We describe here a proposed orbiting interferometer covering the UV, visible, and near-IR spectral ranges. With a 6-m baseline and a collecting area equivalent to about a 1.4 m diameter full aperture, this instrument will offer significant improvements in resolution over the Hubble Space Telescope, and complement the new generation of ground-based interferometers with much better limiting magnitude and spectral coverage. On the other hand, it has been designed as a considerably less ambitious project (one launch) than other current proposals. We believe that this concept is feasible given current technological capabilities, yet would serve to prove the concepts necessary for the much larger systems that must eventually be flown. The interferometer is of the Fizeau type. It therefore has a much larger field (for guiding) better UV throughout (only 4 surfaces) than phased arrays. Optimize aperture configurations and ideas for the cophasing and coalignment system are presented. The interferometer would be placed in a geosynchronous or sunsynchronous orbit to minimize thermal and mechanical disturbances and to maximize observing efficiency

Bely, Pierre Y.

Burrows, Christopher

Roddier, Francois

Weigelt, Gerd

English

NASA Technical Reports Server

HARDI:
N93-1S588
A HIGH ANGULAR RESOLUTION DEPLOYABLE INTERFEROMETER FOR SPACE
Pierre Y. Bely*
Space Telescope Science Institute
Baltimore, Maryland, 21218
Christopher Burrows*
Space Telescope Science Institute
Baltimore, Maryland, 21218
Francois Roddier
Institute for Astronomy
Honolulu, HI,USA
Gerd Weigelt
Max Planck Institute fuer Radioastronomie
Bonn, FRG
Abstract
We describe here a proposed orbiting interferometer covering the UV, visible, and near-IR
spectral ranges. With a 6-m baseline and a collecting area equivalent to about a 1.4 m diameter
full aperture, this instrument will offer significant improvements in resolution over the Hubble
Space Telescope, and complement the new generation of ground-based interferometers with much
better limiting magnitude and spectral coverage. On the other hand, it has been designed as a
considerably less ambitious project (one launch) than other current proposals. We believe that this
concept is feasible given current technological capabilities, yet would serve to prove the concepts
necessary for the much larger systems that must eventually be flown.
* Affiliated to the Astrophysics Division, Space Science Department, European Space Agency.
114
https://ntrs.nasa.gov/search.jsp?R=19930004400 2020-03-17T10:19:49+00:00Z
Jl
The interferometer is of the Fizeau type. It therefore has a much larger field (for guiding)
better UV throughout (only 4 surfaces) than phased arrays. Optimize aperture
configurations and ideas for the cophasing and coalignment system are presented. The
interferometer would be placed in a geosynchronous or sunsynchronous orbit to minimize thermal
and mechanical disturbances and to maximize observing efficiency.
Observational optical astronomy is always scientifically driven to develop telescopes with
fainter limiting magnitudes and higher resolution. However, it is clear that these two goals
cannot be pursued simultaneously anymore. Larger ground-based telescopes have much greater
collecting area but provide little improvements in resolution unless extremely demanding
techniques are used. Space-based telescopes of traditional configuration such as the Hubble Space
Telescope (HST) give great improvement in resolution (being diffraction limited), and a
consequent improvement in limiting magnitude, but further improvements are limited by launch
constraints. It is probably going to be impossible to launch a filled aperture telescope that gives an
order of magnitude improvement in resolution over HST for the foreseeable future.
To achieve still higher resolution, interferometers are the answer. On the ground,
baselines can be very large, but the atmosphere restricts integration time and therefore limits
magnitude. A space-based interferometer, on the other hand, is not limited by integration time
and thus could reach much fainter objects. Furthermore, a space interferometer, although likely to
be limited in baseline initially, gets improved resolution from operation in the UV. (Near the
Lyman continuum, a space interferometer will have about 4 times the resolution obtained from the
ground in the U band with the same baseline.)
Many concepts for space-based interferometers have been proposed, but they are generally
of major proportions, with baselines of 15 m or more that will require extensive technological
development. We believe that the technical feasibility of space interferometry must be
demonstrated before projects of this magnitude can be initiated. A smaller interferometer with a
baseline on the order of 6 m would be less ambitious than the current generation of proposals and
might consequently somewhat limit the science on which they are based. On the other hand, it
would be much lower in cost, risk, and development time, and would serve as a stepping stone to the
larger projects. The validation in space of enabling technologies in areas such as deployment,
active optics, laser metrology, vibration suppression, high accuracy guiding, and pointing, would
ll5
bea majortechnologicalspinofffrom sucha project.Suchvalidationis essentialif the larger
proposedprojectsareto bedemonstrablyfeasible.
Asshownin figure 1, even an interferometer of such a moderate size would offer important
advances over both HST and ground-based interferometers, especially if it can be operated in the
