Weird star shapes. Any suggestions?

[Phillips6549]

I made my first attempt at Imaging on Monday evening.  I thought I'd try for M42 as an easy and visible target

Equipment was Celestron AM130 mounted on an EQ3-Pro mount.  Canon 650D DSLR (2x Barlow on the T Ring to achieve focus).  

My initial impressions of the images (viewed on the camera screen) was that they looked reasonable.  Over the course of  about 50 minutes the image remained in the centre of the viewfinder so tracking was OK.

When I tried to stack the images in DSS, it could not find any stars and, on inspection, neither can I.  What I can see is a sky full of boomerangs!

I've attached a cropped section of a single sub to show what I mean (20s exposure).

All subs were taken with the mirror locked up and using a remote shutter release (infrared) so I was not introducing an manual vibration to the setup.  The focus is probably off but, visually, defocusing produces only the classic airy disc, not boomerangs.

Any Ideas what's going on?

Looking forward to your replies

Mark

Edited January 22, 2020 by Phillips6549
Forgot to mention the barlow.

[Phillips6549]

Gentle bump.  

I inspected my scope and found that one of the bolts securing the focuser tube to the OTA was loose (almost falling out).  Although I couldn't discern any movement of the tube, could this have contributed to my boomerang stars?

[andrew s]

It does look as if the image is jogged every now and again. It could be seeing or mechanical but most likely mechanical given the length of the exposure. 

Regards Andrew 

[vlaiv]

Could be.

I can't really discern what sort of aberrations are mixed in there, but one thing is for sure - it's not single aberration that is causing such star shapes. Might be coma combined with tilt.

I think that first step should be visual inspection of star pattern. Can you take high power eyepiece and point scope to rather bright star and inspect in focus / out focus and perfect focus star images by eye to see if scope is properly collimated.

If star shapes look rather good visually, then I would say focuser would be main suspect.

[vlaiv]

Ah, good point by Andrew. Could you inspect other subs as well to confirm that such star shapes are present in all of them or whether they are changing from sub to sub?

That way we can exclude issues with tracking. I suspect that your tracking is probably fine since shape of stars don't really relate to RA/DEC.

[Phillips6549]

Andrew, Vlaiv

Thanks for the suggestions.  I have checked all of the subs I took and every one has the same star shape and size, regardless of exposure time (10s to 30s),only the brightness varies.  When I was checking focus visually, the airy disk looked reasonable but not perfect.  If we get another clear night any time soon I'll take an afocal image of the inside and outside focus rings.

I attach a full frame for reference.  It looks to me that the effect is less at the top.

Thanks

Mark

[Skipper Billy]

If you can attach a single unprocessed sub I will gladly run it through CCDI for you which will analyze and report back any tilt or curvature etc.

[vlaiv]

20 minutes ago, Phillips6549 said:

It looks to me that the effect is less at the top.

That would mean that tilt is involved. Indeed effect looks like it's getting worse as we move from top to bottom of the image. Whenever you have such linear change of shape - it usually means tilt. It could be focuser tilt or secondary misalignment.

[Phillips6549]

Billy

Thank you, unprocessed sub is attached.  

 

Vlaiv

I'll have to  check the alignment of the focusser and collimation.

IMG_0231.CR2

[Phillips6549]

I checked the focusser and drawtube.  The drawtube definitely flexes in its mount so that is the first issue to address, either by reassembly or perhaps by careful placement of the camera next time to minimise droop.  Something to look at while the cloud lingers.

 

Any other suggestions gratefully received.

Mark

[Davey-T]

How far out is the focuser when focused ? I have found with relatively " cheap " focusers it's better to keep them as far in as possible and extend them to the camera with solid screw fittings.

Dave

[happy-kat]

Te barlow doesn't that add opportunity for aberrations too.

[vlaiv]

1 minute ago, happy-kat said:

Te barlow doesn't that add opportunity for aberrations too.

I totally missed the barlow, and yes - it must be responsible at least a bit for funny star shapes. Tilt and collimation alone should not produce above star shapes, but barlow with tilt might do funky stuff to PSF.

