Star shape changes with focus - collimation error or something else?

[MikeODay]

The shape of stars changes as I change focus on my Newtonian scope:

....

in-focus:  round stars

Clockwise turn ( FeatherTouch focuser ) : stars become elongated in N-S direction

Anti-clockwise turn: stars become elongated in E-W direction

....

It may be just a curiosity ( as I am not aware of it as being an obvious problem - in fact it helps me notice when focus is changing  ) however I suppose it may also be a symptom of a problem that is causing sub-optimal images in some respect.

Does anyone know what might be causing this? 

Might it be collimation error or maybe sensor tilt?

[Stu]

Sounds like astigmatism to me.

http://www.telescope-optics.net/astigmatism1.htm

[drjolo]

If all stars in the field of view behave the same way then it will be astigmatism. You can check if it is primary or secondary mirror - you need to rotate primary mirror in the cell for about 45 degree and check if elongation orientation changes. However it is more probable to be secondary mirror astigmatism - at least I encountered it there more often. 

It that would be collimation error or tilt, then stars shape across field of view would change.

[MikeODay]

Thanks guys.  Just to be clear, when the scope is focused the stars throughout the field are perfect circles.  It is only when the scope is unfocused that the stars change shape and, when they do, it is the same distortion everywhere.  Is this what you would expect with astigmatism?

Thanks

Mike

[Astrobits]

Yup. Classical astigmatism.

Make a pinhole light source and test the primary mirror. Leave it in the telescope, just view down the tube from twice the focal length. examine the image of the pinhole both sides of focus with a 25mm eyepiece. Keep the eyepiece and pinhole as close together as possible. If it's the primary that is astigmatic you will see the same effect as you see on the stars. Check that the mirror cell retaining clips and centring components are not tight as these can distort the mirror.

Nigel

[MikeODay]

Thanks Nigel.  Could it be caused by over tightening the set of collimation bolts or maybe by tilt of the coma corrector / focuser?

[drjolo]

If you have piched optics in newtonian then in defocused image you would have triangles instead of circles. Something like this:

I also think it is astigmatism. 

[MikeODay]

1 minute ago, drjolo said:

If you have piched optics in newtonian then in defocused image you would have triangles instead of circles. Something like this:

I also think it is astigmatism. 

Mmm, not pinched optics then.  What about tilt - can it cause astigmatism?

Thanks

Mike

[drjolo]

I don't think so. In my both imaging newtonians I used to have tilt was visible as elongated stars in one part of image and pinpoint in another part. Both tilt and miscollimation effects in different star shapes across image - when stars are focused. 

Astigmatism in newtonians is not so uncommon. My friend has Orion CT8 also with slight astigmatism visible in defocused images. But he lives with it - just watches images to be focused well. 

[MikeODay]

I don't know if it will help but here are crops from the extreme corners and centre of my latest image.

                                       

                                     

                                      

 

So, when focused I think the stars are not too bad - reasonably round.  What do you think?

I don't want to try to fix what is not too broken because when I do that I invariably make it worse.  I would love to hear your thoughts on how I should I go about deciding if the astigmatism is bad enough to to take the risk of messing with the scope.

 

[Stu]

Mike, maybe also post some defocussed star images so people can judge that too?

[drjolo]

In my opinion they are quite good. Is it from CT12 newtonian? 

One thing you may try to check is which mirror causes astigmatism - like Nigel suggested. However if this elongation is oriented along one of axes of secondary mirror, then it is quite probable it is secondary's fault.

[MikeODay]

2 minutes ago, drjolo said:

I don't think so. In my both imaging newtonians I used to have tilt was visible as elongated stars in one part of image and pinpoint in another part. Both tilt and miscollimation effects in different star shapes across image - when stars are focused. 

Astigmatism in newtonians is not so uncommon. My friend has Orion CT8 also with slight astigmatism visible in defocused images. But he lives with it - just watches images to be focused well. 

Yes I was thinking that as I posted the images above - if I had significant tilt then I suppose I might expect to see differences when comparing the extreme parts of the image.  If there are differences they are too small for my poor eyes.

Living with it is definitely an option I am ok with as I am reasonably happy with the results I am getting and, with my poor skills and understanding, I absolutely have what it takes to make things worse

[MikeODay]

4 minutes ago, Stu said:

Mike, maybe also post some defocussed star images so people can judge that too?

That is a good thought, thanks.  I'll will grab some out of focus images when the sky clears.

[MikeODay]

3 minutes ago, drjolo said:

In my opinion they are quite good. Is it from CT12 newtonian? 

One thing you may try to check is which mirror causes astigmatism - like Nigel suggested. However if this elongation is oriented along one of axes of secondary mirror, then it is quite probable it is secondary's fault.

Yes, CT12 with an ASA coma corrector.

I am trying to remember now but I think the elongation is nearly, but not quite, aligned with the cardinal directions; so not aligned with the spider vanes which are rotated 45 deg wrt to the scope's axes.

[Stu]

3 hours ago, MikeODay said:

Yes, CT12 with an ASA coma corrector.

I am trying to remember now but I think the elongation is nearly, but not quite, aligned with the cardinal directions; so not aligned with the spider vanes which are rotated 45 deg wrt to the scope's axes.

Nigel's method eliminates the secondary so you can see if it is the primary at fault. I guess then it's a question of whether the astigmatism is inherent in the mirror figure, or due to it being supported incorrectly. Rotating the mirror should tell you this.

[MikeODay]

Thanks guys for all your replies, they have been very helpful in understanding what is going on; my scope is suffering from astigmatism

.......

I don't think there is a problem with the manufacturing of the main or secondary mirrors.  The scope was supplied with a test report for the main mirror and I only noticed the behaviour about nine months after I set up the new scope.

It occurred to me that perhaps the problem is being caused by the way I store the scope ...

I designed my pillar and roll-off shed based on the intention of storing my old scope with it parked in the zero position.  This worked out fine and allowed a clearance of about 6" between the top of the scope and the top of the door frame of the shed.  

However, when I purchased my new scope I found that its point of balance was much closer to the mirror than my old scope and this meant that by sliding it forward to achieve balance I effectively increased the total height of the setup when in the zero position.  This, plus the increased length of the new scope, meant that it no longer would fit in the shed when parked in the zero position.  

The solution I have been using is to park the scope with the declination axis rotated 180 degress to point the scope down.  It fits very well in the shed in this orientation.  

Could I be causing stress on the mirror by storing it with the scope pointing down at 33 degrees below level?  

Or maybe for some reason the mirror is 'sticking' to one or more of the clips when it is pointing down and does not rest properly on the supports when the scope is in operation.