Kris.
Pooh-Bah
Reged: 09/16/04
Posts: 1276
Loc: Belgium
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Hi all,
how do i prevent the secondary mirror of my 12" Dob from dewing up? i've never had that problem with the 8" dob, but it has already happened a few times with the 12"?
because i mostly observe mars, i always let the scope cool down by putting it outside before i go to sleep, so it's well cooled when i get up to observe in the morning. i've done this many times the last years and never had a problem with the 8" dob though  i don't think the air is very moist now, it was just freezing a little this morning.
because i thought of it, i let the scope cool down with the end cap on, but with one of those little plastic covers (for an off axis mask i believe) off. i thought this might let it cool down gradually and prevent the secondary from dewing up, but it happened again 
any ideas someone?
thanks in advance!
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Kris
To be old & wise, you first gotta be young & stupid 
8" dob
AstroTech 66ED/APO
TAL 120 newt.
my CN sketch & picture gallery
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earthbot1
super member
Reged: 08/27/09
Posts: 173
Loc: Central Virginia
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I don't own a DOB, but I'd think that you should keep the whole thing covered with just enough venting to let it cool air in.
Maybe construct a tarp cover for it.
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Nexstar 8
Meade/Celestron EPs
Bushnell 90mm Mak-Cass
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Ray4852
sage
Reged: 09/30/08
Posts: 400
Loc: Arcade, NY
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I use astrosystems secondary dew heater. you can wire direct to a small 12 volt battery or plug it into a dewbuster. use 20 gauge wire to connect to a battery.
http://www.telescopes.net/doc/2600/cat/7175/by/manufacturer
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Home Built 18 dobsonian
Panoptic 27mm
Tele vue 16mm 9mm naglers
Tele vue big barlow
Telrad and DSC
deepsky and skytools software
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1105
Loc: Hyde Park, NY, USA
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You don't say what kind of Dob it is, but if an open strut or truss type and you don't have a shroud, try one. I recently got one for mine and the shroud in combination with running the primary fan all the time seemed to be pretty effective, since the shroud directs the flowing air up against and past the secondary. I also use a fan baffle so that most of the blown air is forced up the tube, which likely helps.
It may not be sufficient for really heavy dewing, haven't been out with it enough to find out yet.
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8 + Berlebach UNI 24
Meade 12" LightBridge
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Kris.
Pooh-Bah
Reged: 09/16/04
Posts: 1276
Loc: Belgium
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Mike: it's a standard classic dob, not the open ones.
earthbot1: that's practically what i do; keep the top cover on, with only the small hole (5cm diameter?) open to let air in and out. this has always worked perfectly fine for me with the 8" dob...
i think i've read something about using a hairdryer to get rid off the dew? but won't that disturb the temperature equilibrium of the scope again
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Kris
To be old & wise, you first gotta be young & stupid
8" dob
AstroTech 66ED/APO
TAL 120 newt.
my CN sketch & picture gallery
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5025
Loc: Mobile, Alabama, USA
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If it's a truss-tube such as the Lightbridges, the secondary mirror is much more likely to dew up but even solid tube Dobs are not immune either. The solution is a heater placed behind the mirror, which will keep it clear no matter how damp it gets. I've had this problem crop up with my 10-inch F/4.5 solid tube Dob, then I placed a heater I made behind the mirror and never was bothered by it dewing up ever again.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1105
Loc: Hyde Park, NY, USA
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Quote:
it's a standard classic dob, not the open ones.
Ok, you don't need the shroud, but airflow should help in any case.
A hairdryer will work, that was my solution before I got the shroud. I'm hoping I don't need to use it nearly as often now. It's best to not have dew forming in the first place.
The secondary heater suggestions here are good too. Pretty much guaranteed to work even when other methods don't.
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8 + Berlebach UNI 24
Meade 12" LightBridge
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Tim A.
sage
Reged: 09/19/07
Posts: 239
Loc: 40 30'N 105 3'W
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Make sure also that the tube is horizontal while it's cooling. Otherwise, your secondary radiates its heat into the sky, via the primary.
The Astrosystems Dew Guard is an excellent solution, but may require a different secondary holder, depending on your scope.
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F/5 12.5" Dob (Starbucket)
Celestron CPC800
Celestron CR-150 HD on CG5-GT
Galileoscope
Oberwerk Deluxe II 20x80 & Ultra 10x50
Celestron Regal LX 8x42 (gone AWOL)
Bio-binoculars 1x6
"Me? Crazy? Oh, yeah. Crazy like an ox!"
