Does more integration time affect high signal areas?

[Crushellon]

Does more integration time affect high signal areas or is it just to improve low signal/background areas? The reason I ask is because when I get another chance I would like to collect more data for my HDR Orion image and I'm wondering if I should bother collecting more of the short subs as only the high signal areas from those subs are being used in the final image with the background and low signal areas coming solely from the longer subs.

[gregbradley]

Good question. I think it improves all areas of the image particulary the coloured subs which tend to be noisier.

Even high signal areas will have some noise in them. Short exposures will have shot noise in them which is the randomness of light causing unevenness in an exposure.

But the background and dim areas are usually the weak areas of an image for sure and improve the most from extra exposure. Bight areas will respond better to sharpening if its longer exposures. Even bright areas often have dark sections in them of dust areas etc.

The total exposure needed for an excellent image of our average astrophotography targets does vary depending on aperture, f ratio of the scope and QE of the camera but even with the fastest of both you are still looking at really 8 hours minimum, more like 12 hours for a fast system and upwards of 20+ for a slower system.

Bright targets you can get away with less but if you are going for the best possible quality then the above minimums come into play.


Greg.

[glend]

In my opinion, it depends on your camera and processing skills. Orion is a notorius high dynamic range target, and blowing out the Trap is common when trying to get good fringe emission images. For many imagers the answer is to shoot two exposure series: one long one to bring out the emission detail in the extended cloud areas, and a short set to expose the core without blowout.
I will explain how this is integrated in Photoshop. You bring your images together as layers, and create a layer mask, so that you use the short exposure core and the longer exposure outer area. There are several good Photoshop tutirials online that show how to do this, Ken Crawford's videos on utube are good.

[RickS]

You've hit on an important idea that often gets lost in discussions about SNR. Areas of high signal achieve good SNR very quickly and additional integration time only makes a small difference. It is areas of low signal that benefit most from more integration time. This is a consequence of physics, not an opinion ;)

You don't need a lot of short subs if you're only using the brightest areas for HDR. You do want lots of long subs if you want to stretch enough to bring out the dimmest areas.

Cheers,
Rick.