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Dew on primary, advice requested

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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?





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  • 2 weeks later...

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.



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  • 3 weeks later...

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.



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  • 1 year later...

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