HSC instrument parameters
(1) There are 104 main CCDs, 4 CCDs for auto guider, and 8 CCDs for auto focus. See 'CCD information' for detail. The CCDs are arranged on the focal plane with two different gaps of approximately 12 arcsec and 53 arcsec between neighboring CCDs. We recommend that you should use dither steps of 120 arcsec or larger. For covering a contiguous region of sky, 3 exposures are required at least.
(2) The '20 s' is the time necessary only for reading out signals from CCDs. The 'overhead time' (necessary time from the end of the last exposure to the beginning of the next exposure) is about 30 - 45s, which includes transferring data from HSC to computers, moving the telescope to the next dithering position, and so on.
(3) When we exchange filters, it is necessary for us to move telescope to the zenith position, and rotate the instrument rotator to the appropriate angle (∼0deg). we also close the cover of the primary mirror for safety operation.
(4) After changing filters, the telescope is still located at the zenith position. You may need extra overhead time for moving the telescope to your next target. The maximum slew speed of the telescope is 0.5 deg/s for both AZ and EL directions. Please also note that you usually need to do the focus test, which takes about 5 minutes, just after changing filters.