ZWO ASI 1600 & SW 130PDS - Coma or Poor Collimation or Something Else ?

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Hi All

I’ve just taken delivery of my first ever dedicated astro camera, ZWO ASI 1600 PRO. It’s very exciting, only ever dabbled about with a DSLR before, I’m a beginner really, so I feel like I’ve plunged into the deep end now ! Lots to learn here.

Anyway spent the other night testing it out for the first time, trying to set it up and get things spaced correctly. I am using ZWO filter wheel, ZWO OAG and Baader MK3 CC on a Skywatcher 130 PDS. After lots of fiddling and a couple of fine spacers I’ve got somewhere with it but still have ‘eggy stars’ around the corners.

Attached is a test shot.

I did a quick collimation of the scope and I thought it looked ok, so I did a star test (pics attached), one side of focus seems not bad, the other side seems a little out maybe ?

So..... I am wondering if the eggy stars could just be imperfect collimation ?

Or

Are the eggy stars still coma/incorrect spacing and if so any idea which way I need to go ? Should I add or remove spacers ?

I’ve also noticed when a test image is stretched there seems to be some uneven vignetting, more darkness on the right hand side of the images. That’s probably another issue for another day or perhaps it could be collimation related too ?

Any help would be welcome at this stage, lots to learn with this new camera but if I can get past these setting up issues I might actually get to use the thing ! 

Thanks

Gary

Edited December 18, 2019 by Spaced Out

[SamAndrew]

Looks like coma to me as the stars are all pointing out towards the edges, you'll just have to try adding and removing spacers to see which makes it better. What is your total distance from sensor to the coma corrector? It should be 55mm.

The uneven illumination is a feature of the fast newts I believe, so don't worry about that, it will correct out with flat frames.

[alacant]

Hi. The coma corrector needs to be further from the sensor, either 56.0mm or 58.5mm. This depends upon whether you are using the supplied m42 adapter or the m48 thread on the coma corrector itself:

HTH

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Thanks for your replies, I will persevere with the spacers and see if I can improve things. Trouble with the spacers is (depending on where I put them) they always mess up where the OAG prism sits compared to the sensor. At the moment I have them all squared up nicely but still have some coma.

With regard to uneven illumination, never has this issue with the DSLR, just had an even circle of vignetting all around. With this camera it is definitely worse on the right hand side of frames ?

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Hi again

OK a little update on this one.

I spent last night faffing with spacers, to get things better. Time was limited due to cloud rolling in. The test shot below was about the best I could get it to. It’s an improvement but it still has ‘eggy’ stars in places.

I am struggling to interpret this. The even ‘comet like’ coma in all 4 corners has changed and now these “eggy” stars seem less evenly placed.

I did find this post useful to understanding CC/FF spacing....

https://stargazerslounge.com/topic/271361-star-elongation-and-reducer-spacing-what-does-it-mean/

So.... I am now thinking either I have spaced too far, or there is focuser tilt involved ? It is a heavy imaging train hanging off a standard Skywatcher focuser. I’m just guessing tho so I’d value a more experienced opinion please !

Maybe I am just expecting too much from the Baader MPCC ?

[edarter]

Can I ask where you are sourcing spacers from? I think I need to play with camera spacing more on my 130pds, not got the same camera as you but getting egg shaped stars rings the edge. 

Thanks

 

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4 hours ago, edarter said:

Can I ask where you are sourcing spacers from? I think I need to play with camera spacing more on my 130pds, not got the same camera as you but getting egg shaped stars rings the edge. 

Thanks

 

I'm using these to fine tune the spacing, and for larger threads I just snip them with a pair of scissors and stretch them to fit around, it leaves a gap but still does the job of spacing OK. They have a good range of thickness to help fine tune things.

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

[spillage]

Could be tilt. Make sure that the imaging train is firmly pushed into the focuser and do up the screws a bit each. I found a adaptor like this helps. The baader mpcc should work well though.

Edited December 28, 2019 by spillage

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On 28/12/2019 at 19:34, spillage said:

Could be tilt. Make sure that the imaging train is firmly pushed into the focuser and do up the screws a bit each. I found a adaptor like this helps. The baader mpcc should work well though.

Thanks. I asked on here a little while ago about using one of those adaptors V adding a third thumb screw. Decided to go for the third thumb screw in the end. 

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I'm back on this one !

OK, I've got the MPCC spacing as it should be, turns out I was over-spaced, I've started again and measured it very carefully, it should be bob on now.  

Here is a new test image and I think this shows that I do have tilt going on ? The stars on the left look not too bad, but on the right we have coma like comet tails. I’m guessing there is tilt somewhere and most likely the stock SW focuser as it has a heavy camera hanging off it ?

If this is focuser tilt, what is the best way to fix it ? Is there an easy way of shimming the focuser slightly ? Or is there a better way to sort it ?

Wondering if something like this might help ?

https://www.amazon.co.uk/TS-Optics-ZWO-Tilter-tilting-compensator/dp/B07BZJ4989

Thanks

[spillage]

I added a compression ring, this one I think and also added a third thumbscrew. I find that if I push the focus tube as far down as possible and then push the camera into it from a central point I can slowly tighten up the screws bit by bit.

 

I have just come across this.

Only issue so far is that on my main pc it mentions that the m48 female thread is improper and needs rework (i guess rethreading).