where do i place dew heater on a sct ?

[bill 212]

Hi all . Can someone please tell me where to place a dew heater on a sct with a shield in place. Does it go over the shield and dovetail or can it go back a bit so it clears both and makes full contact with the tube.Does it make any differerence? have got a kendricks.
Regards Bil

[g__day]

I imagine it would go near the glass at the front - its to heat something above the dew point. Dew that isn't on your optic path isn't a real worry!

[mick pinner]

this is where l put mine Bill and it works perfectly.

[bill 212]

Thanks for the photo Mick but i cant quite make out witch black band is the dew heater . Is it the front one or the next one back.

[mick pinner]

it's the front one Bill, on the outside of the corrector plate.

[bill 212]

Thanks Mick. I wasnt sure because my dew shield covers the corrector plate and did know if the heater would penetrate the shield and still do the job. I have not had the chance to use the kendricks since i bought it. You know how it is, every time you buy something for a scope and the clouds wont clear.

[Omaroo]

In my experience, these heaters work FAR more efficiently if they are located inside the corrector flange - NOT on the outside of the tube. You don't want or need to heat all of that material on the outside in order to warm the air directly in front of the corrector plate - which is on the inside. Way too much energy is wasted. It may work, but your batteries will hate you. There's no point in heating a heat-sink! You need the warmth up front too - not behind the corrector plate.

Here are a couple I made up for my 12" LX200 and C8. In comparison to where I used to have them (outside) they just trickle along to do the same work now. The dew shields just slide over as per normal.

[bill 212]

Thanks Chris but how did you get it to stay in place?

[Omaroo]

Bill - they're made by me to be an interference fit - i.e. they are slightly too large (by a mm or so) and by virtue of that stay in place very snugly. I make them with abs plastic strips about 14mm wide and 1mm thick - which gives the resistor strip a "backbone" before I encase the whole lot in 25mm heatshrink. I roll it around in a circle and join the ends with more heatshrink (covering the overlapped ends). This circle is just ever-so-slightly larger than the inside of the flange - so that when you push it into place it's help there under tension. Works extremely well.

The last two shots are the same system being used in a 4" refractor and the final is my C8 with dew heater and shield on. In this shot I am actually running nine (9) separate heaters - for 3 scopes, camera lens, an eyepiece, straight-through finder (both ends) and two red dot finders. One 17Ah battery lasts me 2 full nights.

[bill 212]

Chris , sounds good can i ask where you got the info to make them and do you need a control box or do they run straight off a battery. Wouldnt mind having a lash at it myself.

[Omaroo]

I make all my own stuff Bill - but I've written instructions in this old thread:

http://www.iceinspace.com.au/forum/showthread.php?t=21421&highlight=dew+heater

I also make the modulated pulse width heater controller as described in that article. Running directly off a battery results in a very hot heater.

Originally the heater strips were based on a spreadsheet that IIS member All Sheehan developed - very handy to give you an idea of the nuber of resistors you'l need for any sized scope. I use 390 ohm 1/4 watt jobbies for all my stuff - and before anyone says use nichrome wire - don't!!!! It's awful stuff - you can't solder it and it breaks from time to time - leaving you dewey!

[bill 212]

Thanks very much Chris. I dont mind tinkering around with stuff like this.I am going to have a lash at this. It might be fun.Cheers