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Large refractors and dew systems

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#1 PhilCo126

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Posted 25 October 2013 - 12:53 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 :doah:

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 :p

#2 orion69

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Posted 25 October 2013 - 02:13 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).

#3 GlennLeDrew

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Posted 25 October 2013 - 03:08 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.

#4 Aquarist

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Posted 25 October 2013 - 05:17 AM

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

#5 PhilCo126

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Posted 06 November 2013 - 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 :rolling:

#6 TG

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Posted 06 November 2013 - 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.

#7 Jim Curry

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Posted 06 November 2013 - 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

#8 dkbender

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Posted 07 November 2013 - 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?

#9 Messyone

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Posted 08 November 2013 - 02:00 AM

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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