SCT 9.25 Collimation lines through Airy discs

[YorkieGraham]

I was wonder is someone with the same set-up as me can tell me if this effect is normal for a Celestron 9.25 SCT. I have not had the telescope long so I am still getting used to setting up. I check the collimation before starting each session and have noticed that 2 dark lines are apparent when I have centred the star within its Airy disc. They are like the hands of a clock and quite well defined. Any thought as to what is causing this. I was wondering if the Bob's Knobs are too tight and are causing distortion of the secondary but I cannot find any examples on the internet of this effect to back this idea up. I can't find any reference anywhere to this. Any ideas of what is causing this and how to cure it would be very much appreciated.

[happy-kat]

I think this is your diagonal causing this.

Do you get it if you just insert an eyepiece?

[Steve Ward]

Sounds to me like a heat plume rising from the internal baffle tube , I get this on the Mak180Pro until things have reached equilibrium inside the tube.

An image down the page here of similar effect ... http://www.sergepetiot.com/?p=360

And a discussion here ... http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/615474/Main/612225

[NGC 1502]

Whatever is causing the issue, it won't be the 'Bob's knobs' as they are isolated from the secondary mirror itself.

You could try rotating the eyepiece in the focuser, see if the dark lines you mention turn as well, if so, problem with that EP.

But from what you are saying, the most likely cause is mentioned in post # 3, wait an hour and see if full cooldown cures it.  You can still use the scope while waiting, especially for low power faint fuzzies, where critical definition is least needed, but planetary or double star views are compromised before the scope is fully cooled.

Setting the scope up as soon as you can before observing will help, with these light evenings most of us are waiting around for it to get dark.

If you can store the scope in a cool room or building, that helps too, less temperature difference from outdoors.

Hope you sort it, Ed.

[YorkieGraham]

Many thanks guys for your time. You have reassured me that it is a cooling down matter and not a problem with the optics. I am just getting used to the scope as I had an archromat before getting this, so I didn't have any of this to cntend with. I think I am not leaving the scope long enough before checking the collimation, although I would say that I had left it more than an hour last night as I set up while it was still light. Having said that the temperature did drop considerably last night so I guess the scope was always trying to catch up. I am convinced from what has been said that it is a heat plume causing the effect, not the eyepiece. What I observed was a close match on the first link article photo at the end. Many thanks for the helpful advice.

[Nakedgun]

Also, perform collimation without a diagonal, this will eliminate another source of possible misalignment.

[Peter Drew]

True, but if, as most do, you use a diagonal, if this alters the collimation you need to have an adjustable diagonal or collimate with the diagonal in place.