliminated. The radius of curvature of the
grating was chosen to be consistent with the f-ratio of the telescope and
with the diameter of the grating itself. This latter size was chosen to
give a reasonable number of facets consistent with the size considerations
discussed above and with the constraint that shadowing of the rear facets
by the forward facet be less than 1%. Shadowing effects arise because one
must operate the grating slightly off normal incidence in order to separate
-4-
the entrance and exit apertures.
The grating itself is shown in Plate (1). It is 9cm. in diameter
and the two sets of facets are machined out of brass to a concave spherical
surface of 1.24 meters radius of curvature. Fabrication from brass-ensured
relatively easy machining and polishing of the surfaces. Gold plating of
the finished surfaces was found to increase slightly their far infrared
reflectivity, and also prevents surface deterioration.
The relative alignment of the two sets of facets (stationary and
moving) was achieved by means of a specially designed positioning device
which controlled the orientation of the fixed set of facets by means of
three screws. The moving set of facets is mounted rigidly on a piston
arrangement attached to an Aerotech ATS 301 translation stage. The stage
is driven by a pulsed stepping motor with a minimum 0.0001" displacement.
Each facet is approximately 0.76cm wide and the interfacet spacing was
chosen to be .01cm to minimize loss of the reflecting area consistent with
free relative motion.
Figure (1) shows the optical arrangement. The entrance aperture
was located at the prime focus of the telescope. A dichroic beam splitter
was used to reflect the infrared signal but transmitted the visible light
thereby allowing us to monitor the field of view. An additional plane
mirror. was necessary to fold the optical path in such a way as to package
the instrument consistent with mechanical limitations on the support and
balance of the telescope. This mirror reflected both the incident and
reflected beam from the grating. The optical axis of the grating was at
4° to the axis of the incident radiation. The exit aperture was defined
by the cooled filters preceding the field mirror feeding the bolometer.
The detector itself was a gallium doped germanium bolometer supplied by
-5-
Infrared Laboratories Incorporated. In laboratory testing, a chopping wheel
located at the entrance aperture was used to modulate the radiation from
a Barnes Engineering blackbody placed immediately behind it. In flight,
modulation was achieved by means of an articulating secondary on the
telescope. A throw of 8' arc ensured that 	 the detector looked
alternately at the source and at an adjacent region of the sky beyond the
diffraction limit of the telescope. The entrance aperture was _ 2' which
is equivalent to the diffraction limit of the telescope at 250u. The
response of the bolometer was preamplified at the dewar and then
synchronously detected by means of a lock-in amplifier. The overall system
NEP as measured in the laboratory was < 2 x 10
-12 
watts/Hz1/2 . We were
never able to :-emove completely the attenuation by telluric water vapor in
the laboratory, hence the quoted NEP is an upper limit. The maximum range
of the translation stage is 2.5cm, leading to an overall optical path
difference of 5cm. which corresponds to a limiting resolution of 0.2cm 1.
Figure (2) shows schematically the optical and electronic system.
Infrared radiation from the star passes through the telescope whose secondary
is articulated at a 12Hz rate. -The radiation continues to the lamellar
grating and then to the bolometer by the optical path indicated in Figure
(1). The output of the lock-in amplifier is fed to both a paper tape
pr+ -1' --r for permanent recording and to an A/D converter controlled by the
HP ;1.100 computer on board the aircraft.
Control of the grating is handled by a step driven controller which
automatically sequences the motor with the option of manually interrupting
the sequence. The operator selects the step size (in increments of 0.0001")
at which the interferogram is sampled, the dwell time at each sampling
point and the maximum displacement required for each interferogram. The
NNl	 i	 I
-6-
controller also provides gate pulses to ensure that the paper tape printer
and the computer respond only during times when the signal will have
stabilized at a given grating setting. The paper tape printer records
only once at each grating setting while the computer samples the lock-in
amplifier every 100 milliseconds during the same time. The 100 msec
sampling time in the flight system, together with the 300msec time constant
of the lock-in amplifier,resulted in readings recorded on tape which are
not entirely independent. A magnetic tape recorder stores all these measure-
ments tov.tither with real time computed averages of interferograms and Fast
Fourier transforms of the interferograms. In addition the inflight computer
records auxilliary flight and housekeeping information and provides on-line
information regarding the observations, including hard copies of the fast
Fourier transforms for quick look purposes. Eventually the magnetic tape
was analyzed at our own computing center where any necessary data "massaging"
was applied and final reductions were made.
