6 replies to this topic

[NickWDavis]

My 6” Meade SCT has problems in the cold. I don’t know if it’s astigmatism or on-axis coma but stars appear shuttlecock-shaped at powers greater than 100x.

Intra and extra focal images appear to be collimated correctly.

Again, this is only apparent in cold weather below 32 Fahrenheit.

Does anyone know what might cause this?

Edited by NickWDavis, 06 January 2019 - 12:03 AM.

[Knasal]

Is the scope/ mirror properly cooled to ambient temperature before you start to use it?

 

Kevin

[cookjaiii]

I agree with Kevin.  A warm scope moved outside into the cold will have tube currents that can severely distort your image.  Maks and SCTs are particularly prone to this problem, by virtue of their designs, and can require a looong time to reach thermal equilibrium.

http://aberrator.ast...be_current.html

 

There are threads here on CN that discuss remedies such as pre-cooling vs. thermal insulation.  I used to keep my SCT in a plastic box on my back porch so it would always be close to outside ambient temperature.  

 

Jim

[NickWDavis]

Is the scope/ mirror properly cooled to ambient temperature before you start to use it?

 

Kevin

 

I assume that it was cooled. It was outside for two hours before I started using it.

[Auburn80]

I assume that it was cooled. It was outside for two hours before I started using it.

Even that might not be enough. For instance, taking it from 70 inside to 40 outside takes quite a while. In the mean time, the outside temp is dropping fairly quickly so the scope may never catch up. I see that problem here in the South in the winter. A beautiful clear day may be 45 at sunset and get to 35 in just a couple hours or so. As a result, I keep my SCTs in my garage where it stays around 50-55 in the winter. That way, I have a bit of a head start.

Edited by Auburn80, 06 January 2019 - 11:10 AM.

[jgraham]

Yep, this may be a thermal plume rising from the baffle. A homemade cat-cooler would make a useful accessory.

[NickWDavis]

I’ll try letting it cool longer next time and report back. 

Thanks everyone.