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mvandervorst

Dew on primary, advice requested

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Hi,

 

I have the 12-inch Newtonian located in a remote observatory in San Diego.  This year has been unusually humid.  I currently do not try to image when the humidity goes above 80%.  I'm thinking (actually hoping) that may be a little conservative.  I assume the rear fan will help a little.  

 

I'm curious about others experiences about at what point dew forms?

 

Thanks,

 

Michael

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Hi Michael,

 

Predicting dew formation is a complex subject, but here is some relevant experience:

 

I use a hood on my ASA10, more for stray light exclusion than dew protection, but it does help a little.

 

The fan is a help, and I control the speed, depending on the conditions. It is best not to use it if possible.

 

I regularly encounter very high humidity - often 80% and above. I have successfully imaged when humidity is >90%. My record, at the end of a run I really wanted to complete, was 99%. The alarm on the humidity sensor was flashing and all the outer surfaces of the gear were very wet, but not the mirrors!

 

I don't make a habit of this, but I would expect that you'd be OK up to 90% RH. Just keep everything cold and run the fan for some time after you finish.

 

Mark

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Thank you Mark. 

 

I've been imaging to about 85 to 90% humidity recently without a dew shield.  The results look OK.

 

In your experience, why should the fan not be used all of the time?

 

Thanks,

 

Michael

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Theoretically, you don't want any air turbulence in the OTA, so there's a trade-off. I don't really know where the trade-off is on any particular night, so I just try to keep the fan speed down.

 

Mark

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