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CPC 1100, Primary mirror misting over


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Hi folks, 
This time of year again where cold temps and dew become a problem for our gear but this year, twice now, I've had my primary mirror on my C11 dew over and I do not recall that happening on my last year but I seld took it out in colder weather. 

As you can see in the photo it is there and when I am imaging I can tell it has happened as I would see the histogram drop and would have to increase my gain by 10 to compinsate. No doubt I am loose valuable resolution and clarity. Of course when this happens my corrector plate dews over but I resolve this with a quick blast from a hair dryer and then letting the glass cool for 5 minutes before my next capture.

To get to the point, can anyone suggest anything that might help? 

 

image.thumb.jpeg.807cfa19636f931ef3552109713bf736.jpeg

Edited by Simon128D
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I’m guessing there is moist air inside the OTA, I wonder if one of those desiccant caps might work - it goes in the visual back and keeps the air dry inside the OTA when not being used. I’ve also found my corrector plate dews up a lot less quickly since I gave it a gentle clean to remove the thin film of gunk. I would imagine a mirror prone to dewing would also benefit from a clean, although I don’t think it’s usual to clean an SCT mirror (I’ve never done it). 

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11 minutes ago, Merlin said:

Some SCTs have an optical window at the bottom of the ‘scope to prevent internal fogging.

I have a focal reducer fixed permanently on my C11, which blocks ready access to air. But I’m not sure the rest of the OTA is completely airtight. It’s been in my observatory for over 10 years, with dew heater permanently on, and I’ve never had internal dew problems 

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