Dew - ways to prevent it, / left dirt on mirrors

[Moonshane]

A couple of points to add to the excellent advice:

• Make the dew shield at least as long as the aperture is wide - in your case though I'd agree that about 6 inches will be enough.
  
• Make it slightly shaped (i.e. wider at the top end than the bottom) to ensure it does not intrude into the field of view.
  
• My secondary has only ever dewed up once, when I left the plug out of the empty focuser and went in for something to eat. I find leaving a plug or eyepiece in helps with preventing this but the dewshield will do so too.
  
• I wash my mirrors in their cells unless there's a fan attached etc. The amount of dew won't be substantially less than a good soaking through washing. As long as you ensure the cell is dry afterwards there's no issues. It's possibly better to remove the mirror cell but not essential.
  
• Dismantling your scope helps understand the mechanics of collimation relatively more quickly.
  

[sam mayo]

i haven't attempted collmination yet will be able to afford collminator in a few weeks so will do i then,measured my dw shield its 5.8 inches and isnt interrupting on the fov witch i thought about when i first made it,thanks for the help,

sam

[acey]

A few points I'd add to what has been said.

Dew is caused by heat radiating directly to the sky, and the way to minimise it is to minimise unnecessary exposure of glass surfaces to sky. The aim of a dewshield is to reduce the amount of sky the optics are exposed to, while not intruding on the actual field of view. So the dewshield should be as long as possible, without being too unwieldy or sagging into the lightpath. Any material that holds its shape and doesn't get soggy when wet will do. For best results the interior of the dewshield should be matt black, just like the interior of the scope, to reduce stray light.

Dew forms when heat radiates away from a surface, making it colder than the surrounding air. This won't happen if the air is flowing, because incoming air keeps the temperature equalised. So a fan blowing air steadily across a mirror (not just blowing behind it) will prevent dew. Likewise a hair-dryer (even on a cold setting) will clear a dewed mirror. On breezy nights you're much less likely to get dew forming (though still air inside the scope tube makes it not impossible) and the dewshield can usually be left off.

The glass surface most prone to dewing is the one at the top end of the tube, because this sees most sky - i.e. Newtonian secondary, refractor objective, corrector plate. In a Newtonian, heat is radiating from the "back" of the secondary (i.e. skywards facing part), cooling the whole thing, but it's the moisture on the mirrored surface that we notice. One way of further reducing secondary dewing would be to have the back covered by a material that is a poor radiator: some people have advocated silver foil, though I've never found it necessary to try this.

If a Newtonian is aimed high at the sky for long periods then the primary may be able to radiate enough heat to become dewed - it has happened to me on quite a few occasions (despite the assertions I often hear that primary mirrors rarely dew). On a truss/flex tube a hairdryer can easily be brought close to the primary to clear it, on a closed-tube it may be a bit more awkward.

Eyepieces will also dew if left exposed to the sky, so cover them when not in use. Same goes for books and anything else you might happen to leave exposed to the night sky. Paper is a much poorer radiator than glass, so books dew more slowly - but they get soggy, so it can be more of a problem.

Even if you keep your scope dew-free while observing, it may well become dewed after use, even if you cap all the optics (my scope always dews up on the drive home). In which case you just leave it open in storage until the dewing clears. Some dewspots will remain on the glass afterwards - as has been said, these spots have no real effect on views. How much long-term damage they cause through deposition of salts is a moot point: it was probably more of an issue when coatings were alumina only, most mirrors these days have protective silica overcoating. The way to see if the coating is damaged is to hold the mirror up to bright light and look at the back for "pinholing" - little bright spots where the coating has worn away and light can shine through. You would need to have an awful lot of pinholes in a mirror for it to have any significant effect on image quality.

[Aenima]

Thanks Acey.

Thats some really helpful info. And moonshane, too. Nice1

I actually have had everything including cardboard box dew up and get soggy, and find repeatedly that when a long session involves me going in for ten mins for coffee, the scope is suddenly giving me fuzzy views and no sharp focussed images. The dew seems to hold off while i'm at the eyepiece - warm breath aside - but theres always a point, about 3 or 4 am. that the dew hits really hard and i'm forced to quit sooner than i'd intended. Its when the mirror appeared to be at risk that I decided to go asap for some kind of dewshield.

Ta for all the advice folks, much appreciated.

Time to get my desk fan out.

Regards

Aenima

[sam mayo]

so my netbook isnt going to be a problem if its on because it always warm?think im going to try the hair-dryer for long sessions and thanks for the explanation knowing what causes the problem lets m know how to counteract it will have a think,sam