Image misaligned after meridian flip

[BrendanC]

Hi all,

When I stack images from a session that involved a meridian flip, there's always a slight rotation in the pre- and post-flip images.

I don't mean the 180 degree rotation. I mean there's a misalignment, so that before and after are a few degrees out from each other. It should ideally be pretty much exactly a 180 degree rotation, but it never is.

This isn't a huge problem but it bugs me, especially when I want to make as much of the image as possible for large nebula etc, which I lose because I have to crop out the artefacts around the edges.

I've looked around and found this thread which, after much discussion and diagrams, didn't really end up with a definitive answer or recommendations on how to fix it: https://stargazerslounge.com/topic/280998-rotation-after-meridian-flip-why/

Is this because my scope isn't completely straight on the mount ('orthogonal' is the word, I believe)?

If so, what's the best way to fix it? Would ConeSharp help, for example? (see https://www.sharpcap.co.uk/conesharp)

Thanks, Brendan

Edited January 11, 2022 by BrendanC

[BrendanC]

Any takers?

[vlaiv]

There are two possible things.

1. orthogonality.

I'm really not sure how to handle that because it is deeper mount issue and it means that angle between DEC and RA is not 90°. As far as I know there is no way to adjust that.

2. Cone error

You can fix cone error yourself by adjusting scope on dovetail. Depending on type of scope and dovetail - you might already have what it takes. For example Skywatcher dovetail comes with four bolts:

which you can use to tilt the scope and fix cone error

You can use software to help you with that, but you can also use simple method - pre and post meridian centering.

https://www.cloudynights.com/articles/cat/articles/a-simple-cone-error-correction-procedure-r2447

[BrendanC]

Thank you @vlaiv!

[wimvb]

Polar misalignment is also a possibility, I'd think. The mount doesn't rotate 180 about the polar axis, but about the RA axis.  After the flip, there is a mismatch in pointing, and this is compensated during plate solve by moving in RA (adjusting the 180 degree rotation) and DEC (basically a shift), until the center of the fov is aligned with the pre-flip view. The pre- and post flip angle depends on the DEC angle where you are pointing. Obviously it is worst near the Pole.

So, essentially Vlaiv's answer in the linked thread.

Edited January 11, 2022 by wimvb

[BrendanC]

Thanks! I'm pretty accurate with my polar alignment, at least according to Sharpcap.

This is by no means a showstopper. I'm getting results I'm pleased with. It's just one of those niggles that you'd rather not have and after a while you decide to try and do something about it, if you know what I mean.

I think I'm going to look into cone error, using the ConeSharp utility.

[wimvb]

I thought that I too was accurate,  and never really bothered about the little image rotation that I had. Untill I started imaging ngc2276, qhich is only 4 degrees from the NCP, and rotation was much worse. I will check my cone error as well, and then do a new polar alignment.

[david_taurus83]

The larger the FOV, and higher up in Dec, the worse it is as well.

[newbie alert]

Sounds like cone error and Dione done a excellent video on how to rectify it on his astroshed channel 

[Ohgodwherediditgo]

10 hours ago, newbie alert said:

Sounds like cone error and Dione done a excellent video on how to rectify it on his astroshed channel 

That one.