Instruments and Imaging: How to prevent condensation inside a closed system?

[Astrid]

Dewing of a mirror or object glass can be countered with various devices
and methods, but its occurrence inside a closed system is particularly
problematic. I have received the following enquiry (from an experienced
observer who has used several instruments over many years) concerning
dew on a corrector plate, and would be pleased to hear from anyone who
who has a remedy:

'Having recently obtained an 8-inch Meade SCT I have been having serious
problems with condensation, which persistently accumulates on the inside
of the corrector plate. I have fitted a dew shield and tried a heater,
but these have been only partially successful in limiting and/or
delaying the inevitable.'

 

[M Quack]

Have you tried flowing dry nitrogen over the affected area?

 

[Drakkith]

The INSIDE of the corrector plate? Wow. I have a Schmidt-Newtonian and I use a hand held blow dryer, like the ones women dry their hair with, as a final resort if things get bad. It works pretty well and can even clear the inside of the corrector plate in a minute or two. It just sucks because sometimes you have to apply it every 15 minutes or so depending on how humid it is.

 

[chemisttree]

Your friend needs a http://www.lymax.com/sct/. Run it inside the house where it is dry and warm to dry out the interior of the SCT.

 

[davenn]

...
'Having recently obtained an 8-inch Meade SCT I have been having serious
problems with condensation, which persistently accumulates on the inside
of the corrector plate. I have fitted a dew shield and tried a heater,
but these have been only partially successful in limiting and/or
delaying the inevitable.'

Dew shield/heater units are for condensation on the outside of the corrector plate...
something all us SCT owners suffer from, unless you live at really high altitude ;)

As hinted at by others... you really need to purge the inside of the scope OTA under the driest (read lowest humidity) conditions possible and consider filling the OTA with nitrogen to purge the damp air

this leads to the question tho ... how did moisture get in there in the first place ?
Have you had the OTA apart for main mirror cleaning etc ?

Dave