Flat frame exposure trouble
Posted 07 December 2018 - 03:29 AM
Posted 07 December 2018 - 09:04 AM
Food for thought.
Posted 07 December 2018 - 03:44 PM
I thought pixel count was an issue for CCD cameras. For my DSLR, getting flats to the proper exposure is just a matter of setting the camera to aperature mode-let the camera do the work.
Posted 07 December 2018 - 11:05 PM
CCD or CMOS becomes nonlinear when the count exceeds about 50% of the full well potential. This is why we want to keep the counts lower than 50% saturation everywhere in a flat image.
However, on DSLRs, because we often use high ISOs (400 to 3200). The linear counts we read from the raw images were further amplified by the readout system. Even if you read a saturated count from a pixel (16383, if the sensor has a 14 bit output), it's far from real saturation. For example, a count of 16000 at ISO 400 is equivalent to 4000 at ISO 100. Although 16000 at ISO 400 looks very close to saturation (16383), it is actually very safe.
So, unless you are using an extremely low ISO (which is almost never recommended for astrophotography), all you need to do is to make sure every pixel is below the saturation value. You don't want to get too close to 16383, since there can be fluctuation in exposure between your flat frames. If you get extremely close to 16383 in your first test shot and use the same exposure in all subsequent shots, there is a chance that some pixels exceed 16383 in those subsequent exposures.