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Bigger Cat = more dew

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#1 Kon Dealer

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Posted 10 January 2013 - 09:41 AM

I have a 4" refractor, plus a 6SE and an 8SE.
All have "regulation" length dewshields.
The 4" NEVER dews up, the 6SE occasionally and the 8SE almost always.
(Which is one reason the 8SE never goes to a dark site- I can't use my wife's hairdryer on it!)

Is this usual?
What is other people's experience of this?

#2 bcuddihee

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Posted 10 January 2013 - 10:10 AM

Get a dew heater and fit it under the shield. Also they do mahe 12v hair dryers.

#3 bcuddihee

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Posted 10 January 2013 - 10:10 AM

Get a dew heater and fit it under the shield. Also they do mahe 12v hair dryers.

#4 GlennLeDrew

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Posted 10 January 2013 - 02:49 PM

For one thing, the Schmidt corrector is very thin for its diameter, and so cools more rapidly. How do the diameter-to-depth ratios of each dewcap compare? To reduce the cooling rate on the exposed optic, the longer the dewcap relative to its diameter the better. The idea is to present to the optic a minimal area of sky into which to radiate.

Notwithstanding the above, for those jaunts away from AC current sources, best to invest in active heaters powered by a battery.

#5 Sorny

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Posted 10 January 2013 - 04:43 PM

Yeah, the bigger the scope, the faster it dews up.

Get a dew-heater & controller. I don't need active dew prevention on my C5, but my C11 requires it.

#6 WadeH237

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Posted 10 January 2013 - 09:59 PM

You should see how fast a C14 dews up :grin:

If used properly, a combination of dew heater with dew shield should be 100% effective at keeping things clear. I have a Keating dew heater, which regulates the temperature based on an offset from ambient. I run mine about 5 degrees above ambient and it keeps things clear even on the most humid of nights. The trick is to have the dew strap wrapped around the OTA just behind the corrector (ie. not around the edge of the corrector itself).

#7 dobsoscope

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Posted 10 January 2013 - 10:41 PM

i.e. towards the primary or behind in the sense towards the dew shield?

#8 Kon Dealer

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Posted 11 January 2013 - 10:06 AM

Rats :(

#9 WadeH237

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Posted 11 January 2013 - 10:43 AM

i.e. towards the primary or behind in the sense towards the dew shield?


Towards the primary.

The FAQ on this page has good information on how to place the straps, including diagrams and photos






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