Astro landscapes: aligning the star background stack

[Reisender]

Hello everyone! 

This is one of my first forays into astro landscape photography. I took a stack of five 20" exposures on a tripod and would like to align them for noise reduction. This works pretty well for the foreground with a mean stack, but since the stars have shifted quite a bit over the course of the exposures, it does not work for the background.

So what I probably have to do is process the foreground and background layers separately and then merge them later. But how do I get the stars in the background to align rather than the foreground? I have attached a work in progress, with the brightness adjusted for the foreground.

Thanks!

[John Rostron]

The alignment algorithm wiill be affected  strongly by the building and horizon and hardly at all by the stars.  To align the stars, you will need to isolate the starfields from the rest. You also have what look like clouds. These may or may not affect the alignment, but it would not hurt to remove them from your starfields. You may well have to save the starfields as separate files to get them to align.

With only five exposures, you may find it quicker to simply stack the original images, then align manually.

John

[Reisender]

5 hours ago, John Rostron said:

The alignment algorithm wiill be affected  strongly by the building and horizon and hardly at all by the stars.  To align the stars, you will need to isolate the starfields from the rest. You also have what look like clouds. These may or may not affect the alignment, but it would not hurt to remove them from your starfields. You may well have to save the starfields as separate files to get them to align.

With only five exposures, you may find it quicker to simply stack the original images, then align manually.

John

Thanks for your answer, I appreciate it.

I am rather new to all of this. How would I go about this? I guess I can erase the foreground and clouds on all layers, save them, then run the alignment algorithm again?

I am also not sure how I would manually align the layers. The stars rotate around polaris, which is somewhere to the north west of the picture. How would I rotate my files consistently and accurately?

[John Rostron]

There was a post with a similar problem here which goes some way to solve your problem. It is important to have a good contrast between the star images and the background. Use levels or curves for this. Then try stacking. Look at also at the video on Pin-Sharp Stars.

I realize that the stars will be rotating about a common centre, but unfortunately you cannot expicitly use this. This would be an interesting challenge for a macro which I might accept, but don't hold your breath.

Please re-post if you are still having difficulties.

John

[Reisender]

Thank you! That is helpful. It looks like the process is a bit time consuming, though. I'll give it a shot anyway!