Jump to content


Optical artifact in my C925

Recommended Posts

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.






Link to comment
Share on other sites

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.

  • Thanks 1
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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 by dweller25
  • Thanks 1
Link to comment
Share on other sites

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.

  • Like 3
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.