Fighting Dew

[Rodger King]

Hi

Following on from my last post I have now found my imaging problems are not due to the camera dessicant or even my dirty field flattener but due to dew collecting inside as well as outside the refractor.  I always consider the space from the inside of the lens to the camera to be a “sealed” system and not susceptible to dew but it seems I am wrong.

Last night was very dewy but I took a series of 3 minute images having cooled my camera to -20C and after each image the problem was getting worse. 

I then brought my equipment into the warm house and opened up the filter wheel and noticed a ring of dew on the outer edge of each filter.  I suspect whilst I was imaging, the ring of dew was getting more and more substantive as the cold from the camera was being transmitted.  See image

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I dont have any dew control at the moment.  What would you suggest ?  A mutli-port dew heater and straps on the scope, the finder and somehow on the filter wheel ?   A de-humidifier in the observatory ?    A dew cap with dessicant ?  All 3 ?

I have never needed any dew control before but I moved home last year and this is the first autumn/winter at this location.  Maybe its more susceptible to dew here ?

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[Grant Privett]

Unfortunately, I saw the same problem a couple of days later. See attached. The corrector plate on the CCD started to cloud from the start. Took it off the scope and indoors to sit nosepiece down on a radiator for an hour…. Not an ideal way to treat kit, but with the first clear night in weeks and no moon I really didn’t want to waste more time than I had already.

Later that night there were also hints of condensation inside the corrector plate – so, dew shield now on order. May have to wait ’til a low humidity day and get some air moving through the tube while its in the sun. So, realistically first chance in April?

[Dr Paul Leyland]

If I understand you correctly, the dewing is occurring between the objective and the camera.

If so, would it be possible to cut a hole in the OTA and fit a dessicator (and possibly a circulation fan to be powered up only outside observing sessions to avoid tube currents)?

Sounds brutal, I know, but that would appear to be from where you need the water removed.

[Peter Goodhew FRAS]

I manage to image at up to 100% humidity. I have a dew shield and dew heater strap around the refractor glass. I find that I get the same misting of the glass as you show if I turn the heater on just as I start imaging – and it can take an hour or so to clear. So I turn the heater on well before I start – at least an hour beforehand.

[Rodger King]

I also noticed the inside of the objective lens was a bit foggy so I brought the whole system in, seperated the scope from the filter wheel and from the camera and dried them all out in the boiler cupboard (drastric measures !).  

I have now reassembled and returned them to the observatory and ordered a usb dew heater strap for the objective, coming later today.  If needed I will order another one to sit as close as I can get it to the filter wheel.  Then when we get a night without cloud in what’s left of 2020 I will try it all again.

I have seen an option where you can take off the imaging camera and add a “dew cap” to the end of the scope and inside sits a dessicant pouch.  This should keep the filters and inside of the OTA dry.  The problem with this is you have to take off the camera each time and it spoils any setup (orientation and focus)  I have achieved.  Also it only works until you start imaging and as I have experienced, dew can form in the first hour of observing.

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