How to stop dew on secondary mirror?

[JamesC]

I'm interested in any advice about how to stop dew forming on the secondary mirror of my flextube. I've already rigged together a shroud made from a sleeping mat (I keep meaning to make a better one out of neoprene). Saturday night was really bad for dew. I let my telescope cool for an hour or so (lids on) while I played with my new 15x70s. I then observed with the 'scope until that was brought to a halt by dew after maybe another hour.

I'm not sure I want to deal with batteries, so I'm half wondering about gluing some thin cork to the back of the secondary mirror. Is this a good idea, or will it just end up giving me really bad tube currents? I'll probably just put up with the dew as I'm thinking about selling the 'scope in 6-12 month's time and I don't want to affect the re-sale value.

[James]

I just put a dewshield on the front of my OTA - anything will do such as old camping mat. Works a treat, my secondary hasn't dewed up since.

James

[NGC 1502]

Hi, you already have a shroud, but a tube extension made from "camping mat"

can help. Also a 12v "hair dryer" would be useful.

But the best option is a dew heater on the secondary, keeps it just above

ambient temperature.

Regards, Ed.

[JamesC]

OK, well like I say I was thinking of re-doing my shroud, so I guess I could do a dew shield at the same time.

[brantuk]

If you use a hairdryer direct it round the tube rather than directly on the mirror (in case any rapid temperature change cracks it).

[JamesC]

OK, thanks for the tips.

[Astro_Baby]

A dewshield will fix it almost for sure. I'd avoid the sceondary mirror heater approach (much as I love technology) for the following reasons;

1/ Its expensive

2/ Your create an extra big spider vane on one side cos of the wire leading to the heater

3/ Its one more thing eating power

4/ The big one - over on CN some time ago someone was using a secondary heater that got too warm. The glue holding the secondary let go, secondary mirror did a swan dive into the primary - ouch !

I know we'd all like to think it wouldnt happen to us but I know I get distracted while looking through the eyepiece, its all too easy to turn something on/up(or even off) and forget about it until the moment of doom

A dewshield wil stop it and failing that a quick squirt with a 12v hairdryer will make it go away. Another trick is to hold the secondary mirror holder with your hand for a few seconds. The heat from your hand will make a light dew vanish for a bit.