The AndyCam or "one-shooter" was installed in July of 1995, and is named after the person most responsible for its existence, Andy Szentgyorgyi. The dewar should be filled once every 24 hours. Nitrogen will spill out if the telescope is moved shortly after filling, so we recommend that you fill the dewar at dawn, since the telescope is unlikely to be moved for many hours. The chip itself is a thinned, back-side illuminated, AR coated Loral 2048X2048 CCD. The pixel size is 15 microns, or about 0.315 arcsecs. The quantum efficiency reaches 95% at 7000 angstroms, and is above 70% in the UV. Click here for a ccd qe curve. The dewar has a new corrector, a simple field flattener (plano-concave). Some ghost images from this can be seen from very bright stars, though they are not very bad. The gain is about 2.3 e-/ADU, and the readout noise is about 7 e-. The chip is overscanned by 16 columns and 8 rows. The first and last (number 1040) columns are saturated and should be discarded, particularly when calculating the bias. Thus the bias overscan area should be taken to be something like [1030:1039,2:1023].
The full well capacity is around 40,000 e-, or about 18,000 ADU. Thus in full resolution (i.e., unbinned mode), the counts will peak out at about 18,000 ADU in bright stars. However, once stars are saturated, continued exposure will cause the amplifier to overflow and the counts will again increase up until 32767. So you will see the curious effect of stars having saturation levels between 18,000 and 32767 counts. In 2x2 binned data, the total e- allowed are of course quadrupled to 160,000, and thus the ADU limit of 32767 will be reached before the full well; hence stars will always saturate at 32767 in this case.
Here are some rough count rates for stars in popular filters. For a U=B=V=R=I=20 magnitude star at 1 airmass,
U 3.1 adu/sec B 15.0 V 12.8 R 14.9 I 13.3In 1200s in V, a 22nd mag star can be measured to about 10% accuracy, in a dark sky.
Here are the dark sky count rates, per 2x2 binned pixel.
U 0.28 adu/sec B 0.98 V 2.3 R 3.8 I 18.7
Here are the count rates for dome flats, per 2x2 binned pixel.
U Do not use Dome flats in U B 500 adu/sec V 900 R 1400 I 42001/3 of the light collected in U dome flats is due to scattered white light in the top box. Less than 1% of B dome flats is such light, and even less for the redder filters. Sky flats will work ok for U because the sky is much bluer than the dome flatfield lamps, hence the relative contribution of the scattered white light to the total light collected in a U exposure will be acceptable.
When in unbinned mode, i.e. full resolution, there are charge transfer problems with flats taken at levels above 10,000 counts. Binned mode flats seem to be ok at all levels now. Here are high S/N flats.
First here is a U Sky Flat. Dome Flats in U are affected by unfiltered scattered light in the top box, and hence are not recommended.
Here is the bias, note the 3 bad columns and the slight "rollup" in the first 100 columns, which amounts to about 1 count.
Here is a 20 minute dark frame. After the chip has been UV flooded, there is often an excess dark rate, which declines with time. Thus observers should take darks if the chip has been flooded within a few days. Typically, the dark rate is a few counts in 20 minutes.
Here is an example of the ghosts from bright stars.
There are many bad pixels and bad columns in this detector. Click here for a list of the most prominent ones, in IRAF notation (xmin xmax ymin ymax).
Exposures taken in I will likely show some fringing from the OH night sky lines, particularly near the right edge of the frame.
