Weird star shape. Help requested pls

[PadrePeace]

Equipment: APM 107/700 triplet with a 3” Feathertouch focuser, Atik 460EX mono, ZWO EFW, and for these images an Astrodon Lum 1.25” filter was used. Both HEQ5Pro and AZEQ6 GT mounts with steady guiding have both been tried under the scope with no change to the star shape. It occurs whether or not I’m guiding, and also with flattener/reducers in or out of the optical path. 

What’s telling is that the shape appears well formed on 10sec to 600secexposures so I have discarded trailing. 

I’ve uploaded two star test shots and some raw Lum images taken at both low and high elevations to discount droop. 

Can anyone shed light on what will be causing this star shape across my images and how I might fix it? 

I've added jpegs if these are more suitable than the original .fit files.

The first image is a Cheshire collimator shot of the coherence of the lenses.

Second image is a slightly defocused image of an artificial star test I did over the weekend. 
Added the other extra x 2 and intra x2 focus images from a clear sky session the following night. 

You clearly have to zoom into the other two image to see the pear shape stars I’m concerned with. 

Many thanks for your experienced comment in advance. 

 

 

 

Edited March 15, 2020 by PadrePeace
More images added

[SpookyKatt]

Hi PadrePeace,

There's alot more well informed people than me on the forum but can I just ask re the last 4 images was there any

difference in the equipment being used between shooting the star and shooting the artifical star ?

Kathleen

 

[andrew s]

I would use ccd Inspector from ccdware to look at your images. I think it has a free trial period. This could help you pin down the issue.

Regards Andrew 

[Skipper Billy]

If you can post a single unstretched straight from the camera FITS file - I will run it through CCDI for you and post the result - looks like tilt in the imaging train to me.

[PadrePeace]

Kathleen 

no changes to the imaging rig at all. The Artificial star was resting on top of a SW tripod hence the other random stuff in the shot. 

Edited March 16, 2020 by PadrePeace

[PadrePeace]

Skipper Billy. Thanks for the support. I posted this thread here but got no response.  It has four raw fits files for you to choose from. 

If you want any others get back to me. 

Edited March 17, 2020 by PadrePeace

[PadrePeace]

All

My buddy AdanJ posted this elsewhere on this site which was his take on the issue:

Glad you posted this mate. Ill kick off with my interpretation.

1) I can see the star is pear shaped pointing downwards in the image.

2) You dont mention it but its very important to note that from memory rotating the focuser resulted in the distortion rotating relative to the camera and remaining fixed relative to the object being imaged. For me that rules out sensor tilt in the camera as a cause. I also know you have confirmed that the focuser is aligned using a laser colimator.

3) Its only visible when viewing the image at full size, so not the worst I have ever seen (some horrific star issues get posed on here) but still something is not right for a scope in that price range. I dont think that the flame is the best example of this as poor seeing has disguised the issue a little.  The better example is M109 where you can see something is wrong even with the smallest stars.

4) I dont see anything obvious in the star test, although it might be worth re-doing with the star less defocused and with shorter exposures. I have seensome people use a guide camera and then stack the results as you would a planetary image to resolve more detail in the rings. Seeing can also effect the result.

You are aiming for something more like this (my Esprit 100):

Using a shorter exposure and in better seeing shows many more rings in more detail making it easier to judge collimation, 10 rings are visible or more. Although this is still probably more de-focused than is ideal. Something like a 0.05s exposure from my ASI1600mm pro, not sure what the min exposure is for your ATIK460EX, hence maybe use a guide camera.

On the whole, from the shape I think it might be a case of pinched optics. But as you know I dont have sufficient experience of refactors  yet to be sure or to rule out something like coma or a colimation issue. So hopefully someone else will be able to comment.

Adam

 

Edited March 10 by Adam J

[SpookyKatt]

Ok  thanks for that, hmm well your artifical star images don't look right and I can see an issue in the images but I

am confused why the star images look fine if its always the same setup.  I must admit when I looked at it I wondered

it if was a tilt issue as well but I see Adam's comment implies the distorion remains fixed and doesn't rotate if the camera

is rotated, could there be anything loose or offset alignment wise with the focuser ?  

[Adam J]

1 hour ago, SpookyKatt said:

Ok  thanks for that, hmm well your artifical star images don't look right and I can see an issue in the images but I

am confused why the star images look fine if its always the same setup.  I must admit when I looked at it I wondered

it if was a tilt issue as well but I see Adam's comment implies the distorion remains fixed and doesn't rotate if the camera

is rotated, could there be anything loose or offset alignment wise with the focuser ?  

So you mean the in focus images of the stars look fine or the star tests of real stars look fine? 

There is a chance of additional glint from the artificial star. 

In both cases I would disagree, I see the stars as tear drop shaped or perhaps with a flat on the bottom right side. The star tests would seem to indicate a degree of astigmatism with a slight bulge to the top left of the ring. I would also say that the outer ring is not evenly illuminated. 

Having said all that though I have very little experience in this so hopefully someone with experience of refactor star tesing can comment. 

