| Telescope | MDM 1.3m |
| Date | 11/20/97 |
| Instrument | Echelle CCD |
| Filter | V |
| Exposure Time |
600 sec |
| Other | processed |
Problem:This is a final overscan and bias subtracted, flattened image. Note the terrible large scale gradient and strange streaks. After 2 days of examining flats and biases, the culprit, dark current, was finally discovered with a dark frame image. The CCD was only being cooled to -75C as opposed to -100C due to an error in the software. Upon cooling the CCD to the appropriate temperture, the large scale structure and streaks disappeared. |
Fix:If you see strange structure showing up only in images with longer exposure times (> 30 seconds or so) and if the magnitude of the problem increases with increasing exposure time, you might want to take a dark frame (expose for some amount of time with the shutter closed). If the mean signal is higher than your zero frame levels, you have dark current. Make sure the CCD is appropriately cooled or take dark frames to subtract out the dark current. |
0 sec Zero frame with different structure
15 sec Twilight flat with different structure
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