[Shiraz (Ray)]

Peter pointed out that one of my ASI images had slight hot pixels that had not been fully removed by the calibration process - have been trying to pin down the cause.

It seems that the chip retains some signal processing from it's DSLR heritage - and it removes the hottest pixels. I looked at subs varying from 30 seconds to 10 minutes and at different gains and sure enough, when a single bright pixel exceeds its local background by about 8500ADU?, it blinks out of existence (presumably replaced with data from surrounding pixels).

implications are that dark scaling will not work properly when calibrating images from this chip. Darks must be taken using the same gain, offset, temperature and time as the lights. In addition, for bad pixel mapping, it is probably best to use darks taken at the same gain, offset and temperature but at slightly shorter time than the lights. This will ensure that all of the nasty hot pixels of interest are present in the dark and that there will be no odd hot pixels that have been substituted out by virtue of the read noise tipping them over the removal threshold. Depends on the exact mechanism, but it is also possible that this process may be able to remove stars in severely undersampled images - not tested, but maybe something to be aware of if very fast short fl lenses are used (will not be an issue with normal scopes).

This chip is turning out to be a bit "different" in almost every possible respect - it is taking a lot of time to come to grips with what it does so well and where it requires a bit of extra care.