| Pixels | 765 x 510 pixels (0.39Mpix) |
| CCD size | 6.9 x 4.6 mm |
| Pixel size | 9 x 9 um |
| Implied pixel scale* | 0.677"/pix |
| Implied frame size* | 518" x 345" 8' 38" x 5' 45" 8.63' x 5.75' 49.6 square arcmin 0.0138 square degrees |
An investigation into the bias level and stability was conducted on 14 December 2005. Another set of bias frames was taken on 11 February 2006.
A combination of 12 bias frames at a range of temperatures (-1C to -28C) were used to create a master bias frame. The overall structure is fairly smooth.
Hot pixels: two warm pixels were found at (45, 146) at 441 counts and (45, 144) at 291 counts. Column 45 was also found to have other cooler pixels above the background. Some of the earlier columns also appear to show a kind of vertical interlacing, with an amplitude of around two counts.
Cold and dead pixels: none.
Level: The median bias level on 14/12/05 was found to be 105 +/- 12.3 at -10C and slightly skewed to the dark side. On 11/02/06 it was found to be 874 +/- 12.3 at -32C. On 02/03/06 it was 914 +/- 12.5 at -30C.
Gradient: a 10-pixel-radius Gaussian filter was applied to search a significant left-to-right gradient was found on the background bias. The mean level on the left-hand edge of the chip is 121 counts, falling to 116 by column 45, 110 counts by column 125, 105 by the centre of the chip and 102 counts by the right-hand edge. Care should therefore be taken when placing objects very close to the left-hand edge of the frame. The vertical gradient is insignificant.
On 11/02/06 the levels were found to be 887, 884, 879, 873 and 871 counts, respectively, suggesting little comparative variation.
Bias frames were taken as 0.12s dark exposures - the shortest exposure length available. As a result, they will show some temperature variation due to the dark current.
An average "per pixel" temperature variation of around 4 +/- 1 counts was found between -1C and -28C, suggesting that there is approximately 0.13 +/- 0.03 counts per degree difference. For applications requiring precision, this should be taken into account.
The frame-to-frame bias variance is approximately Gaussian in distribution, with some abberant statistics for various warm pixels, particularly at higher temperatures. The Gaussian full-width-half-maximum appears to be around 14 +/- 2 counts.
As the variance of any master bias frame from a true bias frame will scale as sqrt(#bias frames), a combination of 10 or more bias frames should reduce the variance to below 5 counts.
The temporal stability of the bias frames appears to be fairly uniform, apart from a seemingly highly variable offset.
Based on 3 x 160s exposures, the dark current, per pixel, is approximately 2 counts at -30C, yielding approximately 0.013 counts/s.
At higher temperatures, the bias structure appears bimodal, with around 2 counts/s for approximately 0.3% of the pixels and 0.042 counts/s at +7C.
Based on 15 x 10s exposures, the dark current at 11/02/05 is bimodal, between 7.4 counts/s for the lower half of the image and 7.8-8.0 counts/s for the upper half of the image at -32C. This is suggestive of a light leakage problem, but could also be elevated due to changing bias levels.
As of 02/03/06 using 10 x 10s exposures at -30C, the bias current was calcuated to be 3.8-4.0 cuonts/s, increasing towards the lower-left.
Hot pixels: The hot pixels in column 45 are a current sink and can lead to bleeding down that column.
Current as of 11/02/06. Chip exposure is fairly uniform in all wavebands, with only around 2% vignetting across the chip. Some 7% vignetting is present in a small strip on the top of the frame, towards the left.
Current status (07/03/06): Filters and camera window were cleaned of dust. A quick image in the clear filter shows no obvious dust present. Filters remain scratch-damaged, so there may be some smearing effects.
Images prior to 07/03/06: There are several problematic dust grains apparently on the CCD window. A long, opaque, roughly linear object 0.44mm long lies between (287,55) and (331,77) and obscures roughly 21% of the light(*). A similar grain appears of the bottom of the chip around column 242. Further small dust grains are visible at (269,129), (349,119) and (281,464), but these appear to be mostly transparent. Note that images between 28/02/06 and 07/03/06 show that the larger dust grain has moved.