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Dewing INSIDE corrector plate

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#1 Guest_**DONOTDELETE**_*

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Posted 21 November 2005 - 07:31 PM

Has anyone had this prob.?
I live in a very humid area ...the tropics. +90% all the time.

After reading up on the CAT cooler etc....I realise that such a device will pump moist air into the scope. Then it is possible for the dew to condense INSIDE the scope the next time I take the warm scope outside...as the corrector plate will possibly cool and cause the warm moist air inside to condense. :bawling:

Perhaps it would be better to cool the scope down inside an airconditioned room...(air is dried by the a/c). Its easy to match outdoor temp with an indoor a.c unit where I am.

What do u think?

#2 Rusty

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Posted 21 November 2005 - 09:01 PM

I'd suggest a dessicant cap by Particle Wave - I keep one in each of my Cassegrain scopes.

If your scope is dewing on the inside, cooling it in the A/C won't help - the moisture has to get out of the OTA.

#3 wilash

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Posted 21 November 2005 - 09:20 PM

I have never heard of a scope dewing on the inside. That would mean the air in the scope is warmer than the scope surfaces and why would you be pumping hot air into a cold telescope? At the scope cools, the air inside cools with it - ever heard of condensation on the inside of a camera? I think you are fine.

#4 RichNH

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Posted 21 November 2005 - 09:40 PM

But if the humidity of the air inside the scope is near 100%, as the scope cools and the air inside cools, it's got to dump the condensing moisture somewhere. I think the best thing would be to store the scope in the a/c room, this way it has drier air inside the scope, presuming that you don't leave the focuser without an eyepiece in it for long periods of time while outside. It's not so much having the scope at the right temperature but having drier air inside the scope.

Rich

#5 wilash

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Posted 21 November 2005 - 09:49 PM

Well, this is an interesting question. Any CAT owners ever had condensation inside a scope?

BTW, it has never happened to me and I live in a very humid country.

#6 PMB

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Posted 21 November 2005 - 11:36 PM

Yep, I had it happen to my 10" SCT. Seems some humid air was trapped inside the tube after a very rainy period last spring. It happened a couple of times to me. I took the rear cap off and let the sun "bake" the moist air out. Hard to get out because it will get trapped in the rear cell. You could force some hot dry air into the rear but you'll need to leaved some space for the air to blow by back out. The dessicant idea is probably a must if you live in a humid area. I did that and it seems to have protected against anyfurther inside dewing but then again, it has been really dry here in N TX lately. Not usually a problem here.

#7 Dave LoPresti

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Posted 22 November 2005 - 04:20 AM

Both dew and frost on the inside of my corrector. :smirk:

#8 John_Gillies

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Posted 22 November 2005 - 10:28 AM

There are two ways you can get moisture on the inside of a corrector. Condensation caused by bringing a very cold scope into a warm room without any kind of thermal barrier like wrapped in a cold sleeping bag to let the scope warm up very slowly (24 hours), and having a dew heater set too high in colder temps. This is where a controller that monitors the ambient temp and can be set to within a few degrees comes in handy. The original Kendrick controller that you set at low, medium or high is usually guilty of one overheating their SCT and not realizing it.

#9 Jeff Young

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Posted 22 November 2005 - 04:55 PM

I can't see how a CAT cooler would produce condensation inside the scope. The cooler can't reduce the temperature any lower than ambient. The air going in via the cooler can't be any more humid than ambient.

If the ambient humidity is causing condensation at the ambient temperature, then it's raining.

Dew is when lower-than-ambient temperatures (caused by exposure to the very cold sky) cause condensation.

-- Jeff.

#10 John_Gillies

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Posted 22 November 2005 - 07:24 PM

I guess we could use more details on the situation when it happened. Things like...was a dew heater or cat cooler being used? Was the scope moved to a warm place from a cold place without preperation?

#11 Dave LoPresti

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Posted 22 November 2005 - 08:42 PM

I admit that I've taken the scope indoors from using it all night without taking the proper precautions. :crazy: As in, diagonal not covered..:foreheadslap:


#12 Derwin Skotch

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Posted 22 November 2005 - 09:42 PM

I've had dew and frost form on the inside of the corrector plate on both my 6" Schmidt-Newt and my 9 1/4" SCT. It happened while observing, after observing for quite some time, not when the scope was brought inside or when the scope was brought outside. I don't yet have a heater for my 6" and it happened on my 9 1/4" before I had one for it.

The only way dew (or frost) can form on the corrector is if the temperature of the corrector gets lower than the dew point of the air. With the cooling by radiation of the corrector pointed at cold space, and without a heater that can easily happen.

I wonder why it doesn't happen more frequently on the inside. Is there much of a temperature difference in the glass of the corrector from one side to the other? Is the dew point of the air inside the tube lower than the dew point of the air outside the tube?

#13 Rusty

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Posted 22 November 2005 - 10:20 PM

I can't see how a CAT cooler would produce condensation inside the scope. The cooler can't reduce the temperature any lower than ambient. The air going in via the cooler can't be any more humid than ambient.

If the ambient humidity is causing condensation at the ambient temperature, then it's raining.

Dew is when lower-than-ambient temperatures (caused by exposure to the very cold sky) cause condensation.

-- Jeff.


If the dew heater overheats the OTA, when the heater's turned off, the air inside cools, contracts, and draws in air from the outside - the outside air is humid, moisture gets inside.

#14 Boywtoys

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Posted 22 November 2005 - 10:24 PM

What are some precautions that one can take when bringing scope indoors after a long night observing in the winter chill?

#15 John_Gillies

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Posted 23 November 2005 - 08:02 AM

When I would bring my scope in from the cold, I would wrap it a sleeping bag (that has been outside as well)and stuff the wrapped scope into a hockey duffle bag. Then I would bring it in and not open it until the next observing session or at least 12 or so hours. This allows everything to warm up very slowly and not allow condensation to form on anything inside or out. I do the same with cameras and electonics.

#16 Guest_**DONOTDELETE**_*

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Posted 23 November 2005 - 02:56 PM

I think I shall simply precool the scope in the a/c room with the cat cooler. That way ...dry cool air is inside, filtered by the A/c as well.
Then take it out. That way inside the OTA is low humidity and therefore no condensation.


As an aside....anyone uses any heater on their eyepieces? Or would a little blower fan prevent condensation on em?

#17 John_Gillies

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Posted 23 November 2005 - 03:15 PM

I have heaters on my eyepieces for my binoviewing.

#18 Carol L

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Posted 25 November 2005 - 08:44 PM

What are some precautions that one can take when bringing scope indoors after a long night observing in the winter chill?


I place my cameras, binoculars and scopes into plastic bags and have never had a problem with condensation even though the equipment is brought directly from the deck into the living room where the portable humidifier is. Larger equipment such as my CAT and 22x100s are placed into large lawn and leaf sized bags, my ETX and 11x70s are placed into tall kitchen trash bags, and my cameras are placed into appropriately sized zip-locks.
It's logical that condensation can't form on something if the moisture can't reach it, and the plastic bags create a moisture barrier. I've been doing this for a number of years and have never had one drop of condensation form on anything. Just remember to open the bags in the morning so the equipment can breathe and it'll be just fine. ;)

#19 Boywtoys

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Posted 26 November 2005 - 08:32 PM

Carol,

Have been doing just that. Working great. Thanks for the response.

#20 Carol L

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Posted 27 November 2005 - 03:41 PM

Just ran across this which contains some pretty good common-sense info. :)


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