Stacking images from pre- and post- meridian flip produces poor stars

[Peter Reader]

Hi all,

Please see attached image comparing results of stacking subs before or after meridian flip versus all subs together.

Can anyone comment on why this happens and how I might fix it?

Many thanks

[assouptro]

Hi Peter 

I am not an expert but it looks to me like your guiding isn’t as effective after the flip and there is a little bit of “extra movement” in the mount possibility down to weight distribution, the mount being weights heavy means that it’s fine pushing the worm when pre meridian but when you are on the other side and the weight is heavier the worm is trying to “pull” the mount back and can overshoot the guide star easier.

I don’t know if this is what is going on here and I’m not sure I’ve explained it well, hopefully someone else will chip in? 
 

clear skies 

Bryan 

Edited April 2, 2022 by assouptro
Typos

[scotty38]

Looks like star alignment to me on the face of it, can you try a different tool by any chance?

[almcl]

There's a little bit of field rotation between the before and after, but nothing a competent stacking program shouldn't be able to deal with. 

Which one are you using? 

[Peter Reader]

I used DSS and dumped all files into the same group. Because of the meridian flip and my lack of rotator, the images taken after flip are rotated by 180 degrees. I tried stacking all the same images in ASI studio and got a good result... 

Maybe a bit of defocusing after the flip presumeably due to focuser droop but not huge...

[ONIKKINEN]

Try SiriL, with Lanczos-4 interpolation as the method for registration. Should produce noticeably better stars than DSS which i believe uses bilinear interpolation for its registration.

As a newtonian imager myself, i will also say that your collimation might be different on the other side of the meridian, especially if you have the stock focuser, stock tube, or really stock anything in the mechanical parts of the scope that are not really up to the task of imaging for stock scopes. Guiding is also worse after a flip for me. Not sure how to fix that issue though. Maybe recalibrate guiding after the flip? Still experimenting so wont comment more on that as im just guessing myself.

[michael8554]

Hi Peter

I agree with Bryan, your guiding after the flip wasn't as good.

Compare the guide statistics  before and after the flip.

Michael