Stacking leads to elongated stars in the corners

[c.hengel]

Hi there,

my images suffer from elongated stars in the corners. The stacked image looks like the star position had moved between the subs at the edges and corners, while in the center they remain well aligned. When I blink the individual subs I cannot see a displacement as large as the elongation of the stars in the result image. I double checked with DSS. Using DSS, the stars in the corners look better. I found that if I exclude the images that were taken after the meridian flip, the AA8 image looks better, too.

My questions:

- is there a way to completely suppress image scaling in the star-pattern-align routine?
- is there a way to increase the number of stars used for Alignment to a number >40?
- what else could I try if I want to use AA8 instead of DSS?

Thanks

Christian

[Iver]

Did you try selecting star alignment with translation + field rotation?

[c.hengel]

I did. This is where you can choose the number of stars used for alignment up to a limit of 40.

[fabdev]

Hello, more stars will not help. To solve the problem it will be useful to see some images of the sequence, for example 6 images:
first / mid / last before the meridian flip, and first / mid / last after the meridian flip. If you like, send them by email to: https://www.msb-astroart.com/support.htm (if the images are big you may resize 50%).
Fabio.

[c.hengel]

Thank you, Fabio, I will send the images. Just to illustrate what I am talking about, two images as a reference (https://1drv.ms/u/s!AnXit8pN4lz1hKppsKo ... A?e=B4vLJZ).
Left is the DSS-result, right AA8. 189 subs of 3 minutes each.

Obviously, there is a collimation issue I don't get really solved with my RC 6, and also some focus drift due to temperature change may contribute to the problem. Anyway, in the AA8 result stars almost look like two stars while in the DSS result they still appear as (strongly elongated!) stars. At 66% zoom, they look almost round, which is ok for printing.

[einari]

What stacking method did you use, in DSS or in AstroArt ?

Anyway, I think you should get a field flattener for better edge stars (if that bothers you).

[fabdev]

Hello, meanwhile I received and downloaded your sequence. I confirm that the change of distortions at the edge after the meridian flip (maybe caused by instrumentation weight?) cannot be corrected by AA. By the way a new code for that is already on development for AA8 SP3, I will send you the prerelease in 4-5 weeks.
To compare images, consider the command: View / Equalize visualization / Sigma

[c.hengel]

Thank you for taking care of the issue, Fabio! I am looking forward to the SP3 then
As for your suggestion of using a field flattener: That is what I do. The problem is that APS-C sensor size appears to be the absolute limit for the RC 6 - at least for the two scopes I had the chance to try out.

Best regards

Christian

[fabdev]

Hi, the correction has been included also to Astroart 8 SP2, which will be released next week.
Fabio.