Stacked vs single exposure for deep sky images - which is actually better?

I shot the Orion Nebula last week two ways. First I did a single 4 minute exposure at ISO 800. Then I stacked 20 two-minute exposures in DSS. The stacked image had way less noise and showed more of the nebula's outer gas clouds. But it took me an hour to process all those frames. Which approach do you folks normally use for bright nebula targets?

3 comments

3 Comments

cooper.phoenix15d agoProlific Poster

Wait, you're telling me you shot a 4 minute single exposure and got anything usable on Orion without it just being a blown out core? I've never had luck with anything over about 90 seconds unguided, much less 4 minutes.

willow11415d ago

Gotta jump in here because @amy_foster79 totally nailed it with that aperture mask trick, but I’ve been doing something similar but different. I shoot Orion at 4 minutes unguided with a star tracker and a simple cardboard stop on my little refractor, and the key is dropping the ISO way down, like 200 or 100. That plus stopping down the lens lets the bloated core stars stay contained, and I can recover all the nebula detail with a gentle curves stretch in Photoshop without blowing anything out. It’s counterintuitive because everyone says shoot fast and stack, but a single 4 minute frame at f/8 with low ISO gives you this creamy, natural looking Orion that stacking sometimes muddies with all the noise reduction artifacts. Amy’s 5 stacked 3 minute idea sounds like a brilliant middle ground too, I might try that next clear night to get the faint outer dust without the processing headache.

amy_foster7915d ago

Actually, I've found that aperture masks make a huge difference for bright targets like Orion. I use a Bahtinov mask that I 3D printed from Thingiverse, but sometimes I just cut a circular cardboard stop to fit over my lens. Dropping my 80mm triplet down to about f/8 instead of f/5.6 lets me push single exposures to 90 seconds without blowing out the core. You don't get the same faint stuff as stacking, but the processing time is literally 2 minutes in Photoshop with curves and levels. That said, for anything dimmer than Orion, I still go back to stacking because the signal to noise ratio just isn't there with one frame. For a compromise, I've been doing 5 stacked 3 minute exposures with the aperture stopped down and that seems to hit a sweet spot for both detail and processing speed.