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 4 stars (one in each quadrant). The ACA lost track of the stars during the transit, but the monitor windows continued to function through the test, although their output was saturated by Earthlight for much of the test.Test details
Date: 18 June 2002 Obsid: 61161 Attitude (J2000): RA = 291.5 deg, Dec = 15.25 deg Occultation start (UTC): 2002:169:03:12:40 Occultation end (UTC): 2002:169:03:57:50 Chandra geo-centric altitude: 26350 km (start), 28550 km (end)
ACA Images
--------------------------------------------------------------------------- SLOT ID TYPE SIZE RA DEC MAG YANG ZANG --------------------------------------------------------------------------- 0 209719664 STAR 8x8 291.620135 15.824303 8.87 773.9 -1962.0 1 139858552 STAR 8x8 291.617737 14.663014 9.67 -1492.2 1551.4 2 209730784 STAR 8x8 291.049788 15.144600 10.11 1108.3 1167.1 3 209726448 STAR 8x8 291.849631 15.535374 10.05 -459.6 -1522.1 4 --- MON 8x8 -1300.0a -1200.0a --- -1300.0 -1200.0 5 --- MON 8x8 -900.0a 1800.0a --- -900.0 1800.0 6 --- MON 8x8 1400.0a -1400.0a --- 1400.0 -1400.0 7 --- MON 8x8 1800.0a 900.0a --- 1800.0 900.0
Analysis Assumptions
Earth Radius = 6378 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 Earth limb to the ACA boresight. Curves for both entry in the occultation and exit from the occultation are shown. The entry into the occultation shows a rapid rise in background level at and Earth limb angle of about 3.5 degrees. The exit from the occultation shows a rapid decrease in background until an earth angle of about 5.5 degrees.The solid plot symbols show the point source transmission of the ACA and its stray light shade (SLS), as a function of angle from the ACA boresight. The point source transmission function has been arbitrarily scaled to match the occultation exit light curves at an angle of 4 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.
It can be seen that the shape of the point source transmission function
roughly matches the shape of the light curves, although the instrinsic
dark current from the ACA CCD dominates any Earthlight contribution at
large angles.
Monitor Window Analysis Details
Conclusions
It can be seen that the angle of rapid dark current increase differs for the entry and exit phases by about 2 degrees. This may be because the ACA field of view entered the Earth near the terminator and exited on the bright Earth limb in this test.The Ball scattering model indicates the angle at which the rapid rise occurs corresponds to the angle of transition from double-surface scattering to single-surface scattering in the SLS.
The CARD constraint on ACA operation near the Earth should be set well outside the worst-case angle at which the "rapid rise" occurs. Further tests are needed to explore the range of angles at which the "rapid rise" might occur. This angle may be a function of the phase of the earth, and we should repeat this test for a variety of Earth phases and also for separate cases where the ACA enters and exits on the bright limb. Once the worst case angle has been determined, a fixed angular pad should be added to set the CARD constraint, with an added constraint that the ACA must be able to track stars at the faint limiting magnitude at this angle.
Rob Cameron
Last modified: 2002-June-25