Adam

Edited March 16, 2020 by Adam J

[SpookyKatt]

Sorry Adam maybe I judged the pictures wrongly, for the last pictures attached to the original post I thought 1 & 3 looked like what I would expect 

but I'll bow to your knowledge.   Things certainly don't quite look correct.

[Adam J]

9 minutes ago, SpookyKatt said:

Sorry Adam maybe I judged the pictures wrongly, for the last pictures attached to the original post I thought 1 & 3 looked like what I would expect 

but I'll bow to your knowledge.   Things certainly don't quite look correct.

Problem is that star quality in a final image is subjective in terms of what is acceptable. No need to say sorry. 

[1parsec]

Some of the problems look like a lens when it hasn't been allowed to cool fully.
How long was it from when the scope was taken outside to been tested ?

[PadrePeace]

1parsec

The scope lives in the garage on the mount so is never more than a couple of degrees away from ambient temperature. It was out for 9 hrs until 4 in the morning with temps at 2 deg C with no change of star shape from the start of that session. Bottom line, the scope is always cooled. 

[PadrePeace]

On 16/03/2020 at 10:13, Skipper Billy said:

If you can post a single unstretched straight from the camera FITS file - I will run it through CCDI for you and post the result - looks like tilt in the imaging train to me.

Skipper Billy. Did you find the fits images I pointed out? They are in the imaging forum as I first thought that would be most appropriate. Needless to say I had no comments there so moved over to this forum. 
I’ve reposted the .fits here. If you need any more just ask. 
Thanks for the offer of support. 

Edited March 18, 2020 by PadrePeace
images added

[Skipper Billy]

4 minutes ago, PadrePeace said:

Skipper Billy. Did you find the fits images I pointed out? They are in the imaging forum as I first thought that would be most appropriate. Needless to say I had no comments there so moved over to this forum. 
I’ve reposted the .fits here. If you need any more just ask. 
Thanks for the offer of support. 

I didn't see you post sorry.

I will have a look at it when I get home tonight.

All the best.

David (aka Skipper Billy)

[PadrePeace]

Just had an interesting reply from a fellow in the US on Cloudynights. He said he had a similar problem with the same model scope and APM advised him to slacken or remove the 9x peripheral grub screws that were likely pinching the optics as they didn't need to be there to hold the cell lenses in alignment. Sounds strange but he said he did what APM suggested and the issue fixed itself. I guess his cell hasn't fallen apart yet or he'd have mentioned that you would expect.

What would hold the lens cell together if theses grub screws are removed?

Edited March 18, 2020 by PadrePeace

[Skipper Billy]

Here you go....

The results for M109 show a very small amount of curvature and tilt.

The results for the Flame show significantly more tilt and a little curvature.

Certainly not the worst I have seen!

[SpookyKatt]

Perhaps the retaining ring on the front of the cell would provide enough pressure to keep the

lens in place.  Personally as frustrating as it is I'm not sure its a good idea to remove the grub screws, I think

you should send an email to APM and ask their advice.   

[PadrePeace]

2 hours ago, Skipper Billy said:

Here you go....

The results for M109 show a very small amount of curvature and tilt.

The results for the Flame show significantly more tilt and a little curvature.

Certainly not the worst I have seen!

Thanks David, really appreciate you efforts.  I guess even a feather touch won’t guarantee a tilt free image although this could be the sensor.
That won’t be what's causing these odd star shaped across the field though would it?

The fact that the Flame is worse and being that it was at a low elevation suggests some droop perhaps. M109 by contrast was virtually at the zenith and chosen for that reason so I’ve learned something about the rig if not what I would like to fix. 
Many thanks again. 

[Skipper Billy]

A Feathertouch isn't going to perform well if its not fitted square 😉 

It could be the focuser that isn't square or it could be the sensor. 

CCDI tends to exaggerate tilt so in reality its a very small amount.

If you try to correct it then I am happy to run images through CCDI for you so you can see if its getting better or worse!

[ASTRO1966]

Looks like pinched optics .This image is from a 80mm refractor  I had in 2014  I could have fixed the problem by loosening retaining ring on objective lens but exchanged 

the scope instead .

[PadrePeace]

16 hours ago, ASTRO1966 said:

Looks like pinched optics .This image is from a 80mm refractor  I had in 2014  I could have fixed the problem by loosening retaining ring on objective lens but exchanged 

the scope instead .

Pity, as that had the makings of a nice image. Thanks for chiming in. 

Edited March 19, 2020 by PadrePeace

[jaspalchadha]

Worth a read http://www.loptics.com/articles/starshape/starshape.html

[PadrePeace]

20 hours ago, jaspalchadha said:

Worth a read http://www.loptics.com/articles/starshape/starshape.html

Thanks for this link. Though not for refractors it detailed the common optical issues which is always really helpful. 

[PadrePeace]

As an update for all I have sent the scope down to APM UK where Rupert is looking at it. I’ll report back on what he manages to find and importantly do about it.