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stopping big mirrors dewing

Daniel-K

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Daniel-K

Daniel-K

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I have a 350p flexi and I have a shroud and a dew shield but still finding my primary is dewing over any ideas to stop this? Just ordered some morw flocking paper to do the lower ota.any one else had this problem with a big mirror

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sbooder

sbooder

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I used to have problems with the 10" in a full tube on really cold nights but I found with the addition of flocking and my foam fan baffle it can get as cold and dewy as it likes the mirror stays pristine.

Also do you have any fans or at least one on the back?  These are not just useful for cooling the mirror at the begging of a session but also for keeping the air moving if you see your Primary start to dew a little.

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Mike73

Mike73

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You would think that a bigger primary would dew easily but like Steve I've never had a problem with my 16" but I do keep my fans running all night.

If it happens quite often then I'd look into some heater pads, if it only happens when it gets really dewey then just keep a hair dryer handy.

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kerrylewis

kerrylewis

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I've not got a fan and everything dewed up horrendously last night. I've just bought a cheap 12v hair dryer from the local caravan place which stocks loads of such things. Reasonable forecast tonight so will no doubt try it out.

One of the advantages of Dobs is that there is no need for power so I'm trying to avoid having to install anything that needs permanent power and the occasional use of the dryer is a compromise.

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Moonshane

Moonshane

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the thing with dew is that it's easier to prevent it than sort it out sometimes. I don't normally have issues with dew on any of my (solid tube) scopes but use a long dew shield, occasionally a fan and never leave it pointed at the sky when not in use. last night was very damp with a lot of moisture in the air though.

a fan running constantly may help but sometimes you might find it overcools the mirror (rare to be honest) and this can create issues too.

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crashtestdummy

crashtestdummy

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My 16" lightbridge used to do this quite a lot and as others have said,a 12v hairdryer is the answer.also try to keep your scope horizontal inbetween viewing rather than vertical.I was going to put a horizontal fan blowing across the mirror but never got round to it.it's literally the aperture that the problem as dew just falls down the tube.

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Daniel-K

Daniel-K

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Strange I never had thw issue with the 16" but maybe it has something to do with the open cell design on the back? I have a few fans about I may attempt to rig one up to the back. The design of the mirror and cell won't allow me to use the Kendrick system already looked at this :(

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emadmoussa

emadmoussa

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Strange I never had thw issue with the 16" but maybe it has something to do with the open cell design on the back? I have a few fans about I may attempt to rig one up to the back. The design of the mirror and cell won't allow me to use the Kendrick system already looked at this :(

I was thinking that actually...the open cell. My 12" Flextube primary got dewed up before even though I had a dew shield and a shroud. I was pretty puzzled at how this could possibly happen...

Have you considered closing the scope rear?

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emadmoussa

emadmoussa

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Plus, if you're talking about last night...it was awfully humid that you can actually cut the air with a knife like butter.

I don't mind dewing and all that, but I noticed that every time the mirror got dewed up, there are always some tiny smudges left over on the mirror surface after the dew has gone.

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mitchelln

mitchelln

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My 300P has fitting holes for a fan on the mirror cell, but I have wondered if the fan should blow or suck? Will blowing over the back of the mirror introduce a dust problem? Will sucking just draw down damp air into the tube? Option 3 of course is to not muck about with a fan ;) I have a 12V hair drier, but my how it draws the amps. Need a 2nd tank just to run that.

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faulksy

faulksy

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hi dan, i thought friday was a 1 off, i was out last night and no problems with the primary, all i can think of is your mirror is thin and it cools quicker than the air in the tube, maybe a fan would work mate, i have never put heaters on the primary but i would of thought the heat would have to be distributed very equally or you will have colimation problems, just a thought have you tried viewing with out the shroud maybe more air will pass your primary

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Bart

Bart

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Don't ask me why, but I found on more than one occasion that using a fan constantly through a session brought on dew on the 10". There may have been other coincidental issues ongoing but I put it down to the fan for some reason. Only occasionally use it at all now and only then to cool down quickly. Mind you, its kept in the garage so it's usally ready to rock.

Barry

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Stu

Stu

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Out of interest, does anyone use fans blowing across the face of the mirror to break up the boundary layer? Presumably not whilst observing but may be useful whilst cooling the scope?

Stu

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faulksy

faulksy

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Don't ask me why, but I found on more than one occasion that using a fan constantly through a session brought on dew on the 10". There may have been other coincidental issues ongoing but I put it down to the fan for some reason. Only occasionally use it at all now and only then to cool down quickly. Mind you, its kept in the garage so it's usally ready to rock.

Barry

maybe you fan blows the wrong way, or does mine, mine sucks air out of the tube, which way should it be ?

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sbooder

sbooder

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maybe you fan blows the wrong way, or does mine, mine sucks air out of the tube, which way should it be ?

I have my fan blowing from behind the mirror on every newt I have owned and it has never been a problem, I get no more dust than any other scope I have seen without a fan or with a fan sucking air.

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mitchelln

mitchelln

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While we're at it guys, can somebody direct me where to buy a fan for my 300P please?

I can see there are screw bits on the primary cell, assuming they could work for a fan.. :)

Maplin is probably your best bet. They'll also have all the bits to connect it and a nice grille :)

http://www.maplin.co.uk/p/dc-12v-80mm-sleeve-bearing-fan-rg03d

http://www.maplin.co.uk/p/80mm-plastic-finger-guard-ju26d

They also do ones with ball bearings and extra quiet blades.

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