Combatting dew on primary

[wiseman]

I now have bl77dy dew on the primary.

I already have a 4 way controller, and a secondary heater.

So I think all I want is some tape, for an 8 inch scope. IS that corrct please guys.

So, does this just strap onto the OTA where the lens is attached to the ota or is it more complicated?

I am also getting the rigel quickfinder dewing as well, so I guess I need a special heater for that too?

So that will leave one port on the controller free, for no doubt the eyepiece, which will be fogging up next.

I cant believe I need so much kit, all I want to do is look at the s$$ing stars, not go diving on mixed gasses to 50 m with a rebreather!

Thanks in advance

[John]

Are you using a light shroud ? - I found that doubled as a dew preventer for the primary when I had a truss tube dob.

Sorry that you are having such dew issues - some areas are more prone to this than others. Are you in a river valley ?. We live 300 feet up on a hill and dew is much less of an issue than when I've been observing at lower levels and near a river.

[Adz]

A cheap amount of foam might do the trick. Search on the forum and it should turn up a result for some good stuff you can buy at a warehouse type price. I got some and managed to do a due/light shield (double piece) for my scope, a cover for my telrad plane and my binoculars.

these pics might help

[NGC 1502]

As mentioned, a shroud on your SW 200 Flex tube will help. I've made a foam dew shield for my Dob as in post # 3, mainly to combat dewing of my secondary on damp nights.

I also use a 12v "hair dryer" (camping shop or some astronomy retailers) works to undew eyepieces / finder or gets pointed down the focuser onto the secondary if the dewshield wasn't enough.

Hope you sort it, Ed.

[wiseman]

Sorry, I should have said I already have a dew shield at the top ( some camping mat foam), and have made a shroud over the truss section.

We live in the wettest part of France, and although is farther south than Cannes, and nice and arm, it does have a very high humidity, and I think that's whats causing the problem, so I think I will need a heater, anyone got any experience as to how they fit on please?

Thanks

Wise

[brantuk]

To keep the cool air moving through a dob and help reduce likelihood of dew - some folks fix a small fan behind the primary mirror cell. Something about the size of a computer psu fan would do it. Check out the Dob Owners group there's some pics of that mod somewhere in there

[wiseman]

aHA, I just happen to have an old pc fan unit that I have hidden from my "throw it away" wife"

" scurries off to dob user group"

[wiseman]

err, hang on brantuk, surely ( and I know you dont like me calling you Shirley), the fan will cool it down, and I need it heated up beyond the dew point don't I?

I guess the fan is used to cool down a big lens when you bring an OTA from indoors and set it up outside in the cold?

As usual, everything I say may very well be rubbish

[michael.h.f.wilkinson]

The moving air with tend to remove dew, as brantuk said. Still air is generally the culprit. Otherwise a small strap directly attached to the secondary would have most impact, but the wire would add to secondary obstruction.

[brantuk]

"the fan will cool it down, and I need it heated up"

This is an age old conundrum that allways puzzled me when I started out in this lark. I thought it comical that some folks have a dew band and a fan - contrary or what? lol

As Michael says - still air is the culprit on the primary. In a "solid tube" or a "shrouded" truss dob, dew has to be quite heavy to get to the well protected main mirror. In which case a fan will do the job.

The secondary is much more exposed though and quite prone to even light dew. So a secondary heater is the tool of choice. I've never needed a primary dew band

[wiseman]

Thanks guys, I have a secondary heater ( not installed yet due to warranty issues), however, I have noticed that looking down the tube at the end of the session that the primary is dewed as well.

I am prepared to try anything as long as it doesn't involve hammers, so let me check I have this right.....

The fan is on the outside of the tube ( as putting it inside would be daft).

Blowing air on the outside of the tube, stops dew forming on the inside?

I take the point of the still air causing dew, but the still air is on the inside of the tube, does moving the air on the outside stop the dew on the inside?

As if it wasn't hard enough to get everything working, now its confusing as well.

[brantuk]

You mount it centrally on the back end of the scope blowing over the back of the primary and up the inside of the tube. This moves the air out of the top open end.

[brantuk]

Here's a pic of one from dob mods:

[libraryman]

The small fan from a pc works well, and quickly on my primary, just a couple of points that i noted.

Running the fan to draw air up past the mirror induced some air currents that were visible at higher mags! i'm going to reverse mine!

the other point is that i used rubber isolation buttons to prvent vibration!

Ray

[wiseman]

OK i will give it a whirl, my skywatcher looks more sealed than the photo above, so I couldn't see how the air got inside, I will investigate!