Test Description
Chandra was maneuvered to an attitude through which the Earth would transit. Before the Earth transit, Chandra was put into Normal Maneuver Mode, and returned to Normal Point Mode after the test. During the transit the ACA tracked 4 monitor windows (one in each CCD quadrant), and 2 stars (in separate quadrants). 4 faint stars (magnitude 10.5 to 11.6) were chosen for the test, but the ACA was able to find only 2 of the 4 stars. The ACA lost track of the stars before the geometric limb transit, but the monitor windows continued to function through the test, although their output was saturated by Earthlight for much of the test.The test time period only covered the entry transit onto the bright Earth limb.
Test details
Date: 20 August 2002 Obsid: 61077 Attitude (J2000): RA = 353.0 deg, Dec = -41.0 deg, Roll = 42.129485 deg Test start (UTC): 2002:232:17:40:00 Test end (UTC): 2002:214:18:16:00 Chandra geocentric altitude: 44800 km (start), 48900 km (end) Earth angular radius: 8.2 deg (start), 7.5 deg (end)
ACA Images
--------------------------------------------------------------------------- SLOT ID TYPE SIZE RA DEC MAG YANG ZANG --------------------------------------------------------------------------- 0 1050674328 STAR 8x8 353.295799 -40.464566 10.49 1893.0 885.1 1 1050674136 STAR 8x8 352.055062 -40.797224 11.01 -1429.5 2258.6 <- not found by ACA 2 1050678272 STAR 8x8 353.225869 -41.563353 11.03 -909.8 -1912.8 3 1050808072 STAR 8x8 353.668478 -41.239263 11.61 759.6 -1857.9 <- not found by ACA 4 --- MON 8x8 2100.00a 2100.00a --- 2100.0 2100.0 5 --- MON 8x8 -1000.00a 1000.00a --- -1000.0 1000.0 6 --- MON 8x8 250.00a -250.00a --- 250.0 -250.0 7 --- MON 8x8 -1500.00a -300.00a --- -1500.0 -300.0
Analysis Assumptions
Earth Radius = 6371 km. No velocity aberration. No gyro drift during Maneuver Mode.
Monitor Window Analysis Results
The figure below shows the change in the median background level seen in each of the 4 monitor windows, as a function of angular distance from the bright Earth limb to center of the ACA field of view.To compare the ACA performance for entry onto the bright Earth limb and exit from the bright Earth limb, the results from 3 separate tests are plotted.
For the most recent test, the entry into the earth limb shows the usual rapid rise in background level at an Earth limb angle of about 4.5 degrees. This angle is about 1 degree smaller than the data for the bright limb exit tests. One possible explanation is that the ACA takes longer to recover from saturation effects at exit from the bright Earth, compared to the onset of saturation effects at the approach to the bright Earth. Another possible explanation is the smaller angular size of the Earth for the obsid 61077 test. The proper motion of the Earth for the 61077 test is about 0.18 degrees/minute while the proper motion of the Earth for the other 2 tests is about 0.6 degrees/minute.
The overlaid red plot shows the point source transmission of the ACA and its stray light shade (SLS), as a function of angle from the ACA boresight, added in quadrature to a steady CCD dark current of 12 electrons/second/pixel. The point source transmission function has been arbitrarily scaled to match the occultation exit light curves at an angle of 4 to 5 degrees. The point source transmission data are from a Ball Aerospace model, and were obtained from the Ball System Engineering Report: S95.2879.OPT.223 "Stray Light Performance of Aspect Camera with SLS", J.C. Fleming, 3-Feb-1995.
In each test, it can be seen that the median signal per pixel at
an Earth limb angle of 10 degrees is less than 20 DN/integration
(60 e/second), about 2x or 3x higher than the intrinsic dark current.
The instrinsic dark current from the ACA CCD dominates any Earthlight
contribution at large angles above 15 degrees, and the stray light
from the extended bright Earth dominates at intermediate angles.
The Individual ACA Image Slot Analysis
provides details of the tracking and centroiding performance of the ACA
for the faint stars, plus the time histories of the global and
individual image status flags.
Conclusions
The results of this test and previous tests indicates that the ACA can successfully track faint stars to within 10 degrees of the bright Earth limb, with no perturbation of the ACA performance until limb angles of about 7 degrees.Thus the CARD constraint on ACA operation near the bright Earth could be reduced from 20 degrees to 10 degrees.
Rob Cameron
Last modified: 2002-August-30