Refractor Dewing

Hi:

I am looking for some thoughts, suggestions, ideas about how to deal with a refractor objective that has dewed up. Last night was a typical example. I was viewing in the mountains where it is normally very dry but last night most everything was dewing up and it finally suddenly turned to fog...

I put everything away but when I looked at the objective of my AT-102ED, I saw that not only was there dew on the outer surface but also a spot about 2 inches in diameter had formed between the two elements of the object. I find this quite disconcerting and I have images of the trapped dew sitting there working away on the coatings and glass.

If this happens at home, I will normally use a hair drier to provide enough heat to evaporate the dew. However last night I was in my motorhome without a working hair drier so I finally ended up removing the cell from the tube and using the heat from the forced air heating system to dry out the objective.

So, my question is I am correct in being concerned about dew between the elements of the objective or is it better to just let it evaporate with time? I am quite sure the dew had formed between the elements prior to bringing the scope inside.

Thanks for your help.

Jon Isaacs

The coatings can take it but you don't want the dew to drip and then leave a run mark on the lens especially between the lenses. I find it more likely however that you had the dew not in between the lenses but on the other side of the doublet or triplet i.e. moisture in the OTA condensing on the exposed surface. It is very tricky to examine what is going on inside a doublet esp. at night etc. So my suspicion is that you *thought* it was in between the doublets, and *maybe* it was, but it is *as likely* that it was on the "far side" of the doublet on the side exposed to the air of the tube. In that case cleaning is fairly easy the whole lens assembly will typically come off without much hassle...easier to disassemble (leaving all the lenses in place in the cell) than something like a 24 Pan, for example.

On a four inch your options include:

1. adding exterior insulation
2. taking it into your car and getting it toasty in front of the heater
3. a dew strap

As an sct guy dew straps come naturally to me and it is much easier to keep my 4" and my 5" refractors dew-free than my 14" SCT. All you need is a power supply that's up to the job. I'm including a picture of my simple but works power supply; note that I installed an in-line fuse in the cord. But this is more than enough for a four inch refractor dew strap.

Greg N

Attached Image:

Hi Jon,

My understanding is that the AT line comes from Astronomics and if I recall correctly, that's where you bought the AT102ED. What do the good people at Astronomics have to say about this, can they help?


Mark Costello
Matthews, NC