[Phillips6549]

I presently can't achieve focus at infinity without the Barlow but I agree its likely to be responsible for some of the aberrations.  I may order one of these to see if the additional 9mm back focus is sufficient to reach infinity. 

https://www.firstlightoptics.com/adapters/baader-ultra-short-canon-eos-t-ring.html

 

I disassembled the focuser.  No wonder it moves,  the entire mount is plastic surrounding a pretty flimsy metal draw tube.   There are 3 plastic guides set at 120degree intervals around the mount,  one of which is lined with PTFE tape.  I have added PTFE tape to the other 2 guides which has reduced the flexing  of the tube within the mount quite a bit. 

I knew this was a budget scope but I can now see where the money has been saved - everywhere! 

Also,  as Vlaiv suggested,  the secondary appeared to be misaligned with the draw tube,  I've tweaked this back into line. 

All I need now is a clear night to check the effect of these changes (and to start saving for a new, decent, OTA.)

[Phillips6549]

At last a clear night!  

The ultra-short T-Ring didn't provide enough travel to achieve focus at infinity, but FLO were great in taking it back; thanks guys.

I finally managed to get out again tonight after fettling the scope and the difference is marked - a long way from perfect (or even good) but certainly much better.  I think there is still room for collimation improvement and focus accuracy, the stars are still not round and are too large but at least they're recognisable as stars now.

All further suggestions gratefully received.

Mark

 

[Padraic M]

Hi Mark, that's not a bad result for your setup! I've got similar - Celestron AM130EQ with an old Canon 400D. I've upgraded the flimsy mount that came with the scope to a HEQ5 which makes a huge improvement, but.....

Bottom line is that the SLR is heavy, and won't achieve focus without the Barlow, which makes it stand out long from the OTA. Like a big lever. It's very prone to flex, vibration and wind movement. Focusing is also a pain; at least you've got live view so you have some chance of seeing what you're looking at. 

I haven't come across any option to replace the focuser, or reduce the back focus requirement to reduce the torque of the SLR. I've bought a ZWO CMOS camera with is much better, but with a much smaller sensor, the field of view is significantly reduced. I've also taken to using the reflector as a guidescope,  with the DSLR piggybacking on top with a telephoto Canon lens; or with the ZWO attached to a Canon lens which looks promising.

Padraic.

[happy-kat]

 

That's much better.

If you are into some fettelnjg why not use longer collimation bolts to move the mirror higher, there are posts floating around.

[Phillips6549]

15 hours ago, Padraic M said:

Bottom line is that the SLR is heavy, and won't achieve focus without the Barlow, which makes it stand out long from the OTA. Like a big lever. It's very prone to flex, vibration and wind movement. Focusing is also a pain; at least you've got live view so you have some chance of seeing what you're looking at. 

I haven't come across any option to replace the focuser, or reduce the back focus requirement to reduce the torque of the SLR. I've bought a ZWO CMOS camera with is much better, but with a much smaller sensor, the field of view is significantly reduced. I've also taken to using the reflector as a guidescope,  with the DSLR piggybacking on top with a telephoto Canon lens; or with the ZWO attached to a Canon lens which looks promising.

Padraic.

Padraic

Indeed!  Focus is a challenge even with livewiew, not helped by the fact that I still have collimation issues.  

I have been considering just using a telephoto lens on the camera but, stupidly, hadn't considered piggybacking the camera on the scope - I was looking to mount the camera directly on the mount.  I may give that a try next time the clouds part.

 

15 hours ago, happy-kat said:

If you are into some fettelnjg why not use longer collimation bolts to move the mirror higher, there are posts floating around.

Happy-kat

That's a thought.  I'll look for the relevant threads and see if its feasible with my scope.

 

Thanks both for the suggestions.

Mark

[happy-kat]

Piggy backing the camera plus lens onto the mount and still keeping the telescope on is adding a wind sail and weight may as well go for the benefit of taking the telescope off.

Edited February 7, 2020 by happy-kat

[happy-kat]

If you have an android device then DSLR controller is great for helping with focus as get a bigger screen to see the live view image and control focus depending on lens seeing as you have a Canon, need an OTG cable.

[Phillips6549]

14 minutes ago, happy-kat said:

If you have an android device then DSLR controller is great for helping with focus as get a bigger screen to see the live view image and control focus depending on lens seeing as you have a Canon, need an OTG cable.

This forum is great.  Such an obvious way of making things easier but I would never of thought of it without prompting.  Thanks Happy-kat.