SW 130PDS - Odd shaped stars are doing my head in !

[Spaced Out]

Hi all

 

I’ve been wrestling with my new camera for ages now trying to get nice round stars. I’ve got it as good as I can get but there are still ropey stars on the left, particularly the bottom left. These are squashed looking almost triangular shaped stars with little tails ?!

Initially I thought it might be mirror clips pinching optics so I removed the mirror and loosened them slightly. That made no difference.

I spent ages on CC spacing but I think that is OK now, the right side of the image looks OK.

Next I thought about focuser tilt. I used a bahtinov mask, perfectly focussed a bright central star, then moved the star to each corner, expecting focus to change a little but it didn’t, just remained in perfect focus. A pretty crude test I know but I read somewhere it could show up tilt ?............ Or is that wrong ?

I collimated and collimated again, using a sight tube and a laser. It all seemed OK with a quick star test.

I’m left scratching my head now.

Any ideas ?

Thanks

[ollypenrice]

2 hours ago, Spaced Out said:

 

Next I thought about focuser tilt. I used a bahtinov mask, perfectly focussed a bright central star, then moved the star to each corner, expecting focus to change a little but it didn’t, just remained in perfect focus. A pretty crude test I know but I read somewhere it could show up tilt ?............ Or is that wrong ?

 

 

 

I don't know, but how might you 'test this test?' How about putting your test star in the corner of the image, trying the B mask, getting focus perfect, then turning the B mask through 90 degrees and trying again? If the star is defective in the image but looks good in the B mask in both orientations then I would suspect that the test were useless. In truth I don't think it will prove to be a valid test but I don't understand the optical principles of the B mask well enough to be sure.

If you find that one corner or side shows elongation you could try rotating the camera through 90 degrees to see if the elongation relocates itself on the image or follows the camera. Chips tilted within the camera are not unknown.

I admire perfectionism in imaging but, trust me, we have seen a lot worse!

Olly

[Spaced Out]

14 minutes ago, ollypenrice said:

If you find that one corner or side shows elongation you could try rotating the camera through 90 degrees to see if the elongation relocates itself on the image or follows the camera. Chips tilted within the camera are not unknown.

I admire perfectionism in imaging but, trust me, we have seen a lot worse!

Olly

Thanks for your reply. I will try rotating the camera, what will that prove ? Sorry I'm a bit of a learner/duffer !

[spillage]

Are you using the standard 2" focus thumbscrews. I found adding a third thumbscrew helped and also making sure the focus tube was adjusted to take out as much play as possible. I always push the camera in as far and a firm as possible then just tweak each screw a bit at a time.

Knowing the orientation of the camera would also help work out if its just focus sag.

I think your results are not that bad at all and processing can tighten up the stars.

[Spaced Out]

2 minutes ago, spillage said:

Are you using the standard 2" focus thumbscrews. I found adding a third thumbscrew helped and also making sure the focus tube was adjusted to take out as much play as possible. I always push the camera in as far and a firm as possible then just tweak each screw a bit at a time.

Knowing the orientation of the camera would also help work out if its just focus sag.

I think your results are not that bad at all and processing can tighten up the stars.

Thanks. I took your advice last time, I have 3 thumbscrews and I wind the focuser right in before putting the CC & camera in firmly and tightening each screw bit by bit. It works well. I've got the focuser tensioned reasonably tight with an autofocuser.

I'm guessing if I rotate the camera and the wonky stars stay on the same side of the frame then it could be a camera issue but if the stars move to the other side then it's likely a tilt issue somewhere ?

I know it's not terrible but I'm keen to get things as good as I possibly can before I start using it properly, it's just bugging me at the moment !

 

[spillage]

Are you using delrin spacers? The mpcc might just need a bit more distance. I had a similar issue. Try another couple of mm and see how looks then, it real pain in the behind to do but worth doing on a night that is not quite good enough for imaging but good enough to image a star field.

[Spaced Out]

12 hours ago, spillage said:

Are you using delrin spacers? The mpcc might just need a bit more distance. I had a similar issue. Try another couple of mm and see how looks then, it real pain in the behind to do but worth doing on a night that is not quite good enough for imaging but good enough to image a star field.

Thanks, I've already done this, and at one point I was over spaced but I think it’s good now, the right hand side looks OK at least.

I'm left thinking this has to be tilt somewhere ? It is a heavy imaging rig on a standard SW focuser.

I wonder if a free trial of CCDInspector might help me identify the problem ?

[Skyline]

If the coma corrector is the SW 0.9x reducing CC, then you could try using 58-60mm spacing with a CCD. If your using DSLR you don't need spacing. 

[tooth_dr]

There is the possibly of tilt in the camera chip as @ollypenrice says. This isn’t uncommon. 

[Spaced Out]

50 minutes ago, Skyline said:

If the coma corrector is the SW 0.9x reducing CC, then you could try using 58-60mm spacing with a CCD. If your using DSLR you don't need spacing. 

Thanks. I'm using Baader MPCC Mk-III and I think the spacing is good, I have measured it carefully with calipers and tested by adding & reducing spacers.

[Whistlin Bob]

Have you 'trimmed' the drawtube of your focuser at all? Sometimes it can protrude it over the primary and affect star shapes. 

When it's a big problem it affects stars all over the image and is very obvious, but when it's a small problem it only affects one side/corner. 

To check this get your camera to it's focus point and then have a look down the front of the tube- see how far the drawtube protrudes over the primary mirror. 

I had this problem quite severely (stars like yours in the middle steadily getting worse until they liked like Pacman at the top left corner) and trimmed around 10mm off. This fixed it, but I have to be careful not to wind the focuser out too far!

Edited February 10, 2020 by Whistlin Bob
Typo

[Spaced Out]

36 minutes ago, tooth_dr said:

There is the possibly of tilt in the camera chip as @ollypenrice says. This isn’t uncommon. 

Thanks mate. I'm fairly sure when I rotated this a couple of weeks ago (when I was faffing about with spacing) the ropey stars switched to the other side of the image, would this rule out the camera and point towards tilt at the focuser ?

I need to double check this after the winds have dropped !

[Spaced Out]

2 minutes ago, Whistlin Bob said:

Have you 'trimmed' the drawtube of your focuser at all? Sometimes it can protrude it over the primary and affect star shapes. 

When it's a big problem it affects stars all over the image and is very obvious, but when it's a small problem it only affects one side/corner. 

To check this get your camera to it's focus point and then have a look down the front of the tube- see how far the drawtube protrudes over the primary mirror. 

I had this problem quite severely (stars like yours in the middle steadily getting worse until they liked like Pacman at the top left corner) and trimmed around 10mm off. This fixed it, but I have to be careful not to wind the focuser out too far!

Thanks for this reply. Yes I have cut down the focuser tube but it was for the DSLR I was using at the time and then it was completely out of the tube at the point of focus. With this new camera it does protrude back in a little but not by very much, perhaps 1cm. 

[Spaced Out]

Considering everything discussed here so far I am wondering if my best course of action might be to get a free trail of CCDInspector and purchase one of these ?

https://www.firstlightoptics.com/zwo-accessories/zwo_t2_tilter.html