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Equipment Discussions >> Cats & Casses

Pages: 1
Kon Dealer
professor emeritus


Reged: 01/05/11

Loc: Cambridge UK
Bigger Cat = more dew
      #5615126 - 01/10/13 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?


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bcuddihee
Carpal Tunnel
*****

Reged: 11/04/06

Loc: Cincinnati Ohio
Re: Bigger Cat = more dew new [Re: Kon Dealer]
      #5615169 - 01/10/13 10:10 AM

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

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bcuddihee
Carpal Tunnel
*****

Reged: 11/04/06

Loc: Cincinnati Ohio
Re: Bigger Cat = more dew new [Re: Kon Dealer]
      #5615170 - 01/10/13 10:10 AM

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

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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Bigger Cat = more dew new [Re: bcuddihee]
      #5615675 - 01/10/13 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.


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Sorny
professor emeritus


Reged: 03/15/12

Loc: Southern MN
Re: Bigger Cat = more dew new [Re: GlennLeDrew]
      #5615859 - 01/10/13 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.


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Bigger Cat = more dew new [Re: Sorny]
      #5616361 - 01/10/13 09:59 PM

You should see how fast a C14 dews up

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).


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dobsoscope
sage
*****

Reged: 05/24/06

Re: Bigger Cat = more dew new [Re: WadeH237]
      #5616425 - 01/10/13 10:41 PM

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

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Kon Dealer
professor emeritus


Reged: 01/05/11

Loc: Cambridge UK
Re: Bigger Cat = more dew new [Re: dobsoscope]
      #5616957 - 01/11/13 10:06 AM

Rats

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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Bigger Cat = more dew new [Re: dobsoscope]
      #5617005 - 01/11/13 10:43 AM

Quote:

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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