Optical artifact in my C925

[Greg H 3]

Hello all, my first post here.
I have a Celestron CPC 925, which I bought new in 2008. I used it mainly for visual and some casual astrophotography. In about 2012 the arival of my daughter and busy career took me away from astronomy and the scope was left to gather dust. My interest in astronomy was revitalised by Comet Neowise in 2020 and I've started to get into more serious astrophotography.

But, the scope has an optical artifact/defect in the out-of-focus disk of stars that looks like a crack.
I can see no obvious physical damage to the corrector plate or primary mirror. I've not yet worked up the corrage to take off the corrector plate to inspect the secondary.

I've attached some images, using Procyon, to illustrate the issue. They run from way out of focus to in focus.

I wonder if anyone has any suggestions as to what could be causing this and whether it is a significant problem.

Many thanks.

[Mr Spock 7,698]

It's a heat plume. You need to let the scope cool down more. 

Where is it being stored when not in use? If it's not in a cool place it may never cool down enough. Mine was kept in a cold porch and that would take an hour to cool down.

[Greg H 3]

Michael, Thanks.

Temperature is not something I considered. 
The scope is kept indoors. In this case it had been outdoors for an hour, with an outdoor temperature of about -1 C. I guess that may not be enough.
I'll try out your suggestion on the next clear night, probably not until at least Saturday.
Thanks again.

[dweller25 2,569]

Hello @Greg H

Welcome to SGL

A C9.25 takes a long time to fully cool down - the best way is to remove the front and rear dust covers then point the scope down to allow the warm air to escape - this could take at least 1 to 2 hours.

Or you could wrap the scope with 2 layers of Reflectix to give much better thermal equilibrium inside the OTA -  no need to cool the scope down, it can be used straight away and dew holds off longer.

 Hope that helps and clear skies.

Edited January 4 by dweller25

[Nakedgun 922]

I've seen heat plumes in my C-Nine-Two-Five, but nothing quite like yours. Curious.

[Greg H 3]

Thanks all.
I'll put the telescope outside several hours before turning in to the stars.
@dweller25 I like the idea of insulating the OTA, I might give that a go too.

[Greg H 3]

I put my C925 outside 4 hours before dark. The artifact did not appear, so it must have been a thermal effect.
Thanks all, this had been bugging me for a while. I'm embarrassed at the newbie error, but delighted to be able to identify the cause.

[dweller25 2,569]

Great result

Edited January 12 by dweller25

[Alkaid 415]

Phew!  Great news.