Strange dew problems

[wouter]

Hi all,

I know there are already a lot of topics about the dew problem, so I'm sorry to open another one.

I couldn't find all the answers in the existing topics I read.

Well, I have just purchased a celestron C9.25 on a CG5, it's my first experience with a SCTelescope.

I have been able to use it about 5 times in the last 3 weeks.

At the moment it's quite cold in Belgium, about 4C in the early evening, going to 1C or just freezing at night. (40F to 32F)

Also the air is very humid, usually around 80% or more.

So it's natural dew forms quite quickly under such conditions.

However I'm using a dew shield and I give all my gear about 30min to 1 hour time to cool down.

I always put my scope outside with all the caps on and position the tube horizontaly.

Still I was having condensation on the corrector plate pretty fast, which I don't think is strange.

But, After using a bit more, the primary mirror became to condensate a lot as well.

Also did the finderscope.

I thought that is a strange since I thought that SCT's are a sealed tube.

Also I'm sure it was not the reflection of the dew on the corrector lens I was seeing.

I didn't take off the star diagonal once outside so it couldn't have come from that way either I suppose.

The visual back and all seems to be sealed very firmly.

My ep's are ok, because I just keep the ones I use in my pockets, with the caps on.

My filters are another problem...maybe I should keep those in my pockets as well?

Can anyone tell me why the primary mirror was condensating so quicky as well?

What would be the best way to deal with it under such cold, damp circumstances?

I'm thinking to buy a Dew not heating ring to place around the corrector lens.

But how to deal with the primary mirror and the eyepiece sitting on the star diagonal?

Can I buy heating for that too? Or will just heating for the corrector lens suffice?

I'd like to know since if I have to buy more than one Dew not, I'll have to buy a controller with multiple outputs.

One more question, how to protect my Canon 550D DSLR? I wanted to use it last time on a T adapter with barlow lens on, but finally didn't have the time for it.

I left it outside with all the caps on and the barlow lens which should seal the tube, but when I brought it back in and attached the camera lens again.

The view through the finder was very much blurred as well as the pictures I took immediately after.

Would it be the mirror that had condesated? or something with the sensor?

I seriously don't want to damage my camera. (It's my dad's one!)

Are these Eos cameras even designed to operate around the freezing point or lower?

How can I protect it even more?

Sorry, that's a lot of questions, but I want to take care of all my gear as much as possible and the store I sold my scope from isn't very knowledgable I'm finding out...

If you have any advice or tips or if you found something I may be doing wrong, please let me know.

Thanks a lot

Wouter

[VigdisVZ]

No idea on your primary mirror, not sure if the dew band on the corrector plate will spread enough heat to the primary aswell.

There are products for DSLR's and finderscopes aswell, here is one example:

http://www.kendrickastro.com/astro/dew_cameracozy.html

No expert, so others with SCT experience will probably fill in the blanks for you.

[Islander]

You can buy or make a heater tape for the diagonal but for the mirrors, fit a desiccant cap to dry out the air inside the tube

[bus_ter]

I have the same scope and mount combo, and I too am suffering dew problems. In fact I started one of the earlier dew topics.

Now I haven't had dew on the mirror, but I have had dew on the inside of the front collector glass. This has left permanent staining that I can see no way to clean without dismantling the scope, and I really don't want to do that! My scope is only a few months old and I'm a little annoyed that it has staining already, and I'm very keen to prevent it from getting worse.

In regard to the tube being a sealed unit, well it can't be completely sealed as where would the air go as you move the mirror forward and backwards to focus? This has to slightly push out and suck in air?

Also even if it was sealed when you have the caps on, you're still bringing a tube of warm air outside into the cold. Inside the house the air holds it's moisture, but when it cools down outside some of that moisture can condense on the inside of the scope on the cold surfaces.

Is it best to leave the caps on and let the warm air cool slowly (and replace the cap with an EP), or should you take both caps off and let the cold air circulate inside the tube during cool off?

[bus_ter]

This might help you too.

http://www.optcorp.com/product.aspx?pid=1959

[Islander]

I have the same scope and mount combo, and I too am suffering dew problems. In fact I started one of the earlier dew topics.

Now I haven't had dew on the mirror, but I have had dew on the inside of the front collector glass. This has left permanent staining that I can see no way to clean without dismantling the scope, and I really don't want to do that! My scope is only a few months old and I'm a little annoyed that it has staining already, and I'm very keen to prevent it from getting worse.

In regard to the tube being a sealed unit, well it can't be completely sealed as where would the air go as you move the mirror forward and backwards to focus? This has to slightly push out and suck in air?

Also even if it was sealed when you have the caps on, you're still bringing a tube of warm air outside into the cold. Inside the house the air holds it's moisture, but when it cools down outside some of that moisture can condense on the inside of the scope on the cold surfaces.

Is it best to leave the caps on and let the warm air cool slowly (and replace the cap with an EP), or should you take both caps off and let the cold air circulate inside the tube during cool off?

Again, use a desiccant cap

[wouter]

Thanks for that link bus_ter, there's some good information and advice in there.

Islander, that desiccant cap looks interesting!

Can anyone else confirm this is a good product?

How about my camera question?

How can I protect the inside mirror / sensor more and how to make it more resistant to dew?

Thank you guys!

[Islander]

I don't think FLO would sell them if they didn't work - it's a simple enough principle. In fact you could probably make your own out of a 35mm film canister. They contain sachets of silica gel which is a widely used and very effective desiccant - the sachets can be reused by drying them out periodically on a hot radiator or in a low oven. Use of a desiccant will remove any moisture from the air in the scope tube which is exactly what you're trying to achieve.

As far as the camera goes, if you're connecting it to a scope with dry air in the tube then there shouldn't be a significant problem. The sensor shouldn't dew up anyway - they get warm in operation.

[Herbig]

Yes you can make a dessicant cap with a 35mm film canister, drill a few holes in the end and put a couple of small sachets of silica gel inside and put the lid back on. I have them on both the SCT's I we have.

An SCT is not sealed, the primary mirror has a hole in it, how do you think the light gets from the secondary to the visual back! Once you take of the dust cap from the visual back the inside of the tube is open to the air