Mirrors, cooling and Dob's

[Blackant]

I have a rather silly beginners question, but does a dob cool down faster with it's dust cover off? I'm guessing it does, as it would open up the tube to the air that it is trying to reach equillibrium with.

The reason i'm asking is that the last few sessions I've had a fair bit of dew. My routine for setting up is to take all the covers off, carry the scope outside and then turn the fan on. I leave it cooling for about half an hour while I read the kids a story and put them into bed, and then head out.

My last session the finder scope and the telrad were dewed over before I even got out there, and the secondary mirror only lasted for an hour before that succumbed as well. I've made a foam cover as a dew protector for the telrad, and one out of a plastic pot plant pot for the finder scope (I'm also not taking the cover off those before I'm ready to observe, that was a real no brainer on my behalf :P) but I'm wondering if I could forestall the dew by leaving the cover on the dob too without affecting cooling too much. Do I need to let my eyepieces cool down as well?

Thanks all

Ant

[mental4astro]

Dew can be a real problem.

EPs, don't let them cool down as they will fog up as soon as you put your eye to them. Doesn't mean you warm them, rather keep them from cooling by keeping them in a foam lined case. Should they start to fog up, look at investing a few bob in a 12V portable hairdrier. Only bit of high-tech gadgetry I have plus a 12V car jumpstart pack.

What you did with the finder & telrad, :thumbsup:, good thinking. When they start to succumb to dew, see above.

The same for the secondary. A short blast of the drier is all you can do. Just blow of & a smidgen of warming.

If your primary dews up in a solid tube outfit, pack up & hassel the better half. Just allow the dew to dry of the mirror before closing the tude.

Do keep the dust cover off to allow the scope to cool, just have it tilted a little closer to horizontal so air just doesn't pool in the bottom and not exchange. A little fan speeds up the process.

[erick]

Unfortunately, Ant, you need to spend some money on HEAT! Everywhere but the primary mirror. OK, very mild heat - a few degrees above ambient is all you need.

Lots of threads on dew suppression so search through the fora. Waving hairdryers around - yes we have all done that. I think that is an interim step to something more sophisticated. Be aware that a 12V hairdryer will probably want to pull 8A or more of current and it still won't feel particularly hot (given that is around 100W compared to a mains powered hairdryer of 1500-2400W).

Heater straps and a heater controller and a reasonable size 12V battery or 12V power supply is the way to go.

Though I favour the heated eyepiece box, many cap their eyepieces between uses and pop them into coat pockets to keep them a bit above ambient.

Finderscope objectives will dew up. I've never found dew shields super effective so a heating strap on that is a must.

The secondary - I think you just have to go the whole way and fit a small heater on the back of it. This one:-

http://www.sdmtelescopes.com.au/shop/product_info.php?cPath=27&products_id=46

I created a thread on installing one on the back of a 12" scope secondary mirror. A bit of work with some fun getting the mirror out of the holder - but it can be done, with fantastic results.

Eric :)

[mental4astro]

Come to think of it Eric, I'm now seeing the writing on the wall.

[erick]

Woops, I came over all "evangelical"? My experience is that, once you have active dew control, you never want to go back.

[Blackant]

Thanks for the tips there Eric and Alexander, I thought I may have to think about some heater's at some point. The idea of the hair drier sounds tempting to start with though.

There are so many exciting accessories I'd rather spend my money on such as Naglers, panoptics, UHC filters, maybe even a skywatcher 80mmED grab and go rather than heaters :D

But of course, none of those are much point if whenever I set up everything fogs over, and where I live is a very wet place for a lot of the year :lol:

Regards

Ant