The DECam shutter is a large shutter made by Bonn Shutter (bonn-shutter.de) and consists of two accurately-controlled blades that sweep across the focal plane, the first movement lets light to the focal plane, the second closes off the light. The length of time it takes a shutter blade to sweep across the focal plane is approximately one second. For any exposures longer than one second, the movement of the two blades is sequential, but for shorter exposures the movement of the two blades occurs simultaneously, and the two blades form a “slit” that sweeps across the focal plane. Since the blades are accurately synchronized, very short exposures (a few milliseconds} are possible. The longest exposures regularly taken, through u band or narrowband filters normally do not exceed 1200 seconds although there is no technical limitation to going longer. The number of cosmic ray events on the CCDs starts getting obnoxious for long exposure times.
There are several aspects to be considered if doing short exposures of moving objects, or very rapid variables. Firstly, the the blades move in the East-West direction. Blade “A” is on the west side and Blade “B” is on the east side. So for an exposure assume Blade A covers the focal plane. It then moves to the west position and so the exposure starts, then at the end of the exposure Blade B moves from its east position to cover the focal plane. The following exposure reverses the direction the blades move, and so on. Unfortunately, which of the two situations (blades moving to the east or to the west) applies to any given exposure is NOT written into the image header or to the SISPI log. It is however in the low level PanView log. Since one only needs to know which of the two situations you have for one “fiducial” exposure in a given night, that information can easily be provided upon request. Secondly, the actual epoch at which the shutter opens for a given CCD pixel depends on where that pixel is situated. The time for the blade to sweep across the focal plan is approximately one second, so this effect could be of importancce for short exposure times. Thirdly, the shutter is sited, along with the filters, between corrector elements C3 and C4, a long way from the focal plane. Photons falling on a given pixel are contained in a cone that is some 20 cm diameter by the time it passes through the shutter. And finally, the shutter control is not hardware-synced to the CCD readout.
The complications that might be relevant for short exposures of moving objects, or rapid variables, are likely to be irrelevant for the great majority of DECam programs. The following outstanding specifications mean that very few people need worry about any shutter-related effects on their photometry, and indeed the shutter performance far exceeds the DECam design specifications in all respects.
- Minimum recommended shutter time: 100 msec (non-uniformity is then < 1% over the whole focal plane)
- Maximum recommended shutter time: 1800 sec (but beware cosmic rays)
- Minimum allowed shutter time: 10 msec
- Exposure time uniformity: < 1 msec
- Exposure time repeatability: << 1msec
- Exposure time accuracy: << 1 msec
- Absolute timing: < 1 msec ( internal to the shutter, but the external error in the absolute timing is very much larger due to external connections)
Reference: ACTR-Doc 1444