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SCT Internal Condensation Issue

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#1 kevinbreen

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Posted 18 January 2020 - 09:39 AM

I have left my CPC-1100 and 8SE out in my astro-shed permanently since September. The shed is bone dry, but airy. Nevertheless I have both scopes and their respective mounts draped in large loose wheely-bin plastic bags (an over-cautious precaution, methinks). Today I noticed condensation on the primary mirror and the inside of the corrector of the CPC. No such condensation anywhere inside the 8SE. So I hauled the CPC 100 yards into the house and cleaned the optics, which was a major PITA. Now I'm wondering if I should leave it in the house until the weather gets warmer?

 

Might the plastic bag be trapping moisture and it's somehow getting into the OTA? Sound plausible? -but aren't OTAs supposed to be airtight anyway?? 

 

I'm interested to learn if anyone else has or has had this issue and what steps you took to prevent it?

 

Kev

 



#2 Megiddo

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Posted 18 January 2020 - 10:12 AM

Mine was caught in an early morning rain storm this summer... argh!

 

I ended up buying a bag of Rechargeable Silica Packets and hanging them down the barrel of my Celestron 8SE.   I kinda worked, but it took a long while.

 

No these are not air tight and definitely not water tight (even rain).

 

I've seen discussion on putting ports in them with filters keeping dust out, but I'm not taking mine apart just yet.

 

Long winded way of saying... I hear you and I'm in the same boat.


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#3 BQ Octantis

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Posted 18 January 2020 - 10:39 AM

Crikey, mate! Plastic bags totally trap moisture. I still have lemons from October (it's late January, right?) that are still supple because I've stored them in plastic bags. With lots of condensation that I have to dab with paper towels on a monthly basis. And I live in the Outback, where the dew point is usually below freezing!

 

Get some silica gel packs and put them with your scope. Or better yet, I'll refer you to this post by DMach on how he stores his C14 in Singapore:

 

https://www.cloudyni...-you/?p=9653823

 

BQ


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#4 lee14

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Posted 18 January 2020 - 10:41 AM

My Ultima 8 has been in an unheated observatory for more than twenty years, in a location with periodic swings in temperature and humidity. There is often condensation on the exterior surfaces at the close of an observing session, and this is gently removed with a conventional hair dryer before the scope is sealed in an airtight covering. Once moisture enters the interior, removal can be difficult, so the best strategy is prevention. I use a prism diagonal that remains in place at all times, and effectively seals the rear cell. There's no discernible difference in image quality between a high quality prism and a mirror diagonal, but the latter allows potentially moist air to enter. At the very least, a plug sealing the rear cell should be employed when not in use, the only time the tube should be 'open' is when changing eyepieces.

 

Lee


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#5 dcollier

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Posted 18 January 2020 - 11:45 AM

https://farpointastr...-desiccant-cap/

 

https://farpointastr...-desiccant-cap/

 

One of these should solve you problem.. BTW, Not affiliated with Farpoint.

 

            -Dave


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#6 Megiddo

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Posted 18 January 2020 - 12:37 PM

This thread made me think I didn't put my SCT away with desiccant packets.   I have narrow nylon bag (I think it came from some safety glasses or something)... I ran a sewing machine (without thread) all over the end to get larger holes in it.   Then filled it was 6-8 packets.  https://www.amazon.c...r/dp/B07Q39VZKL

 

I make sure they don't fall past the center tube or getting them back out will be a pain.  Something dcollier's idea would be an issue.


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#7 RedLionNJ

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Posted 18 January 2020 - 12:59 PM

Kev, there is absolutely nothing air-tight about an SCT. Even if you were to seal and snug everything as much as possible, there's still at least the focusing mechanism to let air and moisture in/out.

 

Adding some sort of condensation-catcher on top of the whole shebang is just a plan bad idea. It needs to breathe.

 

I'm at roughly 15 years with all my equipment being in my roll-off roof observatory, year-round. And we get reasonably extreme temperature swings here, although it's usually on the humid side (but not as bad as Ireland).  I don't cover my OTA with anything other than the observatory roof.  What I do have are a couple of stand fans which cycle on for an hour right before the start of potential observing time (just in case the sky does indeed turn clear again some evening).  This has been totally sufficient to prevent interior condensation inside my APO, SCT and Maks.


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#8 Kokatha man

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Posted 18 January 2020 - 07:10 PM

If you're going to leave the scope out in the obs it'd be better to lay a blanket or something that absorbs moisture over it Kev: even then I wouldn't leave it completely un-checked for 3 months.

 

Plastic bags will trap moisture...removing it from the primary & inside face of the corrector is quite easy (more so if you can just position the ota when you have a GEM like us) by positioning the ota upright (or as near you can in your CPC, ie the corrector end face-down as far as possible) & removing any after-market focuser or attachments to to rear thread of said scope.

 

I place a single layer of a Kleenex tissue (ie, peeled) over the open focuser end & secure it with a rubber band just to keep dust etc out then crank up the heating in the room it is in & leave it like that for a day...usually dries the inside out bone-dry. smile.gif

 

Megiddo has a good ploy - you can simply stuff a few packs of silica gel into the leg of a stretched stocking & gently push this down the baffle tube until it hangs out the other end (look in at the corrector end to see the progress) & then tie off the other end of the stocking to something (eg, the primary knob) & use a rubber band & plastic bag to seal the end you inserted this device into.

 

As said these ota's are most certainly not airtight but hanging some gel-packs inside does keep moisture condensation at bay for some time...I've used the stocking trick a couple of times but because I can separate the ota from my mount easily I prefer to keep my C14 inside with the corrector face down on the floor in a dry environment.

 

I wouldn't recommend cycling your scope through heavy condensation cycles too often btw although we've had to deal with it on numerous occasions such as unexpected rain like another poster here, as well heavy moisture ingress when we pumped air through the ota to cool it that way - why we now use ice & keep our rear vents sealed...but I guess that in Eire you don't have the temperature equilibrium issues we encounter here so markedly... wink.gif  


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#9 kevinbreen

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Posted 19 January 2020 - 06:43 AM

Thanks everyone for your time and excellent advice. I'll see about getting some of that silica gel and sortng this out now
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