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FOS Pre-COSTAR Target Pointing Analysis





Cynthia J. Taylor

Space Telescope Science Institute



FOS Instrument Science Report CAL/FOS-126

August 1994







Abstract



A log of Spacecraft Small Angle Maneuvers (SAM) during the Faint Object Spectrograph target
acquisition sequence has been compiled from March 21, 1991 to December 3, 1993. During this
time there was no FOS Aperture Location update to the Project Data Base (PDB).



Analysis of the log shows no obvious trends in: time, V2 direction, or V3 direction. The only
trend seen is a systematic +V2 direction offset for the dominant guide star in FGS 2. There is no
trend seen for the dominant guide star in either FGS 1 or FGS 3.



The lack of trends is due to the combination of several errors; target coordinate error, guide star
coordinate error, position of guide stars within the Fine Guidance Sensors, FOS target acquisition
method error, and other internal FOS errors (Filter Grating Wheel non-repeatability, GIMP, etc.).





I. Introduction





The total spacecraft SAM were calculated for each FOS target acquisition sequence (Binary
Search and Peakup methods only). Models of the Binary Search and Peakup target acquisition
commanding were used, rather than determining the actual spacecraft pointing. The SAMs for V2
and V3 were calculated for each step of the target acquisition sequence, summed, and then
entered into the log. The time, guide star, dominant FGS, and sub-dominant FGS information in
the log was obtained from the SMS.



No moving targets were included in the log. Any observations that had known problems that
would increase the target coordinate error (proper motion errors, extended targets, unusually large
initial peakup scans, etc.) were excluded. However, it is likely that not all such problems were
caught. Failed acquisitions were not included.



No attempt was made to estimate the target coordinate errors due to insufficient resources. No
other errors were calculated due to the gargantuan task of estimating all the errors.







II. Analysis of Pointing Statistics



The mean and standard deviations of the pointing offset magnitude anbd the V2 and V3 pointing
offset SAMs are given in Tables 1-6. Figures 1-12 show the plots versus time for the magnitude,
V2, and V3 pointing offset SAMs. All three FGS, each dominant FGS, and FGS 1 and 3 dominant
were analyzed.



Since FGS 2 dominant acquisitions were biased in the +V2 direction, the combination of FGS 1
and FGS 3 is a more accurate picture than all three FGSs. Section III will discuss the FGS 2
dominant problem.



Precise pointing of the calibration target, G191-B2B, is shown in Figures 10 - 12 and Table 6.
Only FGS 1 and 3 dominant acquisitions were analyzed. All included acquisitions of this target
utilized a four stage Peakup with an expected positioning error of 0.035 - 0.04 arcseconds. While
this scenario minimizes the target coordinate error, there were too few points to determine any
real trend.



In addition, histogram plots with 6 month bins of the V2 and V3 pointing offsets were made for
each dominant FGS and FGS 1 and 3 together. There was no trend any of in the means, although
it is known that FGS 1 and 3 were drifting away from each other.





III. FGS 2 Dominant Problem



Early in the mission, the FOS team established a rule that FGS 2 not be used as the Dominant
FGS. This was based on empirical evidence that showed FGS 2 was not well aligned and placed
the target near the edge of the FOS 4.3 aperture (the target acquisition aperture) and occaisionally
caused target acquisition failures (private communication from George Hartig). In mid-1992 this
rule was revoked because it was believed that the FGSs were then well aligned.



As Figure 8 and Table 4 show, FGS 2 still suffered from a one arcsecond +V2 tendency. This had
also caused a proportionally higher number of target acquisition failures. There were seven target
acquisition errors out of 27 acquisitions for FGS 2 dominant (a 26% failure rate) versus six
failures out of 51 acquisitions for FGS 1 dominant (12% failure rate) and five failures out of 52
acquisitions for FGS 3 dominant (10% failure rate).



The reason for this problem with FGS 2 is unknown. On November 19, 1992, the FOS team
requested again that FGS 2 not be used as the Dominant FGS. To our knowledge no other
instrument has noticed this effect.







IV. Conclusion



Other than the FGS 2 dominant problem, nothing obvious could be determined. A more thorough
analysis incorporating all the errors may provide more insight, however the errors would probably
obscure any trends due to FOS aperture location or FGS drift.



More analysis on the FGS 2 behavior is needed, but is beyond the current resources of the FOS
Calibration team.



Copies of the Target Pointing SAM log will be made available by the FOS Calibration team. It is
on the FOS account on the science cluster in the [fos.ta] directory and is called
"gs_acq_presm.log".









Figure Captions



Figure 1: Magnitude of the V2 and V3 pointing offset SAM versus time for all three FGS.



Figure 2: V2 pointing offset SAM versus time for all three FGS.



Figure 3: V3 pointing offset SAM versus time for all three FGS.



Figure 4: Magnitude of the V2 and V3 pointing offset SAM versus time for FGS 1 and FGS 3
dominant.



Figure 5: V2 pointing offset SAM versus time for FGS 1 and FGS 3 dominant.



Figure 6: V3 pointing offset SAM versus time for FGS 1 and FGS 3 dominant.



Figure 7: Magnitude of the V2 and V3 pointing offset SAM versus time for FGS 2 dominant.



Figure 8: V2 pointing offset SAM versus time for FGS 2 dominant.



Figure 9: V3 pointing offset SAM versus time for FGS 2 dominant.



Figure 10: V2 pointing offset SAM versus time for FGS 1 and FGS 3 dominant and calibration
target G191-B2B. The pointing errors are minimized because it is with precise pointing Peakup.



Figure 11: V2 pointing offset SAM versus time for FGS 1 and FGS 3 dominant and calibration
target G191-B2B.The pointing errors are minimized because it is with precise pointing Peakup.



Figure 12: V3 pointing offset SAM versus time for FGS 1 and FGS 3 dominant and calibration
target G191-B2B.The pointing errors are minimized because it is with precise pointing Peakup.