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Effectiveness of secondary dew heater on open dob

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#1 tezster

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Posted 04 December 2012 - 08:03 AM

I was contemplating installing an Astro Systems Dew Guard heater for my secondary for occasions when dew can be an issue at the sites I observe at. Then I read this.

If your secondary is exposed and not inside a shroud the Dewguard will not be effective in controlling dew.



I technically have a shroud, but due to the nature of my scope's design, there are cutouts for the focuser (and other accessories). Will this somehow reduce, or even completely negate, the effectiveness of a dew heater?

#2 bherv

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Posted 04 December 2012 - 01:03 PM

If the shroud covers the area between the upper and lower tube assemblies you should be fine.
Barry

#3 Mirzam

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Posted 05 December 2012 - 07:49 AM

I recently finished building a travel scope that is similar in style to the Obsession UC's. For the dew heater to work you need the secondary itself to be inside a metal shroud, such as the Astrosystems shrouds. It will be held in place by fiberfill type insulation, which will greatly help to ensure that the heat reaches the mirror instead of radiating away to space. If you have this type of arrangement you just need enough power and everything will work fine.

JimC

#4 tezster

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Posted 05 December 2012 - 08:09 AM

Thanks for the tip - I think I may just forego the option of the heater for the time being due to configuration of my scope.

#5 George N

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Posted 08 December 2012 - 11:31 PM

I recently finished building a travel scope that is similar in style to the Obsession UC's. For the dew heater to work you need the secondary itself to be inside a metal shroud, such as the Astrosystems shrouds. It will be held in place by fiberfill type insulation, which will greatly help to ensure that the heat reaches the mirror instead of radiating away to space. If you have this type of arrangement you just need enough power and everything will work fine.

JimC


I have a 12 year old Obsession 20 with the old secondary dew heater that came with the scopes back then: a heating element that goes behind the mirror and is held in place with the fiber. I use a Kendrik unit to heat it. Works just fine. I've had the scope so covered with dew that it looks like it was hosed down, and yet the secondary remains clear. I've also had the scope frosted completely white, with no frost on the secondary. Last year I was at a star party where nearly everyone with an SCT or refractor was shut down with dew, but my 20 was fine.






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