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Equipment Discussions >> Refractors

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PhilCo126
Post Laureate


Reged: 01/14/05

Loc: coastline of Belgium
Large refractors and dew systems
      #6156711 - 10/25/13 01:53 AM

What's the best place to place the dew heater ribbon on a refractor ?
Looks like the heat might influence the correct focus, so some people use a temperature sensor

Probably best to place it behind the lens (lens system for an Apo-refractor) onto the OTA... placing it in front of the lens might cause more easliy some warmth into the dewshield tube


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Large refractors and dew systems new [Re: PhilCo126]
      #6156770 - 10/25/13 03:13 AM

What is behind and what is in front of the lens?
There seams to be some misunderstanding with that terms in recent thread (same topic).


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Large refractors and dew systems new [Re: orion69]
      #6156801 - 10/25/13 04:08 AM

If there is good thermal contact, i.e., metal-to-metal, whether the heater is at, ahead of or behind the objective makes little difference.

If the dew cap is isolated from the cell by something thermally more insulating, like felt, velvet, etc., a heater on the dew cap will tend to keep the heat more localized there.

The dew heater does its job via conduction of heat to the glass and radiation from the warmed tube. Depending such on factors as;
- objective diameter/mass,
- degree of thermal contact between tube/cell and glass, and
- heater efficiency (more efficient when wrapped so as to reduce losses to air/sky),

the 'mode' of heat transfer (condictive or radiative) will be more one or the other.


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Aquarist
Pooh-Bah


Reged: 08/27/12

Loc: Illinois
Re: Large refractors and dew systems new [Re: GlennLeDrew]
      #6156845 - 10/25/13 06:17 AM

Behind the dew shield, on the diagonal, and on the hand held control for the mount IMO.

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PhilCo126
Post Laureate


Reged: 01/14/05

Loc: coastline of Belgium
Re: Large refractors and dew systems new [Re: Aquarist]
      #6179161 - 11/06/13 12:34 PM

O.K. I'm placing it just behind the dew shield, slightly in front of the first lens of the Apo-triplet

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TG
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Re: Large refractors and dew systems new [Re: PhilCo126]
      #6179639 - 11/06/13 05:05 PM

Before using dew strips, I'd consider wrapping a material such as Reflectix around the metal dew-shield of a refractor. I've found that my 7" triplet never develops dew (this is in the Pacific NW where practically everything drips all the time) while an 11" SCT is almost impossible to keep dew free without active heating. I suspect the thermal mass of the triplet is a key factor.

Tanveer.


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Jim Curry
Pooh-Bah
*****

Reged: 10/29/07

Loc: STL
Re: Large refractors and dew systems new [Re: TG]
      #6179847 - 11/06/13 07:08 PM

You might try extending the dewshield for a 3:1 ratio. I've never had a dew issue with that ratio. No electrics.

Jim


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dkbender
sage


Reged: 09/26/13

Loc: Illinois
Re: Large refractors and dew systems new [Re: Jim Curry]
      #6181167 - 11/07/13 02:15 PM

not to hijack the thread... but what if the OTA tube and dew shield is made of carbon fiber and not metal?

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Messyone
professor emeritus


Reged: 05/02/12

Loc: Down Under
Re: Large refractors and dew systems new [Re: Jim Curry]
      #6182189 - 11/08/13 02:00 AM

Quote:

You might try extending the dewshield for a 3:1 ratio. I've never had a dew issue with that ratio. No electrics.

Jim




Jim, Is that 3x the objective size? Sorry if it's a dumb question….I should know but the brain is elsewhere and a dew shield is looming on the horizon along with a large number of clouds as the f12 frac nears completion.
Matt


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