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matt_baker

C14 Dew on inside of Corrector Plate

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I have a dew heater placed behind the corrector on full power, plus a 2 inch desiccant plug on the visual back whenever not in use, but still everytime I fire the scope up in the obsy, the inside of the corrector plate will always dew, right by the secondary expanding outwards 

Any ideas to help fix this?

Getting a dehumidifier maybe?

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I sympathise with your problem Matt. I have struggled with this for years. I tried Dew shields, multiple bands desiccant in the tube etc. Trouble is the air is so moist in this country at this time of year.

The only way I found to hold it off for a while is to thoroughly heat the tube and corrector with a hair dryer about 15 mins before observing.  It cools down just in time for visual use and a few mins longer for imaging. Then I have 1/2 an hour to an hour before the corrector dews up again.

I have heard some flocking material does a decent job of holding off dewing.  I cork lined an SCT I had many years ago, it was a messy and tricky job and to be honest made no noticeable difference to dewing.

Ian B

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I wonder if it might help to use a fan to circulate outside air through the OTA as the outside temperature is cooling?  It could be that warmer air holding moisture is trapped in the tube and as the corrector cools the moisture condenses out on the inside.  Getting rid of the warm air in favour of colder air containing less moisture might help, perhaps.

Lymax used to make a "Cat Cooler" fan that slotted in down the baffle tube.  I don't know if they still do.  I'm sure a DIY solution wouldn't be that hard to create.

James

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You could try parking it pointing down rather than up as well as the desiccant cap.

I have my dew band on the dew shield in front of the corrector.

Dave

Edited by Davey-T
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14 hours ago, matt_baker said:

I have a dew heater placed behind the corrector on full power, plus a 2 inch desiccant plug on the visual back whenever not in use, but still everytime I fire the scope up in the obsy, the inside of the corrector plate will always dew, right by the secondary expanding outwards 

Any ideas to help fix this?

Getting a dehumidifier maybe?

I run a dehumidifier constantly when the observatory shutter is closed on 60% RH and a dew band on a 8” SCT when open. I’ve never had a dew problem inside, though outside the observatory is a different story. 

https://www.amazon.co.uk/dp/B00474K8SY/ref=cm_sw_r_cp_tai_UnE1Db9XQJ53A

 

1C775FD3-F6E0-425F-9723-70BE7BDD0E51.thumb.jpeg.e690d92bd28f856f9d16b80bfc7df582.jpeg

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I have a small dehumidifier running and a green house heater to keep the ROR Shed dry. 

I'm not too sure about Domes but my ROR Shed has plenty of fresh air circulating. Since I took my C11 from standing in the garden to inside a permanent ror shed I have not had condensation/moisture issues.

If your C14 has Fastar, try removing the secondary mirror and removing the eyepiece from the other end. Use a cool hairdryer to blow air through the C14 to help remove the moisture! I have used this trick on my C11 a few times in the past and it works fine the rest of the night :D 

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I've had a C11 in my obsy for over 10 years and never had dew on inside of the plate. I don't use a dehumidifier in the obsy, I have no desiccant plug, but I do leave a dew heater on permanently (placed around the tube near the corrector plate. But what I do have is an f/6.3 focal reducer permanently in place. I suspect this helps seal the tube and prevents damp air entering.

Having said this, I'm probably now going to experience the problem.... Where's my block of wood?!

  • Haha 1

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I was thinking what about circulating the warm air out by using a hairdryer on a cool setting by sticking it up the drawtube

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It might be worth remembering that hot air rises. However, the baffle tube goes a long way into the main tube. I can't say I've ever had this problem so I wonder if the scope has a particular issue, or maybe its local environment.

Olly

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5 hours ago, matt_baker said:

I was thinking what about circulating the warm air out by using a hairdryer on a cool setting by sticking it up the drawtube

I'm not sure that would work very well because there's no other way for the warmer air to escape.  The Lymax cooler funnels the incoming air down a small tube inside the baffle tube and has gaps where it fits into the visual back so air can be pushed up the baffle tube and out of the OTA.  Without chopping holes in the tube or backplate it seems like the neatest solution unless you have a Fastar unit and are happy to remove the secondary to allow the OTA to cool properly.

James

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This is what the inside of the corrector looked like tonight

When not in use, I point the scope down and have the desiccant cap on, but this just doesn't seem to be working

I have no idea at the moment how to rectify this, it could be to do with the atrocious weather we've had over the last few weeks.

Matt

IMG_20191129_185507931.jpg

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I think getting colder (ambient temperature) air into the OTA would help because it will contain less moisture to start with and have a lower temperature differential when compared with the outside of the corrector when it is pointing up at the sky.

Perhaps bodge up something like this?

http://www.homebuiltastronomy.com/sct/index.html

James

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I have seen some videos on Youtube where someone installed small fans in their OTA tube to counter this - and to more rapidly cool the OTA down and move the air around.

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10 minutes ago, kirkster501 said:

I have seen some videos on Youtube where someone installed small fans in their OTA tube to counter this - and to more rapidly cool the OTA down and move the air around.

I think that's probably the ideal solution, but requires a fair bit of confidence :)

James

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15 minutes ago, JamesF said:

I think that's probably the ideal solution, but requires a fair bit of confidence :)

James

Yes indeed.  But there is noting inherently complex about these scopes.  Other than the insane accuracy of the mirror and corrector surfaces, they are pretty simple devices mechanically speaking.

Issue with fans is the risk of creepy-crawlies getting into the tube.  So if you do this you must make sure of using some gauze or soemthing across the aperture of the fan.

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I'm not sure if this is total coincidence but I tightened up the corrector plate a tad on all of the screws and the dew seems to have disappeared on the inside while in use?

Does that mean there's a seal on the inside that was a tad loose and allowed moisture to creep in?

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I did a test with my dad inside but not near the scope and it seemed to not dew up at all but the next night comes around, he's up there and it dew's back up again.

Might be the issue I think

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