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Tommohawk

Dew and Turbulence: Heating or Fans or Both?

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Hi all.

I'm hoping for some advice on a subject which I know has had a lot of air time, but which I'm still wrestling with.

Firsty, I know I get dew on my secondary mirror and it has wrecked many a DSO photgraphy session. I've fitted a home made heater, and this seems to fix it. It's insulated with neoprene, and runs at a very low wattage (about 0.4W) , so hopefully not too much turbulence.

Next step is to do the same to the guide scope, which also mists up after about an hour, and then maybe the primary with a very low wattage.

The main puzzle for me is this: Some folk say the mirrors are huge heat reserves, and take ages to cool down, during which time they cause turbulence. But other folk say the mirrors drop in temperature rapidly and go below the dew point, and then suffer condensation. I suspect theres some truth in both statements, but this doesnt simplify how to deal with it.

Probably the main consideration is that the primary is much less ikely to suffer dew than the secondary, and so they have different problems and solutions.

It would be easy to say that turbulence is in the eye of the beholder, but for sure when I'm focussing - using the camera and Live View with high screen mag - there appears to be turbulence aplenty even after an hour of cooling and before putting the secondary heater on. Of course this could be atmospheric, rather than tube currents.

Rather than go through all my (muddy) thinking, can I just ask some direct questions please which relate to all this.

1. Is the small amount of heat on my secondary likely to cause any significant turbulence?

2. Is the turbulence that I see on the screen likely to be caused by the OTA?

3. I havent actually seen dew on the primary - but I've read that low grade condensation can affect the reflection. Is heating of the primary maybe just unneccesary?

4. If I heat the guidescope with a belt at about 2.5W, might this adversely affect the guiding property?

5. If I use an exhaust fan to cool the primary, is there any prospect that the air current will:

a. increase dew on the primary and/or secondary by drawing in more damp air?

b.  reduce dew on the primary and/or secondary by evaporation?

I'd be really grateful for any help with this. My next step is to wire up some kind of controller, and it would be useful to know what I'm likely to need to run before doing this.

Many thanks

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Oh, thats disappointing! 0 replies! Maybe I overdid the questions.

Could I settle for some thoughts on just one question please!

Is a rear fan likely to prevent dewing, and if so should it push air up the tube, or draw it down?

Would really appreciate some input!

Thanks

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Hi Tommo,

I think the fan draws the air from the tube. The only reason I think this is that I just turned mine on to see, and I could feel the air from behind the tube when I put my hand close to it. I presume if it was blowing up the tube I wouldn't feel it but I'll stand corrected for anyone better informed?

Also I think a rear fan will prevent dew but I don't think it will get rid of it once the dew has set in. In general the long tube is the biggest dew preventer as it acts as a long dew shield so this plus the fan should be ample protection. This is only bits of info I've gleaned from what I've read though.

Edited by Scooot

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hi Richard

OK thanks - and from your other thread you do have a dew issue on the secondary I think. Have you tried leaving the fan running to stop this?

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Just found this thread, http://stargazerslounge.com/topic/164616-suck-or-blow/

It won't help though lol.

Yes the secondary still dews with the primary fan on in very damp conditions. This is my third winter with the scope and this is the first year I've had a problem with the secondary. I think its because I've taken up sketching so the scope is left in the same position for long periods of time , and I've been sketching high objects. Previously I only viewed things mostly for a few minutes or so I think moving the scope around more regularly delayed the dewing.

Bit more info here as well

http://www.fpi-protostar.com/bgreer/fanselect.htm

Edited by Scooot

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OK, well I've already fitted the secondary heater so I might as well leave that running as it seems to work well even set very low.

Thanks for the link - I read loads but missed that one.... and your right - it sucks. (!) One useful  thing I discovered though - I thought I could reverse it by reversing polarity but this isnt so by the look of it. So I may not have any choice.

Do you notice any discernible difference at all with the fan in terms of seeing?

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OK, well I've already fitted the secondary heater so I might as well leave that running as it seems to work well even set very low.

Thanks for the link - I read loads but missed that one.... and your right - it sucks. (!) One useful thing I discovered though - I thought I could reverse it by reversing polarity but this isnt so by the look of it. So I may not have any choice.

Do you notice any discernible difference at all with the fan in terms of seeing?

I keep the scope indoors so not surprisingly if I use it straight away when it's cold outside it's very poor, but it improves more quickly with the fan on. Once the scope is cooled its difficult to differentiate between the cause of any wobbliness, it could just as easily be my skies as opposed to the scope but I leave it on in the hope it will delay the secondary from dewing a bit longer.

Oh and I haven't used the secondary heater yet, I'm still faffing about putting the various bits together.

Edited by Scooot
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Oh, thats disappointing! 0 replies! Maybe I overdid the questions.

Could I settle for some thoughts on just one question please!

Is a rear fan likely to prevent dewing, and if so should it push air up the tube, or draw it down?

Would really appreciate some input!

Thanks

Hi Tommo.  Lots of differing ideas on that, you may need to experiment to see what works for you, however.....

I know that the large Dob users in my club have said that a rear fan blowing towards the back of the primary reduces or prevents dew when the conditions are bad.  Our club 16" Dob has two fans, one blowing on the back of the primary, the other across the front surface.  

I think that if the air is moving, dew is less likely, and also cool the mirror to ambient temp.

Hope you sort it, Ed.

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Ok thanks Richard and Ed.

So the answer I think will be secondary heater, low setting, fan blowing up the tube but with the option to reverse it. The primary seems fairly resistant to dew, so hopefully the air movement should discourage dew and also prevent the mirror falling below ambient.

Main thing, as you've said before Ed I think, is to get on and enjoy it! Look through it rather than at it, I think someone said!

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