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How do you heat the mirrors on your dob if it came without heater?

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#1 wpostma

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Posted 27 November 2020 - 11:13 PM

I am the happy new owner of a 12" truss rod dob and wondering about whether or not I need to heat my mirrors, especially since I lack the protection of a traditional tube dob.

 

These dobs I have come with a fan to Cool the mirror down, intended to get it to operating temperature sooner, but I'm guessing that there's a good reason to have it actually stay a few degrees above ambient, when theres condensation happening around you.

 

How do you usually heat your dob?  Mine comes in a metal box rather than the typical plywood or mdf boxes that the mirrors usually go in. I'm thinking that some kind of heat tape attached to the metal box may be the ticket.

 

This S&T article seems to have ideas. What have you tried and what worked well for you? Particular dew heater strap tech for dobs?

 

 

https://skyandtelesc...aling-with-dew/


Edited by wpostma, 27 November 2020 - 11:15 PM.


#2 Waynosworld

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Posted 27 November 2020 - 11:54 PM

I don't know everything, but I am in the middle of reading a thread about Refractor or Reflector, they talked about heat and mirrors, I mean they go down to if they touched there dob tube with their bare hand they could see the waves of heat in the dob tube, they wear gloves so that doesn't happen, metal tubes were the worst.

You cannot heat your mirror or your secondary because it will show up in your eyepiece, heat coming off the mirror will look like heat coming off the freeway on a hot day and make the object your trying to look at dance.

 

https://www.cloudyni...r-vs-reflector/

 

I am the happy new owner of a 12" truss rod dob and wondering about whether or not I need to heat my mirrors, especially since I lack the protection of a traditional tube dob.

 

These dobs I have come with a fan to Cool the mirror down, intended to get it to operating temperature sooner, but I'm guessing that there's a good reason to have it actually stay a few degrees above ambient, when theres condensation happening around you.

 

How do you usually heat your dob?  Mine comes in a metal box rather than the typical plywood or mdf boxes that the mirrors usually go in. I'm thinking that some kind of heat tape attached to the metal box may be the ticket.

 

This S&T article seems to have ideas. What have you tried and what worked well for you? Particular dew heater strap tech for dobs?

 

 

https://skyandtelesc...aling-with-dew/

I myself am a newbie, I have a model airplane heater to keep my Telrad dry so I can find things, I have used it once to dry off the primary mirror because of my mirror cover, and a few times on the secondary mirror, when I look thru the eyepiece things are dancing and I go inside for a little while.


Edited by Waynosworld, 27 November 2020 - 11:54 PM.


#3 ad701xx

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Posted 28 November 2020 - 04:37 AM

Fall is a bad time of year for dew where I live. The 16" truss Dob I recently completed has worked great in in these conditions.

 

For the primary, I have 4 60mm fans blowing on the front surface.

 

The diagonal is taken care of by an Astrosystems Dew Guard:

 

https://www.astrosys....biz/dewgrd.htm

 

My scope has been soaked with dew and the mirrors have stay clear all night. This is without using a shroud.

 

 

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#4 ad701xx

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Posted 28 November 2020 - 04:43 AM

A view of the entire scope. If you look closely you can see a dew strap wrapped around the SIPS to keep the coma corrector clear of dew.

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#5 Ihtegla Sar

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Posted 28 November 2020 - 05:48 AM

Fall is a bad time of year for dew where I live. The 16" truss Dob I recently completed has worked great in in these conditions.

For the primary, I have 4 60mm fans blowing on the front surface.

The diagonal is taken care of by an Astrosystems Dew Guard:

https://www.astrosys....biz/dewgrd.htm

My scope has been soaked with dew and the mirrors have stay clear all night. This is without using a shroud.


Nice! After my 20" primary completely frosted over last week I've been thinking of adding boundary fans in the corner of my Obsession. Do you find they help with tube currents during warmer seasons? What type of brackets did you use to hold the fans?

#6 havasman

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Posted 28 November 2020 - 06:27 AM

Do not heat your primary. This article explains the effects of thermal instability on optical performance pretty well -  http://www.loptics.c.../starshape.html

The section you should see is toward the end titled Other thermal effects on optics. Thermal instability in a primary is when the mirror is above ambient enough that heat, unevenly distributed, warps the reflective surface.

 

Our club site is plagued with dew most of the year. All my scopes regularly have condensation on them somewhere and fairly often it is heavy enough to run off the tubes and structures of the Dobs in rivulets or form rime ice if it's cold. But the primary never dews up. I have a 120mm high-spec fan behind the primary of the 16" blowing on the back to promote cooling as the ambient temp falls and the air movement works to keep the dew down. The secondaries of my Dobs are Astrosystems spider/secondary holder/mirror systems with their excellent secondary heater system in place. I lost many observing sessions to dew before putting those in place and they have worked 100%.

 

I also have active dew control on Telrads and eyepieces on the Dobs and eyepieces and objectives on the refractors. I use R-Sky strips and controllers for all that.


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#7 kathyastro

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Posted 28 November 2020 - 08:26 AM

These dobs I have come with a fan to Cool the mirror down, intended to get it to operating temperature sooner, but I'm guessing that there's a good reason to have it actually stay a few degrees above ambient, when theres condensation happening around you.

What you need is to keep it a few degrees above the dewpoint, not ambient.  (If the dewpoint is at ambient, you are in fog, and the night is over anyway.)  Keeping it at ambient is fine for dew protection.

 

The fan is not just for cooling.  A fan will bring the mirror temperature towards ambient if it started warmer.  It will also bring it towards ambient if it is cooler.  So it is not just a cooling fan.  It is an "ambient temperature stabilization" fan.

 

Run the fan during the whole session.  The mirror will absorb heat from the moving ambient air to replace the heat it loses to radiation.  It is not foolproof.  If the mirror is losing heat faster than the moving air can replace it, it will eventually drop to the dewpoint.  However, the fan will postpone the inevitable.

 

A shroud on the truss will reduce the loss of heat from radiation.


Edited by kathyastro, 28 November 2020 - 08:27 AM.

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#8 junomike

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Posted 28 November 2020 - 08:40 AM

First line of defense is a Shroud.  Without this the Primary will Dew over quickly.

Next a Secondary Dew Heater is advised.  IME with a Shroud, I'd never had the Primary Dew over.



#9 ad701xx

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Posted 28 November 2020 - 09:41 AM

Nice! After my 20" primary completely frosted over last week I've been thinking of adding boundary fans in the corner of my Obsession. Do you find they help with tube currents during warmer seasons? What type of brackets did you use to hold the fans?

Thanks! I see you're in the PNW, too.

 

Yeah, my scope got frosted over last week, also. The mirrors stayed clear so I could have stayed out longer if I wanted, but I'm getting less tolerant of the cold as I get older.

 

The boundary fans do make a difference in the summer. I turn them on after I roll the scope out of my house and keep them running all night.

 

Here is a link to my build thread about how I did the fans:

 

https://www.cloudyni...uild/?p=8830209


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