Dew Problems on C9.25 primary mirror

[mattytheshark]

Hi everyone,

Justr treated myself to a C9.25 and got my first look through it last night after three weeks of awful weather in the UK.

It got dew on the outside of the scope as I expected but when I got it inside there was also dew on the primary mirror which worried me. I'm not sure wether it was there while I was observing (as the front plate was dewed up anyway) or wehter it was because I'd brought it into the house.

Please help!! What should I do to eliminate this internal dew???

[michael.h.f.wilkinson]

There are plugs with silica gel that can be inserted into the visual back. Lacking one of those, just put the OTA in a warm, dry place, with the visual back open (and preferably facing downwards so no dust gets in) and wait. That is usually sufficient. I always put plenty of bags of silica gel in the flight case in which I store the OTA (I just confiscate any silica gel pack found in any piece of packaging that enters the house)

[E621Keith]

It's quite unusual to get dew on a SCT primary. The only way to solve this is what Michael suggested. A SCT primary is normally protected from dew by the corrector and it's one of the last place to cool down on the scope. Usually condensation form on the primary when the tube is brought back into a warm house, where the warm moist air in the house cools on the cold mirror. As such the tube should be brought into a house caped.

I store my C925 is a large IKEA plastic box. I taped a tub of cupboard dehumidifier to the base to absorb moisture. Cupboard dehumidifier last much longer than silica gel, but there is a risk of some nasty spill (hence the tape).

You need to store you SCT with the rear cap off when you are demisting the inside, but remember to replace the cap once the inside has dried.

[mattytheshark]

Thanks for those responses guys.

I will cap the tube before bringing into the house in future. It has fully dried out (it took about an hour next to a radiator).

[michael.h.f.wilkinson]

Good advice on bringing the scope in capped. I always do that. I wait until all the condensation on the outside of the tube has gone, and only then do I store it.

[SteveNickolls]

Hi mattytheshark, sorry to hear of your dewing problem. Dewing will occur whenever moisture laden air touches a cold enough surface. While your 'scope is cooling down for viewing heat is radiated off the #scope into space which means your 'scope can get colder than the air around it. I think having a set procedure geared to minimising dewing is the key to your future sucess. Use of a dew shield around the corrector plate helps keep a layer of slightly warmer air next to the corrector plate, helping to slow dew formation for some time. An electrical dew heater will add a small level of heat around the plate helping to keep the surface temperature above the dew point throughout viewing. If I am setting up my 'scope a long time ahead of viewing I will cover the OTA after about an hour outside to lessen the 'scope getting too cold. Also if an observing session is interrupted I likewise cover the OTA. Dew can form on the surface of the scope over a session but (touch wood) I have never had a problem with dewing on the primary mirror (the air inside the OTA acts like an insulted layer above the mirror). I'm sure if the observing session was long and the temperature was low enough then when bringing the cold OTA inside you run the risk of condensation forming inside the 'scope but I have not been troubled by this. I always cover my OTA when bringing it inside and first allow it to warm up slowly in the cold kitchen, then only later moving it into the warmer living room until packing the OTA away the next day. This practice helps prevent the rapid warming of the 'scope when brought inside and associated condensation issues to the primary mirror. My OTA is stored in its original packing plus polystyrene chips and any packets of silica gel I find, I keep the eyepiece fitted on the OTA when in storage.

Hope this helps

Cheers,

Steve

[SteveNickolls]

P.S. I bring the 'scope in with the front cap already fitted in place.

Cheers,

Steve

[mattytheshark]

That's great advice, thanks!

Is it likely then that the condensation on the primary mirror is due to bringing in to the warm house uncaped?

[VigdisVZ]

http://www.firstlightoptics.com/adaptors/flo-125-2-inch-desiccant-cap.html

[M4lcs67]

I have a C9.25, but since I got it a few months ago I have only had a couple of opportunities to use it. However these large SCT's are a problem is some respects with dewing up. Blimey mine was doing it back in the summer with a dewsheild on.. I have bought a heatertape and controller, but I have yet to try them out. As Keith mentioned earlier, put the bag of sillica gel that came in the bag with the ota in, inside your case to keep it dry inside. Certainly when you bring to tube inside, just leave it somewhere to dry out naturally with the covers off then once the ambient temperature in the room has meant that the temp inside the tube has equalized, you should be free of moisture. Then you will be ok to put the covers back on.

However I wouldn't put it next to a radiator, as you don't want to be drying things out too quickly. Just keep it somewhere out of the way to dry naturally.

[damnut]

Just traded my 10" SCT. Never had issue with dew on the primary. Good advice above. I always used the dewshield, however on one occasion the whole of the scope iced up!.. Bring in pointing down all drips off downward, just avoid any of the electronics if on forks.

[brantuk]

I would stand an SCT horizontal to dry out if you have dew inside it. You don't want water to form on the primary nor the inside of the corrector plate or it will smear. Better to have a small puddle drying out in the tube.

As mentioned - don't dry it out next to a radiator - sudden change in temperature causes rapid expansion/contraction in all materials and you don't want to be spoiling the optical figure or cracking lenses/corrector plates.