In the past I have managed to stack sessions with slightly different focal lengths (185 & 200mm).
Is there a limit in how far the 2 focal lengths can be? I get an error when trying to stack 135mm with 200mm subs. I tried using a 135mm sub as reference and a 200mm sub as reference but keep getting an error on the non-reference focal length subs.
I also tried stacking both focal lengths separately, which of course works, and then stacking the 2 stacked files, but same issue.
could you tell us what error? during which operation? how do you process them together?
First of all, you need to know which version you’re using. The dev version can handle distortion, which is very useful for aligning images of different focal lengths.
Secondly, there are no rules. It may or may not work that depends of the images and the reference image you chose.
Processing steps:
Pre-proces all subs of each sessions/focal lenght separately with the sessions dark/flat/bias frames.
Then make a new sequence with all the preprocessed subs. Register them, stack them.
The error I now get, is when I try to register the images. Either the ones taken at 200mm or at 135mm get registered and the others give an error.
I tried the global star alignment, the 1-2-3 star registration and Two-pass global star alignment. I tried all available algorithm options for both registration methods. I tried all the color channels.
The error is always the same: " Cannot perform star matching: try #3 . Image (images number) skipped".
1.2.4 (build 11551)
that sounds like the correct way to proceed to me, can you share one image from each focal length? It’s supposed to work with the global star or two pass, not the 1-2-3 star registration.
Sure, here are 2 files: Unique Download Link | WeTransfer
My workflow would be to run a cropped sequence on the calibrated 135mm images to a slightly larger FOV to the 200mm images.
You should then have no problem registering all images to a 200mm reference frame thereafter.
The alignment between the two images is not great, I think somethink like a third of the image is in common (I checked that by plate solving both images). By cropping the 135mm image and choosing a higher threshold for star detection to have less of them, it was able to register. It’s possible that the astrometric registration of the new version will do a better job at that, I haven’t tried.
For some reason I had to download 4 times from wetransfer, getting a different file every time, until I finally got a non-corrupted one…
Yes, the 200mm was just too tight to have everything within frame, and the 135mm I had to really guess to frame, and I only had the Soul nebula fully inside all subs I believe.
So you say I have to crop every sub of the 135mm? Then register with higher threshold for star detection (which parameters did you change?) all subs of both focal lengths?
Alright! I cropped the subs of the 2nd sessions and then registered both sessions successfully.
Thanks for the help!
I suspect this would have worked if the images had more common stars. I haven’t had a chance to try with the new astrometric registration but that would be the way forward with version 1.4 I believe.