HyperStar and Smart Histogram

[lampcord]

The last 2 nights I've been imaging M31 with my C8HD + HyperStar using SharpCap.

My camera is a QHY183C in case that matters and I'm in a Bortles 7 location.



So I used the smart histogram which has always given me great exposure using the rear camera with or without the reducer.

It told me to set my exposures to 3.4 seconds with 0 gain. I thought that was nuts because that would mean an hour of capture would result in over 1,000 frames.



So I ignored it and ran 30 second subs and got pretty good results. In fact I'd say surprisingly good results considering I was running subs that were 8x longer than Smart Histogram was suggesting.



The next night I added my Optolong L-Pro light pollution filter to see what would happen. This time, the smart histogram told me to set my exposure to a more reasonable 16.2 seconds with 0 gain. Obviously the filter was cutting out a lot of light pollution. This time I decided to listen and ran about 90 minutes at those settings.



The problem is, it resulted in such underexposed frames that any sort of image calibration, even just using bias resulted in calibrated frames that were practically blank. So I had to skip calibration altogether. I've never had this happen before. And the final integrated image had (not surprisingly) far less detail than the 30 second subs with no filter.



Is there some trick to using the Smart Histogram with a HyperStar? Are there settings in SharpCap somewhere that I need to tweak because of the change in focal length.

[admin]

Hi,

I think it is more like a limitation in the smart histogram calculations – the calculations think about way that the noise from the camera interacts with the signal in the darkest part of the images and give recommendations based on that – calculations don't include any consideration of the brighter parts of the image and whether star brightness is will be bright enough for alignment. This would be something interesting to consider in the future update

For now, remember that going to longer exposures than those recommended by the smart histogram won't cause you any problems (unless you start to run up against the limitations of the tracking/guiding of your mount). But the flipside though, longer exposures than those being recommended will not give you any improvement in the detail that you see in the faintest regions of the image.

Cheers, Robin

[lampcord]

Thanks Robin, that makes a lot of sense.

Since I currently have an AltAz mount, shorter exposures are definitely mandatory because of field rotation and the limitations of my mounts tracking. OTOH 15 seconds is the bare minimum I can live with or the processing gets ridiculous.

I'm thinking a solution might be to get a more aggressive light pollution filter. That should allow me to get more detail with longer exposures although the trade off is not being able to capture the natural star colors.