Dew Problem

[lazjen (Chris)]

Last night I had an interesting dew problem that I wasn't expecting.

The setup:

GSO RC10, no dew shield, no dew heaters, cooling fans always on (I leave them running pushing air from the back to out the front on the theory that the air movement should stop the dew settling).

This is what I normally do and so far it's been ok, even on other dewy nights.

Last night however, the primary mirror (and not the secondary!) started fogging up. I hadn't noticed this happening before.

Given that there was nothing more unusual about where I was pointing in the sky to any other (dewy) night, I'm trying to work out what happened.

1. It's possible that on other dewy nights I might not have noticed this and had other problems I was working on distract me. While it's possible, I do remember explicitly looking on occasion when there's been bad dew and not seeing any problems on the primary mirror (or secondary).

2. Maybe it was really heavier dew than I had so far encountered and the airflow from the fans wasn't enough to counter. Everything was very wet last night, but it didn't strike me as unusual from any other dewy night. However I only have subjective data here, so I can't say for sure.

3. The cooling fans cooled too much reducing the surface temperature too low.

4. Something else.

As the dew made it to the primary mirror, I can't see that a dew shield will help. If it's the fans, I can try turning them off and see how that goes.

Any thoughts?

[glend (Glen)]

I used to have a dew problem but think I have it beat now. I have a GSO 12" dob and leave the fan running all the time. The late model GSO dobs seem to have a better fan than the earlier ones and no image vibration problems-so far. I have a dew/light shield but rarely have to use it now. I leave my dob in my unheated shed so its always cooled to the outside ambient temperature, and this helps a lot. Do you store the scope inside a heated structure, if so the mirrors will take a long time to drop in temp and will likely fog up.

[lazjen (Chris)]

There's 3 little fans on the RC10, 12V powered. When I take the cover off the top of the scope, I've been careful to have these fans going. Like you, I haven't noticed any vibration issues with them. Good to hear others are running fans all the time then.

[naskies (Dave)]

My RC8's primary dews up quite easily in winter when pointing near zenith. I have a dew strap around the extension tube between the scope and focuser - works quite well.

Before I did that, I've even had a thick layer of frost over the primary when imaging under dark skies out west.

A dew shield will help by reducing the amount of time that the primary is exposed to the cold night sky.

[lazjen (Chris)]

Do you run the fans on you RC8 as well?

[naskies (Dave)]

The RC8 doesn't have fans for the primary. I uncover the scope once the sun sets far enough for the scope to be in shade, and leave the dew heaters off until astronomical twilight.

[lazjen (Chris)]

I'm going to get/make a dew strap and use it like your setup Dave. Thanks for the tip.

[brian nordstrom (As avatar)]

, Chris the dew must have been really bad for that to happen , was it moisture rising up from the ground getting into the tube ? .
I have never had a problem with my Takahashi M210's primary dewing up and it gets to 95% + humidity up here , yes dew straps work and hope you sort it out .
Brian.

[lazjen (Chris)]

I really don't know, it's possible I guess.

Thinking about the conditions more: we've had scattered showers today, mostly in the morning. It's possible it was extra humid last night with a change coming through.

Or maybe it was because the lawn got mowed that afternoon. Shock value.

[JohnH]

Hi, just to let you know - I have the RC10 as well and on the humid, cold nights we had some 2-3 weeks back (Sydney - clear after rain) both the primary and secondary dew up. I have had this happen with fans running and with a dew shield fitted. Granted it was very heavy dew - the outside of the ota was streaming but I am resigned to a secondary heater and I think I will also put a band on the focuser as per Daves suggestion.