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: 02 August 2002 Obsid: 61099 Attitude (J2000): RA = 269.796 deg, Dec = 31.229 deg, Roll = 225.525992 deg Occultation start (UTC): 2002:214:01:34:20 Occultation end (UTC): 2002:214:02:17:40 Chandra geocentric altitude: 27350 km (start), 27450 km (end) Earth angular radius: 13.5 deg (start), 13.5 deg (end)
ACA Images
--------------------------------------------------------------------------- SLOT ID TYPE SIZE RA DEC MAG YANG ZANG --------------------------------------------------------------------------- 0 341838880 STAR 8x8 269.352298 31.774805 9.01 -452.7 -2347.5 1 341842144 STAR 8x8 269.417603 30.622325 10.02 2378.3 692.2 2 341848904 STAR 8x8 269.226932 31.372861 10.21 852.7 -1614.1 3 343558680 STAR 8x8 270.315802 31.139895 9.79 -895.9 1365.0 4 --- MON 8x8 -1500.0a -1000.0a --- -1500.0 -1000.0 5 --- MON 8x8 -1500.0a 500.0a --- -1500.0 500.0 6 --- MON 8x8 500.0a 2000.0a --- 500.0 2000.0 7 --- MON 8x8 1600.0a -800.0a --- 1600.0 -800.0
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 Earth limb to the window position in the ACA field of view. 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 an Earth limb angle of about -4 degrees. The exit from the occultation shows a rapid decrease in background until an earth angle of about 5.5 degrees. The angle of rapid increase on entry is inside the Earth limb, because the ACA crosses the dark Earth limb on entry. The angle of rapid decrease on exit is similar to the equivalent angle in the June 2002 Earth occultation test.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.
It can be seen that the shape of the point source transmission function
roughly matches the shape of the light curves up to angles of 5 or 6 degrees.
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.
Another feature can be seen in the occultation entry light curves:
a sharp increase and intensity spike at an Earth limb angle of about
-0.5 degrees. The light curves for the stars also show an intensity
spike at this same angle. The various times for the spike
at the different positions on the ACA field of view are consistent
with a narrow band of light sweeping across the ACA field of view.
The consistency of the limb angle for all image positions indicates
it is associated with the Earth. At a geocentric distance of 27000 km,
the 0.5 degree offset from the limb is equivalent to 235 km inside
the limb. The cause of this apparent narrow band of light from
just inside the Earth limb is not known, but atmospheric refraction
of sunlight around the dark limb might be an explanation. The monitor
windows clearly show excess light from the dark limb, before
transiting the terminator. This band of light should not affect
the primary test objective, of reducing the CARD constraint for
operating the ACA near the Earth.
Individual ACA Image Slot Analysis Details
Conclusions
It can be seen that the angle of rapid dark current increase differs for the entry and exit phases by several degrees. This is presumably because the ACA field of view entered the Earth on the dark limb and exited on the bright 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. Another test is needed to investigate entry into and operation near the bright limb, for fainter stars.
Rob Cameron
Last modified: 2002-August-09