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effects of dew?

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Last night I noticed a fairly sudden deterioration in what I could see though my scope (reflector) - I was looking at albireo and over the space of a few minutes it went from being fairly clear to having a definite double misty halo - visually the sky still looked clear so I don't think it was high level cloud / mist.

So my newbie questions are....

- Is that what you would typically expect to see if the secondary or primary had dew'ed (if thats the word) ?

- If so, whats the best thing to do when it happens? and whats the best way of preventing it in the first place?

- If not - what other causes could there be?



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Dewing can certainly happen very quickly and views become dim and fuzzy as a result. You should be able to see it on the objective lens/mirror if it's a problem. You can dry a refractor lens with a hair dryer but a reflector is somewhat more tricky. The best option might be to extend the length of the tube by adding a home-made dew shield. I believe camping mats are the preferred material for doing so.


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You can buy electric dew conrtol boxes that heat a cloth covered wire tape. works a treat. be carefull with hairdryers if 240v. and do not heat refractor lenses without checking the material used , some should never be heated hot and fast..

cheers steve :grin:

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Things usually dew up in the following order:

1) Telrad/finder

2) Eyepiece

3) Secondary

4) Primary

The secondary may dew before the eyepiece if you are swapping eyepieces a lot and storing them in a box when unused. If you store your eyepiece exposed to the air with the caps off then they will dew at the same time as the Telrad. It's unusual for the primary to dew over. Secondary dewing usually manifests itself as image dimming. Eyepiece dewing usually leads to the halo effect. The eyepiece can go more suddenly than the secondary. Some eyepieces are worse than others (those with recessed eye lenses are better). The proximity of your eye can induce dewing (I don't know if it's moisture from breath or from the eye itself).

Eyepieces can be kept warm in a box. Returned to the box often enough and you should be good. Heater strips can be used but may be a pain to work with. Extending the tube with a dew-shield can help keep the secondary dry for longer. You can remove dew from optics with a rocket blower (those camera cleaning things). But you need a lot of air and so a lot of pumping and a big blower. The dew will come back quickly. It is possible to heat the secondary but unless you have an enclosure behind it, there will be a huge heat plume as soon as you turn on the heater (even at a low setting).

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Thanks so much for the advice everyone - what I saw sounds most like the eyepiece halo effect umadog mentioned - and the finder was definitely also getting more difficult to use. I hadn't been swapping eyepieces much - in future I'll be sure to cap them in between viewings, and I'm going to look into making a mini-dewshield for the finder.

Once it happened I basically had to give up for the night - so with clear nights so rare in the UK, keeping the dew at bay is going to be key to making the most of the limited opportunities.

Also I have to admit to a bit of confusion around dew heaters - seems everyone goes to such efforts to cool scopes down before viewing that introducing some heat into the equation must have some negative effects. I'm beginning to realise that this astronomy lark is full of compromises and balancing of competing bits of physics!!

Thanks guys.

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You're correct about cooling. With a Newtonian the key to stable images is making sure the primary mirror is cool. You want it to be within about 2 degrees C of ambient. If you defocus a lot on a bright star right when the mirror is warm then you will see the convection cells drifting across the surface. When the mirror is cool these go away. You'll see that. You can now defocus a lot on a bright star and stick your hand in front of the tube opening. You will see a heat plume coming off your hand.

With the dew heaters you're heating the eyepiece. I've heard this makes less of a difference to the image, but I don't know why. They key is to warm it gently and not overdo it. Ditto with the secondary. Even if you do overdo it a bit, it's better than dewy optics that you can't see through.

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