Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/instrument_news/FOC/Foc_handbook/v5/handbook_v5_ch02.ps Äàòà èçìåíåíèÿ: Fri Dec 9 19:43:50 1994 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 20:14:22 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 5

FOC Instrument Handbook Version 5.0 3
2.0 COSTAR OVERVIEW
COSTAR replaces the High Speed Photometer in the Axial bay of HST, in the \GammaV2, +V3
quadrant (see Figure 7). It is a ``passive'' instrument, in that it has no detector of its
own, its sole purpose being to deploy a set of mirrors in front of the other Axial Scientific
Instruments (ASI). These mirrors, and their associated mounts and arms, serve only to
block the aberrated OTA beam from entering the ASI entrance apertures and to correct
the spherical aberration of a different part of the OTA field of view before re­directing the
corrected beam into the ASIs. A schematic diagram of the COSTAR optics in front of the
F/96 relay is shown in Figure 1. There is a separate set of mirrors that corrects the F/48
channel, mounted on the same arms as those for the F/96 channel.
110mm=6.565'
4.6'
V1
M1
M2
COSTAR Chief Ray
F/96 Aperture
Aberrated OTA beam
Figure 1. Diagram showing the COSTAR correction principle for the F/96 relay. The
aberrated OTA beam (dashed line) is blocked from reaching the FOC aperture
by the M2 mirror.
Before COSTAR, the aberrated F/24 OTA beam at 6.565 arcminutes from the V1 axis formed
an image at the FOC focal plane aperture 110mm from the V1 axis. After the deployment of
COSTAR, the M2 mirror and its mounting arm blocks this light from entering the aperture.
The beam that would have formed an image 4.658 arcminutes from the V1 axis is re­directed
by the spherical M1 mirror to form an image of the OTA exit pupil on the M2 mirror. This
mirror becomes the exit pupil of the OTA+COSTAR optical system, and the anamorphic
aspheroidal figure fulfills three functions:
ffl it re­directs the corrected beam into the FOC, forming an image at the FOC focal
plane aperture;
ffl it corrects for the OTA spherical aberration;
ffl it increases the astigmatism from that present at 4.658 arcminutes to that appropriate
to 6.565 arcminutes because the FOC is designed to correct for exactly the latter
amount of astigmatism.

4 FOC Instrument Handbook Version 5.0
However, there is one major difference between the COSTAR­corrected beam and the un­
aberrated OTA: the F/ratio is increased from F/24 to F/37. This results in a change in
the F/number of the FOC relays from F/48 to F/75.5 and from F/96 to F/151.
Because the names ``F/48'' and ``F/96'' are deeply­rooted in the HST ground
system at all levels, from proposal entry to data archiving, we are forced to retain
these names despite the fact that they do not describe the true focal ratios of
the cameras anymore. As a result, the user must take particular care in reading
this handbook, for whenever we mention the new F/48 and the new F/96 relays,
we are really referring to the relays with new focal ratios of F/75.5 and F/151
respectively.
A schematic diagram of COSTAR when deployed is shown in Figure 2. The FOC M1
mirrors are mounted on an arm which can be tilted in each of 2 orthogonal directions. This
adjustment is necessary to accurately center the image of the OTA exit pupil on the M2
mirror, and was done on orbit. Any error in this centering results in the introduction of
coma into the PSF. Since the M1 mirrors for both of the new relays are on the same mount
(``ganged''), only one channel (the new F/96) can be optimized in this way. It is anticipated
that the tilt of the new F/48 M1 mirror relative to that of the new F/96 mirror was set
during ground alignment to sufficient accuracy that no appreciable residual coma is left
in the new F/48 PSF. Optimization of the M1 tilt was accomplished during the Servicing
Mission Observatory Verification (SMOV) period shortly after the installation of COSTAR.
The M2 mirrors for both of the new channels are also on a single arm mounting, but there
is no tip/tilt capability. Both arms are connected to the Deployable Optical Bench (DOB),
which can be commanded to move over a 16mm range parallel to the V1 direction. This was
used to focus the new F/96 channel during SMOV. Again, it is anticipated that the focus
position of the new F/48 channel relative to that of the new F/96 channel was set to an
accuracy of 0.1mm or less during ground alignment.
The correction of the spherical aberration of the OTA by COSTAR does introduce a small
number of side­effects that were not present in the original design of HST+FOC. Firstly,
the focal plane tilt of the OTA at the FOC entrance apertures cannot be duplicated by the
2­mirror COSTAR system. The tangential and sagittal image planes produced by COSTAR
are tilted with respect to those in the OTA, and tilted appreciably with respect to each
other. This means that perfect astigmatism correction can only be achieved at one field
point. Similarly, the mean focal surface produced by COSTAR is tilted with respect to the
mean focal surface produced by the OTA, so that there is a field­dependent focus variation.
These effects are described more fully in Section 6.1.
Secondly, the HST+COSTAR exit pupil is now only 530mm from the FOC entrance apertures
(compared to 7m for the uncorrected OTA). This causes unavoidable vignetting in the new
F/48 camera for field positions more than about 12 arcseconds away from the optimally­
corrected field point. The effects of this vignetting are discussed in sections 4.2 and 6.9. The
new F/96 channel does not suffer from vignetting.

FOC Instrument Handbook Version 5.0 5
Figure 2. A Schematic View of COSTAR, after deployment, showing the FOC light paths.
For clarity, the FOS and GHRS M2 arms are omitted.
For this reason, it was decided to make the new F/96 channel the `preferred' channel of the
FOC, and attempt to optimize the image quality of this channel.