Thanks for the 90Mb file - I hope you have plenty of disk space. Looks like you are now starting to interpret the histogram - progress.
Any stacking software will be picky about the frames matching, so be consistent with your lights, darks, flats, biases in terms of Output Format, Bit Depth, Capture Area and Colour Space.
The image header of the FITS frame contains the 3 important settings - exposure, gain and black_level. I think for the ZWO black_level is called Brightness (see your capture settings file).
- M44-image-header.JPG (56.26 KiB) Viewed 2320 times
- M44-histogram.JPG (87.52 KiB) Viewed 2320 times
There are 2 areas to address with the histogram. Decreasing exposure (2) will move the histogram away from the right hand side. Also adjusting brightness will shift the whole histogram to the left or to the right (1). How I do this is by using a bias frame to adjust the brightness (black_level/offset for Altair/QHY cameras). Think of a bias frame as a 'zero exposure' dark frame. To capture a bias frame, put the cap on the telescope, leave the gain the same as for the light frames and set the exposure to the minimal value possible. The steps are:
- Decide on the gain to be used, 348 in the M44 case.
- Set the brightness to 100 as in the M44 case.
- Set exposure to the minimum possible.
- Cover the scope.
- Capture a single frame.
- Load the frame into FITS Liberator.
- bias-frame.JPG (99.5 KiB) Viewed 2320 times
In the Image Statistics the min value seems to be around 100 and the max value near 1000 (these are just guidelines). Note that the histogram is centralised and not touching either side.
The idea is for a given gain, use the smallest Brightness setting which gives a centralised histogram.
In an imaging session, decide the gain, set up a bias frame as above. This gain and brightness are used for the capture and the exposure can be adjusted to give a good histogram. It is worth trying the extreme settings for brightness to see how it affects the histogram of the bias frame. A good exercise for in the house as no scope is needed. A good starting point (not necessarily an ending point) for gain is to use the Unity Gain value for the camera. See https://bbs.astronomy-imaging-camera.co ... php?t=6595 where the camera manufacturer suggests gain=139, offset (brightness)=21. It may well be that 21 ends up as the figure you need (for gain 139).
The bias frames will be needed if you want to get the best out of processing (they get used to create the maser flat). In DSS they are just added in and the software handles everything in terms of processing. 50 - 100 bias frames only take a few seconds to generate.
I have seen references recently that using the lowest exposure for bias frames on modern CMOS cameras is not the best. Start off with lowest exposure but if there are issues then exposures up to about 1s are ok for these sort of cameras.
The other point is your histogram is 'gappy', I would expect it to be solid. The gaps mean information is missing and will cause abrupt changes between tones (reference: Photoshop Astronomy, R Scott Ireland, page 43). Once you have the brightness setting correct, using unity gain and a reasonable exposure for the object then that issue should go away.
It would be interesting to see how your bias frame looks and what the settings are.
Dave