Images shifting from sub to sub

[Simon Pepper]

Hi all.

Still learning this hobby and I have another question for the community please. When taking my images I find more often than not that my images between subs are moving in one direction. For example if I start with M31 in the bottom right of frame after 90 minutes of subs it’s moved a good inch to the left of frame. I assume this is not dithering as I think dithering is random and only shifts a few pixels? So my issue must be balance, but when balancing I’m sure it’s balanced (ever so slightly heavier on one side). If anyone can assist I would be grateful.

 

thanks 

Edited November 5, 2020 by Simon Pepper

[almcl]

Could you say a little more about your set up?

Assume you are guiding (since you are dithering) ?  What software are you using?

What imaging scope, what guiding scope, what imaging camera, what guiding camera? 

How is guide scope mounted?

None of these may be relevant or all of them may be involved.

[alacant]

19 minutes ago, almcl said:

all of them may be involved.

Hi

It may also help to have -links to- a frame from the start and a from from the end of the session as well as the guide logs.

Cheers

[Simon Pepper]

Hi all thanks for the responses apologies I was a bit vague. I am not guiding (so not dithering learnt something there) just tripod, redcat, dslr on my ipolar skyguider pro. Not out put through DSS yet will do that tomorrow and upload here so you can see.

 

thanks 

[Adam1234]

Quite possibly your polar alignment is a bit off

Edited November 5, 2020 by Adam1234

[fifeskies]

+1 for this being a Polar Alignment issue as the most likely cause

With guiding the Polar Alignment is not so critical , but unguided you will see this kind of drift in one direction over longer times like a 90 minute exposure.

Before I guided an error in Polar Alignment of about a degree would give me a similar scale of image drift, so its very sensitive to misalignment.

 

If you stack your images and crop the final output frame you can still get perfectly good results despite a bit of drift so all is not lost.

[ollypenrice]

1+ for polar alignment. You could try the DARV method. https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

Olly

[Simon Pepper]

Hi all.

 

Please see attached out of DSS. You can clearly see the walk I got here last night. Surely if it was bad polar alignment would I not see start trailing worse than I have here? This was 80s subs, 2.10 hours total.

 

Thanks

[Seelive]

Guiding will (hopefully) remove any star trailing due to polar (mis-)alignment but any polar misalignment will also result in image rotation as well as shift between the images.  The degree of image rotation and image shift will depend upon the magnitude of the polar misalignment and the offset of the guide star from the true image centre. The star elongation doesn't appear to be rotating about a fixed point but also doesn't appear to be completely linear over the entire image suggesting that some other optical aberration may be coming into play?

[Seelive]

Just noticed that you're not actually guiding (I should read all the thread before replying!) so I agree that it will be due to polar misalignment drift over your 80s exposure time. Look at the single images and you should see roughly the same drift in each of them. Fortunately the drift wasn't significant so DSS still 'saw' them as stars and didn't reject all the images when stacking.

What stacking method did you use? When not guiding and it results in a small amount of trailing, then using the Sigma-Kappa stacking method with a low value of Sigma (say 2 or lower) can help in restoring round stars in the final image.

[Simon Pepper]

Hi all,

So just to confirm we think my polar alignment was off and that has caused the image to shift between frames? If guiding will it correct the above issue?

 

Thanks

[david_taurus83]

Guiding will help correct right ascension errors but not declination. The drift is in declination in your stack. Polar alignment is likely cause though if your mount has iPolar it should be good. An error in setting it up?

[Simon Pepper]

Ok thanks all I reckon this could be an amateur fail in mounting the tripod on less than ideal ground and it’s slipped after alignment. I have seen this before but the above shift is the worst it’s been. I will update the base and give it another crack 👍