III. Results
Figures (3) and(4) show two spectra taken in the laboratory. The
crosses and dots refer to unapodized and apodized spectra respectively.
The first curve shows an unpurged spectrum and the effects of atmospheric
absorption result in a strongly wavelength dependent structure. The
resolution of the feature at 161cm 1 approached the anticipated resolution i
of 4.75cm 1 for this run. By comparison the second spectrum shows the
result of purging the approximately 2.5m optical path with dry nitrogen
thereby removing most of the water vapor. The gross changes testify to
the high degree of absorption by rotational excitation of the water vapor
in the first spectrum. That the purging was incomplete is shown by the
1	
I
-7-
residual absorption features of the spectrum in figure (4).
During the night of November 7, 1965 U.T. we obtained several
extended observations of Jupiter, Mars, Saturn, and the Kleinmann-Low
object in the Orion Nebula. In particular we obtained three interfero-
grams of Jupiter and a typical raw spectrum calculated from one of
these is shown in figure (5 ) . The general features of the spectrum are
present in all three observations.
Many of the absorption features are identified with rotation
bands of H2O and are probably telluric. These include strong features
at 149.1cm 1 , 150.6cm 1 , 132.7cm 1 , 127cm1 , 122.Ocm 1 , 120.7cm 1,
99.Ocm 1, 88.Ocm 
1 
and 73.2cm 1 6,7 some of which are blended at the best
resolution employed on this flight, 3 cm 1 . The spectra of Mars taken on the
same night showed the same absorption lines. Complete analysis of these
data will be reported in subsequent papers.
IV. Summary
The lamellar grating interferometer on the Gerard P. Kuiper
Airborne Observatory has been shown to be a reliable useful system for
the study of far infrared astronomy in the range 50-800u. Its self-
collimating allows for the efficient operation with the minimum of
auxiliary optics.
The authors wish to thank M. Toporek, S. Varlese, R. Lerner, and J. Krassner
for their varicl assistance in this research. Pan Monitor Inc. designed
the mechanical support structure, and G. Lanpher designed and constructed
the electronic sequence controller. We particularly wish to thank the
staff of the NASA-Ames Airborne Sciences division and the ADAMS group
for their enthusiastic support and assistance during our flight series.
Support for this work was provided by NASA under grant NGR33-019-127.
^i	 '	 1	 I	
C	
r
Bibliography
1. Hansen, J . P. and Strong, J. Aspen International Conference ou
Fourier Spectroscopy, 1970, p. 215.
2. Richards, P.L., 1964, JOSA 54, 1474.
E
3. Milward, R.C., 1969, Infrared Physics 9, 59.
4. Strong, J. and Vanasse, G.A., 1960, JOSA 50, 113.
5. Cooley, J .W. and Tukey, J.W., 1965, Mathematics of Computation
19, 287.
6. Hall, A.T. and Dowling, J .M., 1967, J. Chem. Phys. 47, 2454.
7. Randall.., R.M., Dennison, L.M., Ginsburg, N. and Weber, L.R.,
1931, Phys. Rev. 52, 160.
PRECJCMrNG PAGE BLANK NOT pUMED
Figure Captions
Plate 1. Photograph showing the lamellar grating mounted in its
housing.	 The two sets of facets and the adjustment screwz
for the fixed set are clearly visible.
Figure 1. The optical arrangement for the lamellar grating inter-
ferometer.
Figure 2. Flow Chart of the Optical and Electronic System.
Figure 3. Unpurged Spectrum of 1000°K blackbody source. 	 Solid line
joining, dots represents spectrum with triangular apodization.
Crrsses are the unapodized spectrum.
	
Resolutions of feature
near 163 cm-1 are shown.	 Theoretical apodized resolution for
the length of run used is 4.75cm 1 .	 Selected strong water
vapor transitions shown by vertical arrows.
Figure 4. Purged Spectrum of 1000°K blackbody source.
Figur	 S. Raw Spectrum of Jupiter, at 3.2cm l resolution obtained Nov. 1975.
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A spherical lamellar grating interferometer for airborne astronomical observations of far infrared objects

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