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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.