Fine Guidance Sensor Instrument Report # 27
Transfer Function Calibration for FGS #3
B. Bucciarelli, S. T. Holfeltz, M. G. Lattanzi, and L. G. Taff
Summary
Cycle 2 Instrument Scientist calibration proposals #4724 and #5059 resulted in
calibration Transfer Functions through three filters (F583W, F5ND and PUPIL) for
five of the proposed seven positions FGS #3’s field of view. Of these, those obtained
at two of the positions [(-136",709") and (278",663") in demagnified FGS image space;
the center of the field of view is (0",726")] were completely successful while those
scans taken at the remaining three positions were only partially successful due to an
incomplete correction for the Coarse Track to Fine Lock offset. Parameters describing
the calibration curves (shown in Figures 3a - 3dd) are given in Tables 1 through 5.
The calibration curves are also available on the science cluster on
disk$boston_data:[holfeltz.data.go.refdat].
1. Introduction
Two Cycle 2 Instrument Scientist calibration proposals, 4724 and 5059, both
entitled "7 Points of Light", were designed to calibrate the single star Transfer
Function at seven positions across Fine Guidance Sensor #3’s field-of-view (FOV) in
3 filters (F583W, PUPIL, and F5ND). This calibration is necessary since the shape
of the Transfer Function varies as a function of position within the FOV (see Fig. 1).
These variations become important when, for example, one is reducing TRANSfer
mode data for double stars. The parameters of a binary star (separation, magnitude
difference, and Position Angle) are determined by the deformation of the observed
Transfer Function with respect to a pair of reference Transfer Functions (a single star
of known magnitude observed near the same position in the FOV through the same
filter). In order to support TRANSfer mode astrometry, a database of reference
Transfer Functions is needed for every filter at many points across the FOV.
Towards this and other ends, several such "N Points of Light" proposals have
been executed: two engineering "5 Points of Light" tests (proposals #3132 and #3139,
PI W. Brady of HDOS), an Instrument Scientist Cycle 1 Calibration Proposal "15
Points of Light" (proposal #4218), and other Instrument Scientist Cycle 2 calibrations
"7 Points of Light" (proposals #4724 and #5059). In addition, a Cycle 4 "15 Points of
Light" (proposal #5555) is planned. Furthermore, by choosing the points at which the
Transfer Functions will be taken in later proposals to be coincident with the positions
observed in the earlier proposals (see Fig. 2), we obtain a temporal history of the
morphology of the Transfer Functions and are thus able to keep track of long term
changes, if any, in the "instrument" (which may, in fact, originate in the OTA; the
proposals listed above, for example, were not all executed at the same secondary
mirror position).
2. Cycle 2 "7 Points of Light" Tests
In May of 1993, when all the active proposals where re-TRANSed to take into
account the new, slower slew rates imposed on the FGSs, the remainder of Cycle 2
proposal #4724 (which had not yet been executed) was split off into a new proposal,
#5059. Three points (each using three filters) were observed under the 4724 number.
All of the second group of observations were severely off-center in at least one axis
because of an insufficient correction for the Coarse Track to Fine Lock offset (see
Figs. 3a - 3r). This de-centering makes the Neutral Density filter’s (F5ND) Transfer
Scans virtually useless and severely limits the utility of the Clear and Pupil filters’
scans as astrometry calibration curves. A HOPR (HST Observation Problem Report)
was filed but no telescope time for repeat observations resulted. One effect of the re-
TRANSing was the correction of the Coarse Track to Fine Lock offset for proposal
#5059 and two positions [(-136",709") and (279",663")] were successfully observed in
three filters (Clear, Pupil and Neutral Density; see Figures 3s-3dd). Unfortunately,
the two remaining positions were never observed; they were bumped from the
schedule when Cycle 3 was suddenly over-subscribed. Figures 3a-3dd show the
Transfer Scans obtained in Proposals #4724 and #5059; Tables 1-5 list some pertinent
parameters of these Transfer Functions and Figure 4 defines the terms used in the
tables. The signal-to-noise ratios range from about 200 to over 300 for the Clear filter,
from 60 to 120 for the Neutral Density filter, and from 160 to over 300 for the Pupil
filter. The calibration curves are available on the science cluster on
disk$boston_data:[holfeltz.go.refdat].
3. SMOV and Cycle 4
Position within the FOV is not the only parameter that affects the morphology
of a Transfer Function. The position of the secondary mirror can (and has) resulted
in drastic changes to the shape of the reference Transfer Scan. Figure 5 shows a
Transfer Function of the FGS reference star (Upgren 69) before and after the
mirror moves connected with the SMOV. As is obvious from the figures, the Cycle 4
"15 Points of Light" is essential for post-SMOV astrometry. Finally, the sensitivity
of the shape of the Transfer Function to stellar color index had been theoretically
predicted. Once the mis-manufacturing of the primary mirror was publicly known,
we concluded that this minor affect would be un-observable. We were wrong and
direct observations of the effect, with a B - V spread of ~ 0.m75 have been made.
Thus, we will have to expand our list of calibration stars and resurrect the matrix of
Transfer Functions we had originally proposed.