The 0.84 m Telescope OAN/SPM - BC, Mexico Readout error CCD zero-level (bias) ramping CCD bias frame banding Shutter failure Significant dark current Image malting Focus frame taken during twilight IR problems
D. 1. Readout Error: saturated strips across image. The saturated strip on the bottom of the image is due to a read out error. These usually happen in the electronics of the instrument, but they can occur due to a software error as well. Another feature visible on this image are the doughnuts due to out-of-focus dust grains on the dewar window which are a very common phenomenon. The best fix is to retake the image. If the problem reoccurs, the advice of the technical personnel responsible for the health of the telescope to the astronomer would (very likely) be not to touch anything and to call them for assistance. The dust doughnuts will go away when the image is processed with a flat field image. Larger doughnuts due to dust on the filters can potentially be a bigger problem as they may change position throughout the night due to the motion of the filter wheel. See link below to the other example of an image with dust rings for a more detailed explanation. Another example of a readout error:
D. 2. CCD zero-level ramping: zero-level (bias) varies across image. Often on this run, bias ramping structure would show up in low level images such as standard star frames. The ramping structure varied from image to image and was not always present. Although this looks terrible, the bias level is only varying by a small amount (2.5%) which is why it only showed up in short exposures with low signal. The images often can be fixed during the processing because the same structure usually shows up in the overscan region. Subtracting out a fit of the overscan region frequently works well. The cause of the problem, however, lies within the electronics, so checking for things like loose cables might also be a good idea. Overscan region This is a plot along the overscan region showing the same ramping structure. The magnitude of the variation in bias levels is small - 2.5% which is why it only shows up in short exposure images with little signal. This structure will normally disappear during the processing (overscan subtraction) of the image, as long as one produces a higher order fit for the overscan region.
Corrected Bias Ramp Here you can see that the differing bias level problem has been erased. The overscan was subtracted as a fit as opposed to a constant value. The noise structure still seem to differ in the ramped areas, but the sky levels are now the same across the chip. Done.
D. 3. CCD bias frame banding due to electronic interference in the system: dark or bright bands along rows or columns on CCD. These two bias frames (0 sec exp time) show banding features due to electronic interference. Such banding always occured when the telescope and/or the dome were moved during readout. The problem was a result to insufficient shielding of the cables which caused interference between the motors of the telescope and dome and the CCD readout electronics. The easiest fix is to simply not move the telescope or dome during the readout. If the problem persists regardless of dome or telescope motion, most likely the shielding of the cables is inadequate which is a much more involved fix.
D. 4. Shutter failure: bright stars have trails in the direction of the readout of the CCD. The bright trails of the stars in this images are due to the fact that the shutter was not closed when the CCD was read out. During the readout process, light was still able to hit the CCD and produced the star trails which will therefore always point in the direction in which the CCD is read out (in this case, toward the bottom of the image). It is readily apparent that brighter stars produce much more of a trail than fainter ones which, if they are faint enough, do not produce any kind of noticable trail at all. The fact that the trails do not extend from the location of the star all the way to the top of the chip implies that the shutter closed at some point during the readout process. There is no easy fix to this problem. If the shutter is actually failing, the best solution will most likely be to contact technical assistance.
D. 5. Significant dark current: image has a high level background structure which cannot be corrected for with the bias or flats. 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 -75 C as opposed to -100 C due to an error in the software. Upon cooling the CCD to the appropriate temperature, the large scale structure and streaks disappeared. 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. Raw image with dark current This 600 second exposure has a lot of dark current which is apparent in the strange large scale gradient (notice especially the upper left hand corner) and horizontal streaks. Ignore the filter vignetting around the edges which was caused by using small filters. This same structure was not found in either flats or zeros. Cool the CCD to the correct temperature! If that does not work, take dark frames to subtract out the dark current. This involves taking exposures for some amount of time with the shutter closed.
400 second Dark frame The temperature to which the CCD was to be cooled was set incorrectly in computer software and rather than being cooled to -100C it was only cooled to about -75C. After being unable to flatten or zero subtract out the strange structure, a long dark exposure was taken and revealed the missing structure which had to be subtracted out. Make sure the CCD is appropriately cooled or take dark exposures to subtract out the dark current. 0 sec Zero frame with different structure The gradient of the zero second exposure does not show the same large scale structure or other features as the images which have dark current. Cool down the CCD or take dark images (take exposures with the shutter closed).
D. 6. Image Melting The stars in one part of the image appear to melt, i.e., have elongated and streaky appearance This star streaking phenomenon occurs with the CCD chip. When the sky level is so high that it approaches the saturation limit of the detector, some charge transfer problem seems to set in. No fix possible, except for reducing the sky level (e.g., by reducing the exposure time).
D. 7. Focus frame taken during twilight: thin, bright, vertical streaks across image; stellar images only visible in a thin strip on one side of the field. This focus exposure (multiple exposures of the chip with small offsets in between exposures) was taken during twilight. The vertical strips across the chip are most likely a saturation feature, perhaps a saturation of the CCD potential wells, but not the A/D converter (if the A/D converter reaches saturation, the image would be uniformly affected). The reason for the saturation is very likely the bright twilight sky. Some pixels seem to be more affected than others; stellar images are visible in the top left corner. The problem was "fixed" by just taking another focus sequence after twilight (with darker sky background). One could also reduce the exposure time.
D. 8. IR problems: Infrared image with multiple problems: circle shaped ghost images of bright star (small and large size), wavy interference patterns, stripe across image, and dark blotches This image, which consists of a 2x2 mosaic of images read out by 4 amplifiers, has multiple problems, none of which are easily fixable: 1. Fixed pattern noise. This noise is visible throughout the bottom two quadrants as a wavy pattern (low frequency component), and as a white stripe toward the bottom half of the top two and bottom two quadrants (high frequency component; harder to see in this image). 3. ghost images of the same star, visible as small rings or doughnuts in the quadrants not containing the star itself. These are created by amplifier cross talk, ie., electronic interference between the 4. amplifiers. A stripe between the bottom and top half of the image. Dark blotches of unknown origin throughout the middle part of the image. 5. Dark blotches of unknown origin throughout the middle part of the image. None of the problems are easily fixable. Problems 1 and 3 are due to the electronics of the setup and may not be fixable at all. Problem 2 is a reflection problem, the origins of which are usually not easy to track down and eliminate. It is not clear what caused problems 4 and 5.