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Stacking accuracy

Q: How can I get the sharpest images in my stack using Nebulosity? How does Nebulosity compare to other stacking tools?

Nebulosity has several means of aligning the images prior to actually stacking them. We can use simple translation, translation + rotation, translation + rotation + scaling, and Drizzle. I've covered Drizzle in an article for Astrophoto Insight, so I'll focus on the more traditional methods here.

The big difference between "translation" and "translation + rotation (+ scaling)" is that when doing a translation-only alignment, Nebulosity does not resample the image. It does "whole pixel" registration. This sounds worse than "sub-pixel" registration. Isn't it better to shift by small fractions of a pixel? Well, it would be, except for the fact that when you do so, you need to know what the image looks like shifted a fraction of a pixel. That means, you must interpolate the image and interpolation does cause a loss of sharpness. So, you're faced with a trade-off. Keep the image exactly as-is and shift it by whole pixels or resample it and shift it by fractional pixels.

Now, toss into this the fact that our long-exposure shots are already blurred by the atmosphere (and to a varying degree from frame to frame) and you've got a mess if you try to determine which is better from just thinking about it. So, we have what we call an "empirical problem." Let's get some data and test it.

I took some data I had from M57 shot with an Atik 16IC at 1800 mm of focal length and some wider-field data of M101 shot on a QHY 2Pro at 800 mm. I ran the M57 data through a number of alignments and Michael Garvin ran the M101 data through several as well.

Here are the images from M57 (click here for full-res PNG file). All were processed identically, save for the alignment / stacking technique.


Here are the images from M101 (click here for full-res PNG version). Again, all were processed identically. Here, the image has been enlarged by 2x and a high-pass filter overlay used to sharpen each (all images were on the same layer in Photoshop so the same exact sharpening was applied).


So what do we take from all this? Well, first, there's not a whole lot of difference among the methods. All seem to do about the same thing. To my eye, adding the "starfield fine tune" flag in Nebulosity helps a touch and using the resampling (adding a rotation component) hurts a touch, but these aren't huge effects. Someday, I'll beef up the resampling algorithm used in the rotation + (scale) version. Comparing Nebulosity's results with those of other programs again seems pretty much a tie. I can't pick out anything in their stacks that I don't see as well in Nebulosity's. Overall, these images seem to be limited more by the actual sharpness of the original data than by the stacking method.