Note: The example depicted in Figure A.1
was chosen for its clear demonstration of the phases of the acquisition. This example is atypical, as the 7˝ difference between the expected location and the true location of the target is unusually large. More typical miss-distances are 0.3" to 1.00".
The acquisition begins with the “WalkDown to FineLock,” or simply the WalkDown
. The FGE commands the FGS’s IFOV to a position offset or “backed-off” from the photocenter (determined by CoarseTrack). Here at the very beginning of the WalkDown, while the IFOV is far from the fringe, the FGE collects data from the two PMTs on each of the X and Y channels to compute an average sum (SUM) and difference (DIFF) of the PMT counts on each channel. The integration time is 0.4 sec or one FESTIME, whichever is larger. The FGE then uses these values to compensate for any difference in the response of the two PMTs on a given axis. Thus, the X-axis Fine Error Signal (FES) for the remainder of the Position Mode observation will be:
are the average photon counts/25 msec (from PMTXA and PMTXB) integrated over the FESTIME, and DIFFx
are the average difference and sum of the PMTXA, PMTXB counts per 25 milliseconds (as computed at the start of the walkdown). The Y-axis FES is computed in a similar fashion.
shows the instantaneous value of the normalized difference of the PMT counts along the Y-axis during a WalkDown to FineLock. The fact that the null lies to the positive side of the Y-axis S-Curve (S(y)
> 0.0) clearly demonstrates the need for the DIFF-SUM adjustment to locate the true interferometric null. The reference to “pipeline corrected null” in Figure A.2
refers to the true values of DIFF and SUM as computed by the astrometry pipeline, which uses the photometry during the entire WalkDown to achieve a better signal-to-noise for these values. (See Chapter 7
for additional details on the FGS pipeline.)
During the WalkDown, the IFOV creeps towards the photocenter in a series of equal steps of approximately 0.
in X and Y. The IFOV is held fixed for an FESTIME after each step while the PMT data are integrated to compute the fine error signal on each axis. If the absolute value of the fine error signal for a given axis exceeds a specified threshold for three consecutive steps (satisfying the 3-hit algorithm
), the FGE concludes it has encountered the S-Curve on that axis. From this point, a continuous feedback loop between the star selector servos and the FES value governs the repositioning of the IFOV along that axis. For the remainder of the observation, the FGS continuously adjusts the star selector positions by small rotations every FESTIME to set the FES to zero. This repositioning of the IFOV ensures the wavefront at the face of the Koesters prism has zero tilt.
When the S-Curves on both axes have been encountered and the 3-hit algorithm satisfied, the FGS is said to be tracking the object in FineLock. Figure A.3
shows the IFOV’s position for both the WalkDown and FineLock tracking in local detector space. Notice the interferometric null is first encountered along the Y-axis. The IFOV must walk an additional 0.
2" along the X-
axis to find its interferometric null. This is typical FGS3, and is due to a bias in the CoarseTrack photocenter and the FineLock (interferometric) null. FGS1r has a different bias, with the X-axis null encountered some 0.440" before the Y-axis null. These biases do not degrade the instruments scientific performance.