, the Scientic Capabilities of these Interferometers
Edmund Nelan
a
, Olivia Lupie
a
, Barbara McArthur
b
, G. Fritz Benedict
b
,
Otto Franz
c
, Larry Wasserman
c
, Linda Reed
d
, Russ Makidon
a
,
Lauretta Nagel
a
a
Space Telescope Science Institute, Baltimore M
b
University of Texas at Austin, Austin Texas,
c
Lowell Observatory, Flagstaff, Az,
d
Raytheon, Danbury Ct.
ABSTRACT
The Fine Guidance Sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing inter-
ferometers that offer a unique capability to astronomers. The FGSss photometric dynamic range, fringe visibility,
and fringe tracking ability allow the instrument to exploit the benets of performing interferometry from a space-
based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated
as the Astrometer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of
its eld of view, (2) determine the true relative orbits of close (20mas) faint (m
v
=15) binary systems, (3) measure the
angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system
objects, as well as (6) photometrically monitor stellar ares on a low mass M dwarf. In this paper we discuss this
unique instrument, its design, performance, and the areas of science for which it is the only device able to success-
fully observe objects of interest.
Keywords
: interferometry, astrometry, Koesters prism, FGS, binary stars, parallax, mass luminosity ratio
1. INTRODUCTION
The Hubble Space Telescope is well known for its superb images and spectroscopy. It is less well known as a space-
based interferometer. Indeed, HST has not one but three interferometers onboard, the Fine Guidance Sensors (FGSs).
The FGS is a clever solution to a fundamental dilemma, i.e., how is it possible to provide HST with 2 to 5 milli-sec-
onds of arc (mas) pointing stability when HST is diffraction limited at about 40mas? An imaging device is certainly
not the instrument of choice for this task, but a white-light shearing interferometer, such as the FGS, can meet this
requirement. The fringe pattern of a guide star observed by the FGS provides HSTs pointing control system (PCS)
with simple, yet precise ne error signals used to measure and correct the small pointing errors of the spacecraft to
maintain its attitude on the sky for prolonged exposures by the science instruments. The FGS is also well suited for
use as a science instrument to study areas in astrometry and astronomy not possible by any other instrument or facil-
ity. In this paper we discuss the design of the FGS, its interferometric performance with HSTs spherically aberrated
wavefront, its operating modes, capabilities as a science instrument, and the areas of scientic investigations for
which there is no alternative.
2. INSTRUMENT DESIGN
As an interferometer, the FGS is quite different from a long-baseline Michelson Stellar Interferometer. Long base-
line interferometers determine the angle between a luminous object on the sky and the interferometers baseline by
measuring the difference in path length of the coherent beams collected by separated apertures. The FGS
1
measures
the angle between a star and HSTs optical axis by presenting the stars collimated and compressed light to a polariz- ing beam splitter and a pair of orthogonal Koesters prisms. These prisms produce output beams with relative intensi-
ties dependent upon the angle between the wavefronts propagation vector and the entrance face of the prism.
Figure 1. FGS Field of View in the HST Focal Plane with FGS (x,y) Coordinate
System Related to HST (V2,V3) System
.
The FGS can acquire and observe any sufciently bright object (m
v
<17) over a 69 square arc minute eld of view
(FOV). Figure 1 shows the FGSs in HSTs focal plane. The FOV is a quarter annulus at the outer perimeter of HSTs
focal plane with an inner and outer radius of 10 and 14 arc minutes respectively. However, only a 5x5 arc second
(asec) aperture, the instantaneous eld of view (IFOV), samples the sky at any one time. The location of the IFOV
within the instruments FOV (gure 2) is determined, with sub-mas precision, by a dual component star selector servo
system (called SSA and SSB). Only the light from objects in this IFOV is presented to the polarizing beam splitter
and Koesters prisms for interferometric fringe construction.
