The latest information of the ANDICAM IR channel can be found in The OSU Andicam Instrument Pages . Here is some older information, that may be useful:
In July 1999, the Ohio State team installed the IR array, a 1024x1024 HgCdTe HAWAII array from Rockwell. This permitted a checkout of the IR channel end-to-end. The internal mechanisms (dither/focus mirror and filter wheel) have been thoroughly exercised and appear quite reliable. The dither/focus motion in particular functions very well; the motions are accurate and repeatable, which should allow for straightforward assembly of dithered images. The IR channel is sufficiently parfocal with the optical filters that no internal refocus amongst the possible filter combinations is required (that is, the IR and CCD channels are "locked" together in focus, so only the telescope focus should ever be adjusted).
The array has very low read noise (~11 electrons) and is flat to ~20% (most of
the variation is a gradient towards the right-hand-side or north edge
of the array). There is a group of about 300 dead pixels and ~50
scattered dead/hot pixels over the array.
The system throughput is about
as expected, suggesting the array has reasonable quantum efficiency for
devices of this type. Measurements of a grid of standards spanning all
four detector quadrants and ~15% of the area of the array show very
good reproducibility (<1% rms in the photometry). These tests were
limited by weather, and a larger grid covering
more of the detector is planned.
| Useful Numbers | |
|---|---|
| Gain | 7 e-/DN |
| Read Noise | 11 e-/pix |
| "Bias Level" | 400 DN |
| Linearity | 10,000 DN (<1% non-linear)1 |
The JHK filters were made to follow the CIT system. Filter transmission curves have been determined by the OSU team. Note that the sky rates vary considerably (by factors of 2 or so) depending on conditions (temperature, humidity, cloud cover, etc.). Any exposure longer than 10 sec or so is over the read noise. Use 30-60 sec for K exposures. All observing is done in queue-scheduled mode by a telescope operator/observer.
The dither pattern is done in groups of 7. The dither/focus mirror is held in place by tension against three actuators. The dither pattern starts at the original position. The next three dithers are made by moving each screw (while leaving the other two screws at 0) the requested number of "scale" steps (dither positions 2, 3, & 4). The final three dither positions are made by simultaneously moving two of the screws by the requestd "scale" steps (positions 5, 6, & 7). After exposures the screws are returned to their original position so motions are not cumulative. Because of the locations of the actuators on the mirror and the mirror's placement in the optical beam, one of the actuators moves a star more than the other two. All this together results in an asymmetrical pattern centered on the original position. If more than 7 dither positions are requested, the pattern is just repeated and the dithers go to the same postions. If you want to have unique dither positions, you must break your observations up into groups of 7, and move the telescope between each group by a fraction of the dither step.
The dither steps are not in arcseconds but in encoder units. The encoder units are almost exactly twice the number of arseconds moved. Thus a requested dither of 30 means that the dither move is about 15 arcseconds.