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3469
Loc: Cambridge, MA, USA
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With truss-tube Dobs, it's impossible to keep the entire unit sealed. So truss-tube Dobs come with little bags that you can place around the secondaries to keep them dust-free during storage. They also work very well to keep dew from forming while the scope is unattended. I'm not sure what they're made of -- no doubt any lint-free material would work fine -- but they work very well. Maybe try contacting one of the premium Dob vendors and ask them what they use?
That won't help while you're using the scope, of course. There, I've found two solutions. Holding the *back* of the secondary holder in my bare hand for a minute or two supplies enough heat to keep the dew away for another 20 minutes or so. But if electricity is available, a blast from a hair drier is better by far.
The secondary is so far from the primary that you don't have to worry about destroying the primary's cool-down. You may have slightly degraded images right after using the hair drier, but the secondary cools down really fast -- which is why it dewed up in the first place.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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tedbnh
sage
Reged: 11/14/07
Posts: 302
Loc: New Hampshire
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When I "borrowed" the flexible dew shield from my 8" SCT and started using it on the front of my 8" dob, I never got dew on the secondary again even without a secondary heater. The notch for the dovetail even matches the base of the Telrad on the dob. Try this (and I agree, when your scope is cooling keep it horizontal).
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Ted
Hampton Beach, NH:
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Celestron C8 (orange tube), WO ZS 80ED 545/6.8
Orion XT8i + COL 1200/5.9
iOptron Minitower, AT Voyager
Bunch of EPs
Stardust observing chair
"Gladly wolde he lerne, and gladly teche" - Chaucer, Canterbury Tales
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aa5te
Genial Procrastinator
Reged: 08/30/08
Posts: 428
Loc: Clinton, TN
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Quote:
running the primary fan all the time seemed to be pretty effective, since the shroud directs the flowing air up against and past the secondary. I also use a fan baffle so that most of the blown air is forced up the tube, which likely helps.
It may not be sufficient for really heavy dewing, haven't been out with it enough to find out yet.
A fan alone has been effective for my 10" dob's primary and secondary mirrors, even on at least two relatively heavy dew nights. I even turned the fan off on one night for about an hour and dew formed on the secondary within the hour; turning the fan back on high speed eliminated it within about 15 minutes. This has kept me from dragging out the hair dryer, so it's much simpler.
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Shane
Binos: Apogee 25x100 / Burgess Optical 25x100 / Pentax 20x60 PCF WP / Tasco InFocus 10x50
Tripods: Sunpak PlatinumPlus Ultra 7500TM / Quickset Samson
Refractors: Meade DS-2102AT-TC 102mm f/7.8; Sears Discoverer 3 (4454) 80mm f/15; Jason 313 Discoverer 60mm f/15.2
Reflectors (Dob): Hardin Optical DSH10 10" f/5
EP's: Zoom: Zhumell 8-24mm & Circle K 7.5-22.5mm 0.965"; 9-52mm Plössls; Zhumell 2x barlow
Kodak Z760; Orion SteadyPix camera mount; Orion LaserMate Deluxe Collimator
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Kris.
Pooh-Bah
Reged: 09/16/04
Posts: 1276
Loc: Belgium
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Quote:
Make sure also that the tube is horizontal while it's cooling. Otherwise, your secondary radiates its heat into the sky, via the primary.
hmm that could be it, i point the scope almost vertically when cooling down. i thought it would be quicker this way, next time i'll try it horizontally!
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Kris
To be old & wise, you first gotta be young & stupid
8" dob
AstroTech 66ED/APO
TAL 120 newt.
my CN sketch & picture gallery
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David Castillo
scholastic sledgehammer
Reged: 09/09/06
Posts: 882
Loc: Carmel Valley, Ca
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Try a dew shield. You can cobble one up from carbdboard and test it out. My 10" dob's is out almost 10" above the secondary when it's fitted. The bonus is that it blocks out stray light from around the neighborhood, boosting contrast.
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Dave
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Greg Stone
member
Reged: 04/29/06
Posts: 80
Loc: Southeastern New England
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FYI - Orion ( and I suspect others) sells a 12-volt dew gun - handy when you don't have AC nearby to plug in a hairdryer. I've used this on the secondary of my 15-inch DOB when I've forgotten to hook up the heater ont he back of the mirror ;-)
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Canon 12X36 IS
PST
60mm Televue
16X70 Fujinon FMT
80mm Celestron Onyx
100mm ED Skywatcher (Voyager)
120mm ED Skywatcher on T-Mount
10-inch Orion Dob
15-inch Obsession Dob
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