Distorted stars in Meade sn10 image

[meade expert]

I need some advice from experienced people with Meade SN10 scopes.
Tried out new scope sn10 last night but got unusual star images
(i checked collimation at start if imaging and stars ok.
I used Baader MPCC with modded 350D.
I have attached an image..any ideas whats causing these strange star images?

[Moon]

I see star elongation at about 11 O'clock and 7 O'clock.

Did you get the same problem with short exposures or just long exposures?

[multiweb]

I don't think it has anything to do with collimation. The field is uniform. Your guiding and/or stacking might be off a bit. As a result the overall picture is also slightly blurry. How as the seeing when you took that pic?

[meade expert]

I am autoguiding with 80ST and PHD , imaging through SN10 (1016mmFL) using baader MPCC and 350D.
Here is another shot of thors helmet magnified so you can see the distortion.

[multiweb]

Looks a lot out of focus. You said you did a start test and collimation was ok so it could be a camera tilt may be?

[meade expert]

May have found the issue see link below

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/6/Number/3100454/Main/3098338

I did recenter the corrector plate a few days ago because it was loose and tightened the screws somewhat !

Let me know what you think !

[multiweb]

:eyepop: Got it now. Ok un-tighten it straight away. You might be pinching the glass. You're lucky the weather is hot and it's summer. If you did that in winter you could have split the glass from cold back to ambient temp indoors. The corrector only needs to be held in place. Nearly loose. If the glass move use cork or paper shims, not metal (alu can or others). And most importantly do not apply any pressure on it when fitting it back in the cell.

[astroboy]

A problem I would have at times when I had the scope is tube currents distorting stars , switching on the fan fixed that , did you have the fan on ?
Also collimation is critical so use a good laser you can trust and an auto collimator.

[meade expert]

No I have to make up the lead for the fan !
I have now lossened the retaining ring bolts so that they are just starting to press on ring , well see tonight?

[multiweb]

That should do it :)

[meade expert]

Yes , the stars are now fairly round after rough collimation and loosening the retaining ring.
Took 1.5s of data at F4 of IC434 and here is the result.
Need better collimation , anyone know of a good auto collimator.:D

[astroboy]

The Cats Eye gear is the best I've seen works great on any scope no matter how big or fast even the 24" F3.66 is no problem.

Zane

[meade expert]

I encountered a strange issue again with triangular stars in my image after having collimated (using 4mm nagler on star).

In and out of focus the stars look round and concentric but approaching focus they go triangular??

I am at a loss what might be causing this?

I made sure the corrector plate retaining ring was loose , the corrected plate was shimmed with some thin cardstock where the cork pads are to stop the corrector sliding (missing paper compression gasket?).

I did find by accident last night that when I moved my dew heater from just behind the cell that holds the corrector plate (around white tube) to around the corrector plate cell itself the stars started to become rounder rather than triangles.?

Any thoughts welcome

[tlgerdes]

OK, I had something similar to this, astigmatism. Your focal plane of the camera may not be completely square. Adjust focus either side and see if your star image elongates in directions 90degrees to each other.

[meade expert]

Yes it does flip 90 deg!!:(

[tlgerdes]

It is not :( it should be :)

"If" it is astigmatism, then it just means that the camera is not sitting square to the focal plane. This shouldn't be too hard to identify and fix.

In my case I found the M42 T-Ring insert wasnt square, unscrewed it and tightened it all up and things were 100% better.