UV. For example, this would allow bright quasars, Seyfert galaxies, or stellar chromospheres at
Lyman alpha to be imaged at several times the resolution of HST. For several scientific areas, the
resolution of HST is just marginally inadequate (e.g., imaging the narrow emission line region
in a variety of QSOs). Clearly, also, it will often be necessary to pursue the study of discoveries
made with HST and the new large ground-based facilities at higher resolution.
We present here a first attempt at defining the main characteristics of an instrument
corresponding to this rationale. We call this instrument "HARDI", for High Angular Resolution
Deployable Interferometer. We also describe the various configurations and technological options
that we plan to examine in detail as part of our ongoing preliminary study of the instrument.
Aperture Configuration
The optimal aperture configuration of an interferometer depends on a number of factors
such as scientific goals, complexity of the observed objects, synthesized points spread function,
deconvolution, speckle or phase closure techniques, and practical constraints. To determine the
best configuration for our proposed instrument and scientific applications, we plan to do a
comparative study of three typical configurations. The three aperture configurations, labelled
Type I, II, and III have an outer diameter of 6 m and are very diluted with less than 6 percent
filling factor.
Type I is composed of six 40-cm-diameter mirrors on six arms and a 1-m-diameter mirror
on axis (5.4 percent fill factor). The aperture configuration is highly redundant with the intention
of supplying a high signal-to-noise ratio (SNR) 1,2.
Type II is a pupil function proposed by Cornwell 3. It contains nine 40-cm-diameter
mirrors arranged on a circle with a 2.8 m radius (4 percent fill factor). Its advantage is excellent
instantaneous UV plane coverage which could have applications in the observation of ephemeral
phenomena or microvariabilities.
115
Type III offers complete coverage of the UV plane by aperture synthesis. It is composed of
six 60-cm mirrors (6 percent fill factor) arranged in such a fashion as to lead to a quasi uniform
uv plane coverage when the entire telescope is rotated half a turn around its optical axis.
Type III has not been described elsewhere tO our knowledge. In it, the mirror locations are
such that the density of baselines increases roughly linearly with the separation. The idea is that
the object spectrum for any system with unresolved bright components is close to fiat. Therefore,
one wants approximately equal coverage of the UV plane out to the diffraction limit to get the same
SNR ateach frequency.As longerbaselinessweep a greaterareawhen rotated,thereneeds tobe
more ofthem togiveequal coverage.
A number of optimal Type III configurations were obtained for various numbers of
subapertures and different subaperture sizes using the Monte Carlo method with more than 10, 000
trials each. The subaperatures were constrained to lie at equal distances from the optical axis, so
that they can be fabricated by replication. Each subaperture was divided in 10xl0-cm elements and
the moduli of the elementary baselines were binned in 10-cm intervals. The optimization criteria
was to minimize the rms of the spread of the modulus distribution with respect to the ideal function.
We have selected a configuration with six 60-cm-diameter mirrors as being a good compromise
between the number and size of subapertures.
Figure 2 shows the three configurations described above together with their corresponding
UV plane coverage and point spread function in the image plane. We are planning to conduct
computer simulations and laboratory experiments to evaluate these configurations as a function of
the type of object to be resolved and the point spread function deconvolution algorithm.
Interferometersused in opticalastronomy are generallyofthe Michelson type. This
designsuffersfrom a lackoffield4and poorthroughput especiallyinthe UV due tothe large
number ofrelaymirrorsrequired.Our proposed instrument isofthe thinned apertureor Fizeau
type (figure3). This interferometerconfigurationuses a smallernumber ofreflectingsurfaces
and offersa sufficientfieldofview topermit guidingusing offaxis"bright"stars.
117
Thefinal numerical aperture of the system is determined by the necessity to match the
angular resolution of the system to the detector's pixel size. Using the Nyquist criterion, the final
numerical aperture of the system must be F/D = 2p/k, where F is final focal length of the system, D
the overall aperture diameter, p the pixel size and k the operating wavelength. Table 1 gives the
minimum numerical aperture of the system as a function of the wavelength for current typical
pixel sizes.
Table I
Wavelength Resolution Detector F/ratio
(pro) (milli-arcsec) pixel size for optimal
(ttm) match
1.0 42 50 i00
0.6 25 15 50
0.24 i0 15 125
0.12 5 15 250
Since a fast primary surface is essential to minimize the overall length of the telescope,
obtaining such slow beams directly would lead to an impractical Cassegrain magnification. This
is exemplified by figure 4 which shows the influence of the Cassegrain numerical aperture and
that of the primary surface on the major optical parameters of the system. The tradeoffs are
complex and will require an indepth study, but for the purpose of our conceptual study an t71.2
primary and f/12 Cassegrain appeared to be reasonable combination. Optical relays will be used
for reimaging onto the three detectors (UV, visible, and near-IR) with the appropriate scale. These
