Elliptical focus disc in SCT

[Bugdozer]

I am not sure if this has always been there in my telescope, or whether it's new, but I think it hasn't always been there and it doesn't quite make sense to me. 

In my Celestron Nexstar 5SE, when I am inside of focus, the "doughnut disc" appears slightly elliptical. It's only very slight, but it's definitely there. When I adjust to be outside of focus, it also appears elliptical by the same amount - but with the long axis of the ellipse at 90 degrees to how it was when inside of focus. 

Give how SCTs work, the whole optical system should have radial symmetry, surely? And before anyone immediately accuses collimation, this doesn't look like a collimation effect - the black centre of the doughnut is exactly in the middle, symmetrical in every direction. Because it's smaller, it's hard to tell if the black centre is itself slightly squished, but I think it is. When I am as close to perfect focus at high magnification on a bright star, the diffraction rings aren't offset from the Airy disc as would be seen with a collimation problem. However, the diffraction rings seem stronger in a sort of cross pattern, like you would get from spider vanes but more subtle, rather than being an even circle around the disc. 

Can anyone explain this? 

[Peter Drew]

It could be worth loosening the 6 screws that hold the corrector retaining ring and then retightening them with just light finger pressure.  The mild astigmatism that you describe has often been reported due to the over tightening of these screws.    🙂

[davidc135]

Edit. Another part of the optical train to check is the diagonal if Peter's suggestion doesn't help. Try the scope without.

David

Edited January 8 by davidc135

[Ricochet]

It could also be a transient astigmatism present only while the primary or corrector is cooling, which is more prominent now the temperature has dropped than it was over the summer months. 

[Bugdozer]

All good suggestions, I will see if I can track down any of these as the culprit. Interesting that I have only begun to notice it as it's got colder, but also since my diagonal fell off the scope into a plant pot. 

[Rusted]

8 hours ago, Bugdozer said:

All good suggestions, I will see if I can track down any of these as the culprit. Interesting that I have only begun to notice it as it's got colder, but also since my diagonal fell off the scope into a plant pot. 

Were you hoping your 1.25" would grow into a 2"?

[Bugdozer]

1 hour ago, Rusted said:

Were you hoping your 1.25" would grow into a 2"?

Ha ha, I wish! No, it just suddenly slid out while I was in the middle of changing eyepieces, probably because of slight contraction in the cold - fortunately I had just taken out the eyepiece and was holding it. The diagonal bounced off a tripod leg and the plant pot saved it from hitting concrete. Unfortunately I did have to clean out some mud from inside it, and the chance of it being knocked askew from either the impact or my cleaning is certainly a possibility. I'm going to test the diagonal first because it's the easiest thing to check, if no joy with that then I will try the retaining screws as Peter suggested. If that doesn't work then I will wait until we get warmer weather later in the year and see if it goes away naturally - it's not so bad that it's ruining my views. If none of these things work, then I guess I will have to get it looked at by a professional. The strange thing, if it's due to tight screws or cold weather, is that I had never been aware of it before. Still, I suppose things can shift a bit over time. 

[Bugdozer]

Gold star for Ricochet! I believe this is a thermal effect. I got the scope all set up and ready to go indoors and then took it out and pointed it at Rigel as quickly as I could, before it had any opportunity to cool down significantly. The image both sides of focus was a perfect circle. I tried with and without the diagonal, and it made no difference, so fortunately its death plunge into the pot did no damage. 

I might still try relieving some of the screw pressure to see if that minimises the contraction pressure. Still, good to know it isn't something that's gone permanently wonky.