Photostacking - Why does this technique fail for my astrophotos?

Question

Last week I went to Tahoe to do some astrophotography.

I watched this video (https://www.youtube.com/watch?v=Rydg7JGTAbw) about image stacking to reduce noise in my photos so I decided to give it a shot.

My setup: Sony A7ii with Batis 2.8/18 lens. I shot all my photos with ISO 1600 with a tripod for 25 seconds of exposure. My aperture is f/2.8 and I set my focus distance to my hyperfocal distance for all my shots

This is what the sky of my first image looks like:

And I followed the youtube video: I aligned just the sky and converted it to a smart object and applied the median filter. This is what it looks like:

Upon applying the median filter, I realized that aside from the center of the sky, most stars have disappeared in the newly stacked smart object.

I suspected that the movement of the stars could have contributed to this. so I changed the stacking mode to maximum and saw the star trails:

So here are my questions regarding my photos:

1. If the stars moved significantly between each frame, how can auto-align function properly?
2. How can I apply photo stacking to my current set of photos?
3. What can I do in the future to prevent this from happening?

EDIT

Would you guys mind submitting answers so I can reply individually?

Answer 1

You have to remap the images such that the stars are aligned precisely, this requires pixel level aligning throughout the entire picture. This is best done with specialized software, such as Hugin (which is free of charge). With Hugin you can mask out the foreground and align the sky, you can choose specific points that the program can use to get to alignment, in this case you have a sky full of stars to choose from. The Hugin program gives tight control of the algorithm that is used to do the remapping, you can e.g. input fixed parameters for the barrel distortion, or you can allow these parameters to be adjusted during the fitting.

Answer 2

1) From the difference between median and max you can see that alignment didn't work and the pictures were not aligned. Why this is so, is hard to tell. Comparing the pictures in the tutorial with your picture, it seems contrast is higher and more "structure" is present in the stars field. I would try to ramp up contrast before aligning. Try an extreme setting.

2) I would try using a software written to align astro-photos, like DeepSkyStacker or PixInsight. DSS detects stars, gives feedback on how many are detected und allows setting the threshold. Note that stacking might amplify vignetting or other artefacts (like dust on the sensor). You will have to tweak image settings thoroughly.

3) You could use a mount, which counteracts earth's rotation, like Vixen's Polarie or Skywatcher's Star Adventurer. These allow to take exposures of 1-3 minutes at normal ISO settings between 400-1000, greatly reducing noise in the first place.

Answer 3

As I see things, you have a few problems here.

First of all, although it's impossible to say with certainty, I'd guess that the combination of trees in the foreground (probably moving somewhat randomly) and the stars in the background (moving predictably, but almost certainly not in sync with the trees in the foreground) is probably getting Photoshop confused--it can't find any way to shift/rotate things so it all lines up to give a better match than just leaving things alone, so it...leaves things alone.

Second, your individual exposures are long enough that the stars have quite visible movement in the individual frames (pretty easy to see in the first frame, if you look carefully). So, even if you get the frames aligned well, you're still going to have fairly visible trails (just shorter ones).

Finally, for this you're almost certainly better off using raw files instead of JPEG¹. If memory serves, the A7ii can produce raw files that use lossy compression. If so, you definitely want to use the uncompressed raw format--this is exactly the sort of picture that will trigger visible artifacts if you use lossy compression.

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^{1. Of course, maybe for the real thing you are using raw already, and you just posted JPEGs because browsers and such don't support raw--if so, my apologies.}

Answer 4

The median filter is really good for handling noise in stacked images because it discards statistical outliers, e.g. if a plane was passing the sky while you shot, the plane would be completely removed after a median stacking, whereas a mean/average filter would leave a trace of the airplane.

But if the images are not rotated, so the stars align before stacking, the stars themselves just become statistical outliers (in that particular pixel between shots), and that's why they disappear.

If you are Mac user, there is an app, StarryLandscapeStacker, that can handle the rotation/stacking pretty well.

This photo was produced with the app (with a bit of post processing in PhotoShop) from 12 individual exposures.

Answer 5

• My first guess was that camera has moved between or even during exposures (for whatever reason) mostly diagonally, then automatic aligning would align the frames preferring sharp central part of frame and then these trails appeared because of how image is projected by rectilinear objective. Inspect the trees to see if it is the case - they would tell better unless it was windy that night. If trees appear in different positions, this is the case.
• It could also be that Photoshop tried to stretch images, not only rotate and move them. Check your aligning settings if there are any.
• And my last and best guess is that it is natural distortion of sky. Rectilinear objective will only preserve geometric shapes if the object moves without tilting against optical axis. This will be apparent if you do not point your camera roughly the at Pole star (the north center of sky rotation) and if you use ultrawide angle objective (18mm is enough) and if you use long enough exposure (total exposure time). In this case you will both move/rotate and distort images to fix it (it will be good enough but it won't be exact match).

I may try it myself if you upload high resolution JPEGs (straight out of developer).