We describe the FGS as composed of two logical sub-systems, the forward steering section and the 2 orthogonal
interferometers (gure 3). The forward steering component brings the IFOV to the desired location in the FOV. It is
composed of a plane pickoff mirror which intercepts light from HSTs optical telescope assembly (OTA) and re-
directs the beam into the FGS. The beam passes through focus and the encounters the Aspheric Collimating Mirror
which collimates and compresses it (60x) to 42 mm. The beam next encounters the Star Selector Servo A assembly
(SSA), a rigid assembly of two mirrors and a 5 element refractive corrector group which can be rotated about HSTs
optical axis (for reasons to be made clear below). The corrector group compensates for the design optical aberrations
from both the OTA and the asphere. From the OTA come astigmatism and eld curvature, while the asphere contrib-
utes astigmatism, spherical aberration, and coma. Unfortunately, the spherical aberration from HSTs primary mirror
is not corrected (it was, of course, not in the design to which the FGS was built!).
14.6'
14.0'
10.2'
9.0'
+V3
+V2
Sun Side
Optical Control
Subsystem
x
y
x
y
x
y
FGS #1R
FGS #2
FGS #3
FGS reference frames
shown in object space
+U3 Figure 2: Locating the IFOV in the FOV
After exiting the SSA, the beam passes through a eld stop which minimizes scattered light and narrows the IFOV.
The beam next encounters the Star Selector Servo B assembly (SSB), which consists of a rigid assembly of four mir-
rors which, like SSA, can be rotated about an axis which is parallel to HSTs optical axis. Upon exiting the SSB
assembly, the beam encounters a fold at mirror, the lter wheel assembly, another fold at mirror, and nally the
polarizing beam splitter.
The rotation angles of the SSA and SSB assemblies completely determine the location of the IFOV within the
instruments full FOV. Together, these assemblies transmit to the polarizing beam splitter only those photons originat-
ing from a narrowly dened direction, masking out all but a small 5x5 asec patch of sky (gure 2).
The polarizing beam splitter, the Koesters prisms and their associated eld stops and photo-multiplier tubes com-
prise the interferometer section. The polarizing beam splitter divides the incoming unpolarized light into two linearly
plane polarized beams with orthogonal polarizations, each having roughly half the incident intensity. Each beam is
directed to a specic Koesters prism.
The pyramid shaped Koesters prism (gure 4) consists of two halves of fused silica joined along a surface coated to
act as a dielectric beam splitter. By reecting half the photons while transmitting the remainder, the dielectric per-
forms an equal intensity division of the beam. This division also introduces a 90 degree phase difference, with the
transmitted component lagging the reected. This division and phase shift gives the Koesters prism its interferometric
properties. The beam reected from one side of the prism, when joined with the beam transmitted from the other side,
constructively or destructively interfere to a degree which depends upon the angle between the incoming wavefronts
propagation vector and the normal to the prisms entrance face in the plane perpendicular to the dielectrics surface.
This angle is dened as the tilt of the wavefront. Thus, the Koesters prism emits two exit beams whose relative inten-
sities depend upon the tilt of the incident wavefront. Each exit beam is focused by a positive doublet onto a photomul-
tiplier tube (PMT) which records the number of photons encountered during a 25 msec interval.
~90
o
Star Selector A
Star Selector B
Instantaneous
Field of View
Total
Field of View
7.1'
7.1'
10.2'
14.0'
Optical Telescope
Assembly Axis A
(5" x 5") Figure 3. FGS Optical Train
For the FGS to sense angular displacements in the plane of the prisms dielectric surface, another, orthogonally
aligned Koesters prism is required. This prism intercepts the other beam emitted by the polarizing beam splitter.
Small rotations of the SSA and SSB assemblies alter the direction of the wavefronts propagation vector, and hence
the tilt of the wavefront at the face of the Koesters prisms. Since the degree to which the transmitted and reected
beams within the prism constructively or destructively interfere with one another (and hence determine the intensity
of the exit beams) depends upon the tilt of the incident wavefront, the PMT counts will vary as the SSA and SSB
assemblies scan the IFOV across the object. By mapping the normalized difference of the PMT counts to the loca-
tion of the IFOV for small tilts (less than 100 mas), the interferometers fringe pattern emerges. Explicitly, if A is the
number of photons recorded during a 25 msec interval by the A PMT on the FGSs x channel, and similar for B,
then at every position of the IFOV, the fringe, or transfer function, is given by;
S = (A - B) / (A + B)
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