relays should be coated to minimize reflecting losses in each of the wavelength bands.
The Cassegrain combination will be of the Ritchey-Chretien design to produce a large
enough field of view. A total field of at least 10 arcmin in diameter in required to give a good
probability of finding a pitch-yaw guide star in the 14th magnitude range. We would expect rol
control to be achieved using fixed head star trackers. As in the case of the science field, reimaging
will be required to produce a proper scale.
Cophasin_ and coali_ning system. In view of the very tight tolerances on the respective
position of the optical elements and the focal plane and the lack of external shielding, one cannot
118
relyon the dimensionalstability of the structure,eitherpassively(with insulation),or actively
(with structuralheaters).An activesystemis requiredto "freeze"the imageduringthe exposures.
Our proposed active optics systems is composed of actuators on the primary mirrors and the
secondary mirror served to a laser metrology system controlling the internal optical path lengths.
This metrology system, using a Dyson5 interferometer, is described schematically in figure 5.
The active optics system is bootstrapped by observing a bright star in the focal plane and
coaligning and cophasing each primary aperture in successive pairs. Each primary would be
depointable to remove its contribution from the focal plane. This is desirable to allow for failures
on orbit in any event. The metrology system is then activated to "lock-in" the optical pathlengths
between the various optical elements and the focal plane.
In addition to serve to cophase the inteerferometer, the active optics system will also be
integrated in the pointing control system of the spacecraft. The pointing system will be composed
of two layers. A traditional spacecraft attitude control based on gyroscopes and star trackers will
be used for slewing and coarse pointing. The fine pointing (guiding) will be done by using the
active optics system to steer the optical beam based on the information supplied by a guide star in
the field.
Spacecraft General Design and Orbit
As shown in figure 6, the supporting structure is composed of a central tower and six
articulated arms. These arms are braced with telescopic members which extend for deployment
and confer axial rigidity to the structure. Once open, the moments of inertia around the three axes
are nearly equal, thus minimizing the attitude control requirements.
The entire interferometer assembly is protected from the sun by a sunshade located on the
rear of the spacecraft. These are no side baffles. This leads to a considerable simplification of the
spacecraft structure, but the price to pay is that the pointing has to be limited to about 45 degrees from
the antisun direction. The solar arrays are attached to the sunshade to avoid the low frequency
excitations that a steerable system would create.
The entire telescope structure and optics will be passively cooled by radiation against the
sky to allow near IR observations. Preliminary calculations indicate that a temperature on the
order of 100 K may be attainable.
ll9
As for the orbit, we are conducting an indepth study to determine which of the possible Earth
orbits would be the most favorable for the proposed instrument. Factors such as thermal and
mechanical disturbances, sky coverage, radiation level, observing efficiency, baffling, and
communication are being considered. So far the main contenders appear to be the 6-pro
sunsynchronous and geosynchronous orbits which offer significant advantages over low Earth
orbit.
The overall mass of the spacecraft is estimated at 3 tons, which is compatible with the
payload capacity to sunsynchronous or geosychronous orbit of medium-sized launchers such as
Ariane 4.
We have outlined here our approach to going beyond the resolution of HST. It seems to us
that a space interferometer is eventually going to be a necessary next step. Even with a modest
baseline the scientific drivers are enormous. The concept we are developing forms the basis for a
cost effective first attempt in this direction.
References
I* Roddier, F., 1987. Redundant v.s. non-redundant beam recombination in aperture
synthesis with coherent optical arrays, HOSA A(4)1396.
o Reinheimer, T., 1988. Fleischmann, F., Grieger, F., Weigelt, G. 1988, Speckle masking
with coherent arrays, Proc. ESO Conf. High-resolution imaging by interferometry,
Garching, p.581.
o Cornwell, T.J., 1988. A novel principle for optimization of the instantaneous Fourier plane
coverage of interferometric arrays, IEEE Trans. Ant. Prop. 36(8):165.
. Harvey, J.E.,1987. Performance characteristicsofphased array and thinned aperture
opticaltelescopes,Proc.SPIE 751:62.
5. Dyson, J. 1957. Common-path interferometer for testing purposes, JOSA 47:386.
120
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121
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dimensional UV plane.
122
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reflecting surfaces.
123
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final beam numerical aperture, the secondary mirror position tolerance and astigmatism are not,
at least to the firstorder. All effectsare shown for 1200/k and assuming a Ritchey-Chretien
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124
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the primary mirrors.
125
_: Artist's viewof theproposedinterferometerduringdeployment.
126


HARDI: A high angular resolution deployable interferometer for space

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