Tilted corrector vs faulty corrector

[codemonkey]

I have astigmatism + tilt which is manifesting in elongated stars from the top left of the image, to the bottom right.

If I rotate my coma corrector and camera (and the filter wheel, OAG, spacers etc in between them) as one monolithic unit the aberrations stay fixed relative to the sensor. If I then rotate the camera independently of the corrector, the aberrations move relative to the sensor.

I understand that a coma corrector that is tilted in the focuser can/will cause astigmatism in images and it doesn't take much to do so. How can I distinguish between a faulty corrector and a tilted corrector?

Since this is astigmatism and it causes elongation either side of focus, I'm currently mitigating the issue by correcting the tilt between the corrector and the camera. If I remove that tilt adjustment and instead try to tilt the corrector in the focuser's drawtube at the same spot, it makes no difference to the star shape, it just gets more defocused. This has me thinking it's the corrector itself rather than how it's registered in the focuser, but before I go and spend $900 on a new corrector, I'd like to see if anyone has ideas on how I can easily confirm it's the corrector itself.

[rbronca]

Can you test without the corrector? The coma and star sizes should be consistent in each corner if the sensor tilt is good. That would quickly rule in or out the corrector.



ASTAP (free) has an Image(CCD) inspector option that will allow you to work with actual numbers whilst your isolating the issue.



Since there are so many different pieces in the chain can you run just the camera and nothing else and start from there and add pieces and see where things go wrong? You may need a good spacer collection.



Is everything screw threaded?



Are you certain the spacing before and after the corrector is correct?


Collimation?



Sanity check time:

Primary mirror cell gap to the bottom of the tube look even?

Secondary mirror gap look even all around? (Use a small mirror to see)

Does the secondary look square to the focuser?



Any slop in the focuser?
Does the elongation change depending on telescope orientation?



Is the spider tight?


You didn't mention which corrector your using?



Does it have any elements that aren't centered, damaged?

The glass etc. should appear to be even all the way around.


I hope it isn't an Orion UK one...

Paul Haese is the local expert on testing wonky Orion coma correctors :lol:


Hope this helps you to isolate your culprit.

[codemonkey]

Thanks for the detailed advice, appreciate it :-)

I think I can safely rule out anything in the scope by virtue of the fact that the issue rotates with the corrector+imaging train. If it was collimation, or one of the mirrors or anything like that, the aberrations would not have rotated with the corrector+imaging train and thus the position of the bad stars would have changed relative to the camera and they did not.

I'm convinced that this is either a tilted corrector or a faulty one, it's just a matter of which and I don't see a way I can prove either one with a great deal of confidence. I think I'm just going to buy a new Paracorr Type 2.

As for what type of corrector, I purchased it secondhand but it's an oooooold Paracorr with no tunable top... no Tele Vue markings or manual or anything with it so really it could be anything, just relying on the honesty of the seller there.

[gregbradley]

It seems you have clearly established its to do with the corrector.

But it may be that the threads on the corrector are not orthogonal.
Stephen Buda has done work for others fixing out of square threads.

It may be better to open up a communication with him and see if he will remachine the threads on your corrector and fix them up.

Whose to say if you get a new corrector you simply get the same problem?

Greg.

[multiweb]

I feel for you Lee. This tweaking can be a real pita. Don't have anything else to comment on tilt that you haven't already covered but just more thing to consider. The orientation of your primary. Try to rotate it 90 degrees in its cell then check the field again until you zero in the sweet spot.

[codemonkey]

Cheers Greg, good enough that someone agrees with my line of thinking.

I thought about contacting him to see if he can fix it, but ultimately that's guaranteed money and maybe it can be fixed.

I decided to just buy a new Paracorr Type 2 which I've just ordered.



Thanks Marc... yeah sometimes I wonder if this hobby is the reason I'm going bald more than the genetic factors I first assumed ;-)

[gregbradley]

[QUOTE=codemonkey;1490156]Cheers Greg, good enough that someone agrees with my line of thinking.

I thought about contacting him to see if he can fix it, but ultimately that's guaranteed money and maybe it can be fixed.

I decided to just buy a new Paracorr Type 2 which I've just ordered.

Yes I can see your logic there. Its often better to start with something fresh.

Greg.