Fixing these star trails?

[Penguin]

So I have polar aligned the HEQ5 Pro as best I can on its pillar, taken 40 60-s subs of NGC 7789 through the SW150P with the 450D, stacked in DSS and processed in PixInsight and ended up with this image. It clearly has elongated stars, which I frequently get, and I think this should be my priority to sort out. I think the solution could be

Guiding: would adding guiding be able to compensate for errors in balance or alignment? I have an SP900 webcam and am considering attaching it to a finder/guide scope. Is that feasible?

Balance: I have not been precise with my balancing. Could this be causing the trailing?

Alignment: Do I need to just spend more time and effort getting the mount aligned?

 

NGC7789_Processed.tif

[alacant]

Hi. Are we sure it's due to tracking? You have round stars toward bottom right and the trails vary in length elsewhere. Is there anything in the imaging path? A cc maybe? HTH.

**perhaps best not to post big tifs. Here's a jpg; you'll get more answers that way;)

[bobro]

I agree with @alacant : there are round stars so tracking isn't suspect. More likely an optical issue such as collimation, coma or coma corrector alignment, possibly in conjunction with polar alignment. To eliminate tracking and polar alignment just take a short exposure (e.g. 20 sec) at a high ISO of somewhere in the Milky Way to ensure there are lots of stars. No processing required - just a jpeg of the image.

[Penguin]

That explains why the image didn't appear in the post, thanks.i will try to remember to post jpegs in future.

No, there is no coma corrector or anything like that in the path, just the scope and camera. I had not considered collimation, will look into that as well as polar alignment and post my findings.

[Penguin]

It occurred to me last night that not only is the posted image processed, it is also cropped! So here are two of the individual unprocessed subs, taken at 21:47 and 22:07 which may help diagnose exactly where I need to concentrate my efforts. The amount of movement between the two says 'polar alignment' to me. Flicking through the subs, there seems to be consistent drift in one direction.

[bobro]

Certainly coma (stars towards corners having tails pointing away from the image centre). Also looks like polar alignment error - this results in rotation about where the guidescope is pointing as guiding ensures the guidestar stays in the same position.

A shorter exposure will reduce the rotation, but of course better polar alignment is necessary. How are you achieving polar alignment at present?

[Penguin]

Thanks Bob. Just to be clear, there is no guiding at present. Looking closer at those individual subs, I did spot that there are two issues going on, as you have pointed out:

1. a consistent trailing upper left to lower right throughout the frame. I assume this is polar alignment. I may need to do some physical fixes to my rig in order to sort that out, as well as learning alignment methods (see below).
2. a 'shadow' towards the edge of the frame, particularly noticeable in the top-right and bottom-left corners where it is at 90 degrees to issue #1. I assume this is the coma you are talking about and a coma corrector will be the only way to get rid of this if I want to use the full frame.

I aligned the mount quite some time ago, maybe even last year, and have not touched or done anything to check it since. I have not found an alignment guide that I can fully understand so my technique may have been way off. Also, my mounting plate is a home-made affair with a bolt as the pin that the HEQ5-Pro alignment screws push against and it's quite possible that bits may have 'moved'.

[dph1nm]

Check if the motion is in RA, Dec or both. If there is any significant motion in Dec then it is almost certainly polar alignment. If it is only in RA then one would suspect the tracking (periodic error etc).

NigelM

[Penguin]

Thanks Nigel, I will check this on the next clear evening (which might even be tonight!). I believe I know how to do it:

1. Orient the camera so stars drift across the long side when not tracking. Time roughly how long they take to traverse.
2. Place a star at the starting edge and start an exposure
3. When enough time has passed for the star to travel 1/2 way or so across the frame, start tracking
4. Continue the exposure for the same time again

That should tell me which direction is which in the frame and which direction any alignment issues are in, I think.

[bobro]

6 hours ago, Penguin said:

Thanks Bob. Just to be clear, there is no guiding at present. Looking closer at those individual subs, I did spot that there are two issues going on, as you have pointed out:

1. a consistent trailing upper left to lower right throughout the frame. I assume this is polar alignment. I may need to do some physical fixes to my rig in order to sort that out, as well as learning alignment methods (see below).
2. a 'shadow' towards the edge of the frame, particularly noticeable in the top-right and bottom-left corners where it is at 90 degrees to issue #1. I assume this is the coma you are talking about and a coma corrector will be the only way to get rid of this if I want to use the full frame.

Sorry - was in a bit of a hurry this morning and forgot you had already mentioned no guiding. 

1) Drift alignment is a very good way to achieve accurate polar alignment and especially useful as you have a permanent setup and won't need to keep repeating it.

2) Yes, this is coma. Just for fun a piece of cardboard can be used to reduce coma - see my post for info which shows the same image with and without coma : 

Regarding using the webcam for guiding, yes it can be used (I started out using a Microsoft webcam for guiding) though lack of sensitivity can be a problem with needing to rotate the guide camera in order to find a guide star. Depending how you feel about cost and DIY, a sensitive guide camera can be made from an AR0130 module - this is what I use and it finds a star without needing to be rotated. Good guiding can't make up for poor polar alignment though as rotation would be the result. Info on a guide camera :

You've probably guessed by now I like coming up with ways of making things - I made my own coma corrector as it's difficult to find one for a 1.25" focuser : https://stargazerslounge.com/topic/288608-diy-125-coma-corrector/?tab=comments#comment-3163837

 

Hope you get the alignment sorted out.

 

 

 

[Penguin]

That's very interesting on the coma corrector! Once I have the alignment sorted out to enable longer exposures I may try that out on some brighter targets.

Here's the result of tonight's investigation It's about 2 minutes of non-tracking, followed by another few minutes of tracking. I can't remember what the target star was but it was a little east of south, around 60 degrees up. It looks to me as though, when tracking, the stars are drifting up the frame. I think I will watch some polar alignment videos and try again from scratch once I have a better understanding of what is going on.

 

[bobro]

That's a very neat way of diagnosing a polar alignment issue!

[Penguin]

Thanks Bob, and I'm also very interested in your home-brew guide camera article. Definitely something to look into once I am happy with the alignment.

I'm planning on following this guide to get the alignment sorted: Drift Alignment by Robert Vice since it should be easy and quick to verify if the adjustments I make are correct.