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Position of C6 OTA During Cooldown

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#1 coopman

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Posted 23 November 2012 - 12:53 PM

I've read here that the best thing to do is to point the tube down towards the ground, so that the visual back is at the top of the tube for air passage. The problem is, this places the only security screw that I have on the dovetail below my mount's dovetail jaw instead of above it. I'm kind of nervous about doing this because, if for any reason the dovetail slips out of the jaw, I could potentially end up with a broken OTA. What can I do to improve this? Should I try to drill and tap another hole in the dovetail and install another security screw? Thanks.

#2 Javier

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Posted 23 November 2012 - 01:34 PM

Clay,

On both my C6 SCT ad 8" EdgeHD I setup my mount, aligh nthe index marks and get Polaris in the center of the FoV while the scope is cooling. I also install the dew shield and wait an hour or so. Both scopes cool just fine. I've never had a need to point the VB up to allow the scope to cool.

#3 mclewis1

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Posted 23 November 2012 - 04:47 PM

An SCT will cool only a little bit faster if the tube is pointed downwards. There is some "chimney effect" (hot air rising and in this case getting out the baffle tube opening) but since the baffle tube extends at least half way into the ota it's not as effective as openings on the rear cell would be. Most of the cooling is from radiating heat from the rear cell and tube itself ... and that doesn't much care what position the ota is in.

However, one other benefit of pointing the tube slightly downwards is keeping dew off the corrector.


Clay,

I wouldn't worry about the saddle clamping with the ota pointed slightly downwards. First there's not a lot of force pulling the dovetail out of the saddle if the angle is fairly shallow and second if you are at all concerned just give the saddle bolt a quick turn to ensure it's snug before you turn the scope downwards.

#4 Goodchild

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Posted 23 November 2012 - 06:56 PM

Larry at Universal Astronomics makes great dovetail bars with security bolts at both ends.

#5 Asbytec

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Posted 23 November 2012 - 07:50 PM

Tube down is not the best because of any chimney effect. In fact is probably the worst. Warm air simply rises toward the primary and stays there. It does not magically crawl 4" back down the outside of the baffle and sneak out of a 1" hole. Instead, warm air pools against the primary mirror which is not exposed to ambient air.

The best solution is probably pointing the tube up and allowing the air to pool and cool against the much thinner corrector that is directly exposed to ambient. This also allows the warmer air near to primary to rise directly away the relatively massive mirror. It's replaced by cooler air the primary can dissipate more heat into. The same thing happens when the tube is horizontal.

#6 mgwhittle

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Posted 23 November 2012 - 10:41 PM

I agree with Norme.

I have found that my SCTs seem to me to cool faster with the corrector pointed skyward. Think about how fast that corrector dews up in use. That's because it's temperature gets to ambient pretty quickly. I've always thought that the heat goes up to that THIN corrector and dissipates more rapidly than letting the heat rise up to that thick primary and all that metal in the rear cell which I would think only heats them up more before getting a chance to go out that little visual back hole.

All this IMHO of course.

#7 Asbytec

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Posted 24 November 2012 - 02:03 AM

Think about how fast that corrector dews up in use. That's because it's temperature gets to ambient pretty quickly.


Yes, exactly. Excellent point. It's exposed to ambient, the primary mirror is enclosed. Let heat find the path of least resistance.

#8 coopman

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Posted 24 November 2012 - 11:25 AM

Thanks for the great advice, which has solved my dilemma. Corrector up during cooldown from now on. :)

#9 REC

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Posted 24 November 2012 - 02:43 PM

Good question as I wondered about that myself. Are you supposed to leave the cover on the corrector plate? Will this speed up the dew process if left uncovered while cooling down the scope?

Thanks,

Bob

#10 orion61

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Posted 24 November 2012 - 02:53 PM

I usually have mine level and give it an extra 20 min or so cool down. Tube upp or at 45 degrees makes more sence,
the corrector is thin but the tube walls are thinner and a better conductor of heat transfer, main thing is to get the trapped heat out from behind the rear cell and mirror.
This is my problem with my 12" Meade SCT there is always a bit of current itn the top, I cant afford a cooling system right now being disabled, but have thought of a coupl well placed small holes on top and bottom.

#11 Asbytec

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Posted 24 November 2012 - 02:53 PM

Well, for passive cooling, I think that's best. Expose the corrector to ambient air facing up with the dust cap off, if you're comfortable doing so. If not, then leave the dust cap on and lay the scope horizontal. The aluminum skin will perform a similar function radiating warmth directly into the air. I put a reusable medical ice pack laid on a hand towel to control moisture on the horizontal tube. This chills the scope a tiny bit and is fine for moderate temps.

For the chimney effect to work it requires buoyant air to rise. Some will sneak out of the baffle at first with the corrector down. But as a temp gradient forms, the weak venting stops and warmer air will pool toward the primary and cooler air on the corrector. The chimney effect relies on cool air entering the tube, ideally from the bottom, as the warm air evacuates through the top. The corrector end would need a vent to be more efficient, much like a fireplace is open at the bottom.

There are active coolers around, like the cat cooler, that draw cooler air in while creating a little high pressure in the tube. Presumably this high pressure in the tube also forces warmer air out and to mix with the cooler incoming air. That sounds pretty efficient. I believe venting is permitted by the design of the cat cooler's plumbing. Anyway, the scope is not air tight, but it's not exactly air "loose", either, with the visual back plugged.

A little prep will certainly help get the scope to ambient, initially. From there so much depends on local conditions. The greater the difference between the inside and outside ambient temp, probably the more aggressive cooling you need to have to get to ambient quickly. If conditions change slowly over a small range through the night, you should be fine. If they change dramatically or more rapidly than the scope can react, then there will likely be some issue with thermals. This would be true for most, if not all, passively cooled scopes.

Orion, great idea venting at an angle allowing the warm air to evacuate the primary Assembly. That sounds very reasonable.

As for leaving the dust cap on, that would retard efficiency trapping a pocket of relatively warm air. Ideally that warmer boundary layer needs to be blow off with a slight breeze. It might induce faster dew forming after the session begins, well, because the scope is in observing configuration long before it actually starts. I prefer the horizontal (or tilted) cooling and allow the aluminum skin to do the work given time.






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