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Will a dew shield increase tube currents

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#1 Nippon

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Posted 22 August 2014 - 04:47 PM

With classic Cassegrains, Tak Mewlons, Vixen VMCs and similar. Will a dew shield increase or decrease tube currents?



#2 TxStars

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Posted 22 August 2014 - 09:22 PM

If the mirrors have not cooled off then adding a dew shield could increase the time which warm air is in the tube.

Usually the scopes mentioned are not used with a dew shield unless under excessively humid conditions.



#3 Patrick

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Posted 22 August 2014 - 11:15 PM

With classic Cassegrains, Tak Mewlons, Vixen VMCs and similar. Will a dew shield increase or decrease tube currents?

 

Just an opinion here...

 

All three of your scopes have open tubes essentially like a Newtonian...especially with a dew shield.  The only difference would be the placement of the secondary in the OTA. 

 

So, has your dew shield been sitting in the sun and is it warm?  Or have you been letting it cool down along with your scope?  Do you have fans behind your mirror blowing air up the tube?  In my tubed Newtonians, I liked to direct air up the tube to disrupt the boundary layer and any tube currents.  Anyway, barring that, and if you have tube currents present with no active cooling, then I would say a dew shield would tend to work against you regarding tube currents.  It would allow the tube current to stay in the light path for a longer period of time than without it.

 

Just my 2 cents. :)

 

Patrick



#4 Nippon

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Posted 23 August 2014 - 02:40 PM

Thanks Patrick, I wish I had all three of the scopes I mentioned, especialy the Mewlon. Letting the shield equalize with the scope, yes, fans, no. Mine is a VMC 200. I have been thinking as you said the dew shield could be contributing to tube currents.  It has a very good star test and I have had a couple of nights looking at planets when the image was very sharp with lots of detail but more often then not planets are soft. I finally concluded that the scope may be very suseptible to tube currents and perhaps to bad seeing as well due to the thick spider vanes and rather large CO (40%). But I have seen it give extremely good performance when it is in the mood. I've used a dew shield on it since I bought it a couple of years ago since dew here in Central Florida is severe. I'll start using it without the shield and see. I've never actually tried it shieldless perhaps it really doesn't  need it.








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