Hi Dave and Robin,
I've been running a few more experiments which may or may not be of interest. I appreciate the time you both have given my questions and
I've certainly satisfied my curiosity about why you need to increase offset with gain, at least assuming you always want the least offset possible. I must admit to not being totally convinced about the 14/16 bit thing, since it's a totally reversible transformation. Obviously it matters to DSS and I totally accept that.
To summarise, here's what I've learnt about SharpCap etc in regard to the "Save 10/12/14 bit images in FITS file without stretching to 16 bit ...."
1 - If your using the ZWO native driver (ASI071 MCPro), the histogram output by SharpCap is unaffected by the above setting. This means when SharpCap is working in it's default 16 bits FITS mode, the histogram does not represent the actual FITs file. (Although it would be easy to manipulate it with Exccel so it would)
2 - If you use the ASCOM driver rather than the native driver, the above setting does not appear to make any difference. The file is created as if the tick box is not set. The resulting histogram file has 64K lines (ie 2^16) and the histogram does represent the contents of the FITS file.
3 - The graph below shows the histogram output for the ASI071MC Pro, 0.1ms exposure, Gain=90 (unity), Offset = 20 with the above setting not ticked. The solid lines shows the actual offset (median value = 17) before the 14 to 16 bit conversion, but the dotted lines come from an ASCOM histogram with the same settings showing the histogram after the 14 to 16 bit conversion accurately representing the FITs output.
Also, although not directly an issue with SharpCap, but perhaps of interest
Some time back ZWO removed the ability to set offset in their ASCOM driver for the ASI071Pro and related cameras. The offset was fixed at about 50 regardless of gain. I've attached the result of this with a gain of 240 (optimum read noise), and the offset is clearly insufficient. I note however that ZWO have put back the offset setting - I dare say there were complaints. This histogram is certainly banging into the LHS.
Files attached (in zip file) are as follows (I don't expect a forensic investigation here, they are attached as requested)
1 - 0.1ms_G90_Off20_14Bit_00000.CameraSettings.txt (Ie 0.1ms Exp, Gain=90, Offset=20 & NOT stretched to 16 bits - ZWO driver)
2 - 0.1ms_G90_Off20_14Bit_00000.Histogram.csv
3 - 0.1ms_G90_Off20_14Bit_00001.png (FITS liberator view)
4 - 0.1ms_G90_Off20_16Bit_00000.CameraSettings.txt (Ie 0.1ms Exp, Gain=90, Offset=20 & stretched to 16 bits - ZWO driver)
5 - 0.1ms_G90_Off20_16Bit_00000.Histogram.csv
6 - 0.1ms_G90_Off20_16Bit_00001.png (FITS liberator view)
7 - 0.1ms_G90_Off20_16Bit_ASCOM_00000.CameraSettings.txt (Ie 0.1ms Exp, Gain=90, Offset=20 - ASCOM driver)
8 - 0.1ms_G90_Off20_16Bit_ASCOM_00000.Histogram.csv
9 - 0.1ms_G90_Off20_16Bit_00001-ASCOM.PNG (FITS liberator view)
10 - 0.1ms_G240_16Bit_ASCOM_00000.CameraSettings.txt (0.1ms Exp, Gain=90, FIXED ZWO ASCOM Offset ~50 -
older ASCOM driver)
11 - 0.1ms_G240_16Bit_ASCOM_00000.Histogram.csv
12 - 0.1ms_G240_00001-ASCOM.PNG (FITS Liberator view - Did this cause complaints due to fixed offset ??)
Many thanks
Steve