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Major Dew Problem on SCT Corrector Plate


gfletcher
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Hi Guys


I have had my Meade LX10 OTA  out this evening since 6pm.


I had my dew shield secured.


Just been out to do some observing at 9.45pm and the corrector plate is totally dewed up!!


I have only had this OTA a few weeks and this was my first real observing session.


Is it that this evening is particularly bad ?


Do any of you have this problem ?


Do any of you use dew heaters to overcome this?


 


help needed


 


Graham


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Yep, corrector plates are dew magnets, I remember a few years ago at our original dark site at Sawley, some chap turned up with a scope similar to yours and he only lasted about 50 minutes, I've never seen anything dew up so fast!

You can get dedicated dew heaters for your corrector plate, will see if I can dig out a link.

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Darn it, so I was confusing SCT's with RC's and what I was thinking of was this...

https://www.altairastro.com/product.php?productid=16480&cat=243&page=1

Doing a spot of research it looks like your only method of stopping dew on a SCT is a dew band wrapped around the out side of the OTA, but from what I have read, that could cause convection currents within the OTA if you have the power turned up too high, so in essence you are heating the whole OTA and not just the corrector plate.

Tough call but I think it is your only option.

Hopefully somebody will either agree or correct me if I am wrong.

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Apparently it has been 95% plus humidity tonight!


 


A dew band will help, but you could also try a cheap DIY job of lengthening the dew shield.


 


While your scope is cooling outside point it slightly down from horizontal. It will dew up sooner if you leave it pointing skyward.


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95% humidity, wow that seems very high, no wonder I had major problems.


My dewshield is probably about 15" long so it should be long enough.


Its a good point you make about pointing the ota slightly down from horizontal though.


 


Graham


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15" is good, but I think Darren will tell you that he has about a 40" long dew shield (length of the OTA) on his dob, and the mirror has still dewed up on a bad night. So if it is really bad you can only do your best as suggested above.


 


A hairdryer to blow away the dew is a quick fix if all else fails.


Edited by Tweedledee
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the corrector plates are dew magnets, my 8" sct is ok for a while with just a dew shield on but once they start to dew up they become misty and wet very quickly and are then difficult to clear, prevention is the best option, i always put a dew band on the front and turn it up to at least a quarter heat at the beginning of the night, this keeps it clear or the other method is a little hair dryer, you can buy small camping style 12 volt ones and can be a life saver when your observing from a dark site.


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Guest Tweedledum

Corrector plates attract dew like flies to doggy doo doo. 15" dewshield ain't much against 8" dia plate. As above double if poss, then a dewstrap. Never clean off the dew, leave till dry. Also try 12v hairdryer....

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I have a C11 wandering around the skies


I use both heater and dew shield and tonight i had dew problems then i realised that the heater extension needed re soldering


 


I have found with this and the 8" SCT I need both to keep the dew at bay


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Just remember to switch the heater on at the start of the session to give it a chance. On my c11 I had a long shield plus heater, on really bad nights it kept it clear but if I forgot to switch on then it would last a couple of hours only

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I bought a kit from dew it yourself specially for a SCT, it fits round the inside of the corrector plate rather than round the ota. I've not yet made it up, so can't comment on it.

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Cazz can you give a link to that?

I've just tried to find it, and the website seems to have vanished, I should have checked first before posting about it. It is/was a small company that made dew heater kits you soldered yourself.

The SCT one has the resisters soldered end to end, these were then put through a rubbery type tube, this is put round the inside edge of the corrector plate next to the lip. I'm guessing it's natural response is to straighten, which then keeps it in place.

Apologies for my very technical terms, I'm not very/at all techincal.

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