Star Trackers - Frequently Asked Questions
Q: What are the fundamental differences between the Altair HB+ and RIGEL-L star trackers in the format of the CCD / APS and the #F of the optics?
A: ALTAIR HB+ (CCD based)
768 x 512 (9um) pixels
FOV 15.5 x 10.5 deg
25mm # 0.95 COTS lens
1s exposure
RIGEL-L (APS based)
1024 x 1024 (18um) pixels
FOV 22.6 x 22.6 deg
47.7mm # 1.4 CUSTOM Lens
66ms exposure
a) Both sensors have approximately the same instantaneous FOV - Each pixel sees ~ the same angular resolution.
b) The quantum efficiency (QE) of the APS sensor is comparable to that of the CCD, but the pixel size of the APS is 4 times larger than the CCD. This implies that the APS sensor is actually more responsive than the CCD, i.e. has a better ability to convert light into a voltage.
c) For all sensors Dark Signal, Dark Noise and Dark Sing Uniformity are the driving factors on performance. All of these are a function of time. Long exposure periods should be avoided; the APS uses 66ms, while the CCD uses 1000ms. For this reason the APS actually produces less noise. In addition the APS sensors have an image processing ASIC that further rejects noise and improves the linearity of the APS, which the CCD does not have.
d) The CCD use a COTS lens that is not optimized at infinity focus. This implies that the lens does not have a well controlled Point Spread Function across the FOV. The APS sensors uses a CUSTOM lens designed and optimized for focus at infinity, providing a well controlled Point Spread Function across the FOV.
Overall, the APS design has a considerably improved performance over the CCD design, although this is due more to the design of the lens than the sensor.
Q: What is the angular accuracy of the Altair-HB+ and Rigel-L star trackers?
A: ALTAIR-HB+: 10" across, 60" around bore-sight (68%), 1Hz update, CCD< 15 deg/C
RIGEL-L: 2" across, 10" around bore-sight (68%), 1Hz update, APS< 15 deg/C
RIGEL-L: 3" across, 28" around bore-sight (68%), 4Hz update, APS< 15 deg/C
RIGEL-L: 5" across, 40" around bore-sight (68%), 16Hz update, APS< 15 deg/C
Q: How many pixels are there on the chip?
A: ALTAIR-HB+: 768 x 512
RIGEL-L: 1024 x 1024
Q: How many stars are in the database?
A: ALTAIR-HB+: 5000
RIGEL-L: 5000
Q: What is the maximum visual magnitude of a star?
A: ALTAIR-HB+: Catalogue limited to Mv 6
RIGEL-L: Catalogue limited to Mv 6
Q: What is the image rate (in fps)?
A: ALTAIR-HB+: <= 1 Hz per CHU.
RIGEL-L: <= 16 Hz per CHU.
Q: What is the optic's speed (f-nr.)?
ALTAIR-HB+: 0.95
RIGEL-L: 1.4
Q: What is the Noise Equivalent Angle?
A: ALTAIR-HB+: NEA ~ 30" for a single Mv 6 Star @ 15 deg/C at 1 Hz (68%)
RIGEL-L: NEA ~ 30" for a single Mv 6 Star @ 15 deg/C at 16 Hz (68%)
Q: What is the image acquisition time?
A: ALTAIR-HB+: 1s
RIGEL-L: 63ms