Dew Control; help needed

[big_dav_2001 (Davin)]

Hi Guys and Girls ,

Its been a while since i posted, and i hope you could help me out yet again.

Im looking at investing in some dew control gear, but im not sure exactly what i need.

Im looking at heating my 12-inch solid-tube GSO Dob, as well as possibly my eyepieces and Telrad finder, while the eyepiece and telrad heaters are pretty self-explainatory, i just have a few questions regarding the heating of the OTA / Primary.

- Would i need a dew heater band for my OTA, or would a dew shield work?? (or both??)... ive heard that shields only work with Cassegrains...

- What brands would you recommend, and are the different brand's parts interchangeable??... eg: a Kendrick band and a Dew-Not controller??

- Is it necessary to use a controller, or can the bands be hooked directly into the power source??

- ( stupid question alert ) Where exactly does the heater band go?? is it wrapped around the OTA down close to the primary??.. wrapped around the circumference of the primary its-self??.. or around the middle of the tube to radiate the heat evenly?? (ive also seen heaters that are glued onto the back of the primary mirror...would these be more effective??)

- Does the secondary need to be heated also??

Any help you can give would be hugely appreciated, thanks in advance

Davin

[erick (Eric)]

Davin, what have you observed does happen? What gets dewed up? The secondary mirror? The primary mirror? Both?

Jumping ahead, it is more likely that you need to provide some heating to the secondary mirror (as well as eyepieces, telrad, finderscope etc.) There are a couple of ways of doing that with heaters, one requires dismantling and fitting a heater to the rear of the mirror (depending on the style of mirror holder) and one is by fitting a specialised heater to the secondary holder. Otherwise, next best may be the occasional blast through the focusser draw tube with a hairdryer.

The primary is an entirely different matter. We spend all our time trying to get the boundary layer absent by keeping the mirror as close to ambient temperature as we can, so few people heat their primary mirror. The approach that most adopt is one or more "tangential" fans - blowing from the side directly across the top of the primary. Another easier approach that might work is to keep the primary mirror cooling fan on (located behind it, centrally), but seal off the end in some way so the airflow passes around the mirror edge and up the tube. That can help scour the air away from the mirror surface and discourage dew.

Here is an example:-

http://www.iceinspace.com.au/63-400-0-0-1-0.html

I've done an 8" with a piece of wood fitted around the fan and the knobs.

Also, an upper dew shield may help protect the secondary mirror (just like an SCT shield). But one important thing is to tip your scope down so it doesn't point to the open sky, when you are not observing - leave it in a more horizontal position. Don't encourage that primary mirror to feel cold and attract condensation!

[big_dav_2001 (Davin)]

Hi Eric, thanks for the reply mate.

So far, im not having any problems with dew on the secondary, but the primary seems to dew up pretty quick some nights, hence the thoughts of a dew heater.

So as long as i can somehow get the air moving across the surface of the primary, there would be no need for me to buy a heater??

That article seems like a good idea (and a lot cheaper than dew heater gear..lol), i might give it a try

thanks mate

Davin

[erick (Eric)]

If you search, you'll find where people have cut holes in the tube just near the mirror and fitted a fan to blow across the mirror's surface. Seems rather a desperate move! Jut try the cover on the rear and run the cooling fan continuously. But do check carefully with fan off and on. Sometimes they introduce vibration which you see as blurred star images.

[ZeroID (Brent)]

Use a small PC or laptop fan, suspend or isolate it from the OTA somehow and run it at 6 or 9 volts rather than 12. Slower, less chance of vibration. I have one slung beneath the dob on fishing nylon from each corner. You don't even know it's there.

[big_dav_2001 (Davin)]

Ive got a small standard fan mounted to the back of it at the moment, ive used the scope while its been running on 12V and couldn't see any major vibrations.

If theres no problems with the fan as-is, would it be possible to keep it as it is and seal the end off around it?? or would that cause it to vibrate?? im thinking if it aint broke, dont fix it and simply working around what ive got whilst its not giving me problems.

Ive seen the articles on cutting holes and stuff in the sides, using vacuum fans, etc, it all looks a bit drastic to me...hehehe

[erick (Eric)]

Nope, you're fine. Just seal around the fan so the air has to hit the back of the mirror, go around the edge and up the tube. An easy mod and hopefully a positive result.

[big_dav_2001 (Davin)]

Great, i'll give it a try, thanks a lot for your help mate

[big_dav_2001 (Davin)]

Just finished making and fitting a cover on the rear of my scope, i made it out of thin 2mm plywood and added the interior baffle to the tube.

Everything works great with no vibrations and the airflow is pretty strong through the OTA, although when i took the mirror out, i noticed its pretty grubby, so may have to clean it sometime in the near future.

Thanks a lot for your help. Problem solved

Davin

[erick (Eric)]

I hope it performs well - let us know.