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Dew formation on the inside of an SCT

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#1 EFT

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Posted 23 November 2012 - 06:36 PM

I am curious to hear from people who have experienced dew formation on the inside of their SCT, particularly Celestron Edge HD models. I have heard from a couple of people down south in areas like Florida where the dew can be very bad this time of year. They have reported difficulties with dewing on both the outside and inside of the corrector even when using a dew heater. Dew did not appear to be a problem on the primary.

Have others run into this? If so, where are you and what are your conditions like?

Thanks,
Ed.

#2 end

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Posted 24 November 2012 - 07:38 PM

Hi Ed, I've noticed this on my 11" Edge HD, but the problem only cropped up on exceptionally wet nights and only when I had my cooling fans turned on. For the record I'm in Houston.

#3 Footbag

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Posted 24 November 2012 - 08:15 PM

Now that tou mention it, I've noticed considerably more dew buildup on my Edge 800 then my CPC 800. I didn't pay much attention to whether it was inside but the vents have benefits and drawbacks. I havent purchased a dew heater yet, but its on my list.

#4 wolfman_4_ever

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Posted 24 November 2012 - 10:10 PM

Yup, happens all the time here in Los Angeles..

I use dual dew strips and a shield and it still happens..

Since my temps don't fluctuate much, I try to cool the scope down as the sun is going down... as the humidity builds, I shut off the fans..

#5 rmollise

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Posted 25 November 2012 - 10:40 AM

I am curious to hear from people who have experienced dew formation on the inside of their SCT, particularly Celestron Edge HD models. I have heard from a couple of people down south in areas like Florida where the dew can be very bad this time of year. They have reported difficulties with dewing on both the outside and inside of the corrector even when using a dew heater. Dew did not appear to be a problem on the primary.

Have others run into this? If so, where are you and what are your conditions like?

Thanks,
Ed.


Down here on the Gulf Coast it CAN happen--including dew on the primary. Not often, but once in a while; usually after an all nighter at a star party. No harm done. I generally leave the rear port open after dawn to speed drying-out. You can't do that with an Edge, but removing a Fastar secondary will accomplish the same thing... ;)

#6 cavefrog

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Posted 25 November 2012 - 06:52 PM

tell ya what... I wont leave the fan running on my 7" Mak for 3-4 hrs no more!
I wanted to just peek at Jupiter last night after messing with the settings on The ETX 125. I ended up resetting the ETX about a dozen times trying to get the training perfect.
by the time I got it 3-4 hrs passed by. I thought the big Mak should be plenty cooled off by then. I pulled the dust cover off and stuck in an EP... ouch! 3 min later the corrector was fogged inside and out. plus the primary was already fogged.

cut the fan after 20 mins, and turned on the dewbuster tonight.
after 6 nights out with it I should be able to see him!

Theo

#7 RichD

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Posted 26 November 2012 - 12:09 PM

Don't think i've ever had internal condensation on my Mak, but to be honest I have never used dew strips and just rely on a dew shield to keep the dew off the outside of the corrector as long as possible. By the time the corrector dews i'm normally ready to head in and wouldn't know if the inside has dewed as well anyway.

I think it's very unusual for the primary to show condensation in nornal use in a CAT.

#8 dscarpa

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Posted 26 November 2012 - 07:31 PM

It was a dark and foggy night... It wasn't completely foggy when I was out with my recently purchased-5 years gone by-regular C-9.25. It was building in fast from low down and I was trying to get in a few more minutes of moon viewing. When I brought the scope onto the patio and turned on the light I was none to pleased to have a good bit of condensation on both sides of the dew collector AKA corrector plate. The front was easily taken care of with a hair drier but the interior dew stayed put. Jerry at Scope City where I had bought the scope took pity upon me and took the corrector off, dried it and got the optics lined up fee free. I've used the C-9.25 on getting foggy nights since then with no issues. There was something odd about the night my scope did dew out front and back. The fog wasn't rolling in from somewhere but forming around me. David

#9 Asbytec

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Posted 26 November 2012 - 09:20 PM

It was a dark and foggy night...


"Sultry," the night was sultry. :)

#10 Tori

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Posted 27 November 2012 - 04:09 PM

As you know I've had it happen on my 14" EdgeHD in New Hampshire on the really humid nights, even with a dew shield and dew heater going full blast. Now that it's colder and things are frosting rather than dewing, the problem seems gone for the moment and my dew shield and heater are sufficient to keep the dew [frost] from both the inside and outside of the corrector and secondary.

#11 EFT

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Posted 27 November 2012 - 04:25 PM

My original reason for asking the question was to determine if it would be worth it to go back into production of a dessicant air drier that I use to make for a DSI cooling modification. A very small hose bib could either be added to the TEMP-est fan vents or to any closed-tube OTA and then connected to a dessicant chamber and small aquarium pump allowing very dry air to be pumped into the OTA. Driers like these can lower the dew point to at least 50 degrees below the ambient dew point thus eliminating any possibility of dewing inside the tube.

There are clearly times when dew is impossible to get around. The story from one poster about fog forming around him is the perfect example. The air is supersaturated at that point and it is probably not worth trying to view. The question becomes whether there are enough people for whom such weather is so frequent, that it is worth trying to actively do something about it rather than wait for better conditions like Tory did. Dewing on the outside of a tube is a fact of life for some people, but if dewing inside the tube is a real problem for people, then there is a way to solve it.

#12 wolfman_4_ever

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Posted 27 November 2012 - 04:53 PM

I might be interested... Might be cheaper than the scuba tank I keep in the observatory... :p

#13 teskridg

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Posted 21 December 2012 - 05:49 PM

Hi Ed, I've noticed this on my 11" Edge HD, but the problem only cropped up on exceptionally wet nights and only when I had my cooling fans turned on. For the record I'm in Houston.



I remember driving in Houston on a cloudless sunny day and having to use windshield wipers!!
I'm not surprised. Tim

#14 azure1961p

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Posted 21 December 2012 - 10:09 PM

Observing over concrete or asphalt or from it rather can help too. Tho it can kill your local seeing if its still to hot from daylight heat exposure.

Pete






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