Was this a good or a bad idea? Dew-heater glued on and primary mirror covering

[Jannis]

I've been struggeling more and more with dew recently on my explorer 200 pds, not only on the secondary mirror, but also on the primary and the MPCC - so i've done some small modifications.

First thing i've done is i've taken the homemade heater-staps and instead of just having them loosly mounted/taped around the scope and finder, i've used a glue-gun to glue them on directly to the scopes, trying to get it was close to the scope as possible and leaving no air gaps between.

Originally the main scope heater was fitted very loosely around the scope without even being taped on, so it left a lot of space between the heater and scope itself. it still seems to work, but recently on very moist nights, not even increasing the power to ~35W would keep the dew away from the mirrors.

The finders heater originally wasn't even mounted on the finder, but the finder bracket. Never had any issues with that at all, but thought i might as well redo that also before the winter with it's lovely -20c arrives.

I know the glue isn't a good heat conductor, and the glue isn't sticking on well to the metal frame of the scope, but it's certanly better then air at least.

I also put the heater a bit futher in on the scope, so that i have room to use this in addition to a dew cap as well at the same time.

But my main concern change is this:

i've had issues with my flats, and i've read that others have had the same problem, and it turned out to be the lack of a cover place behind the primary mirror. Earlier models had this, but not my model. I don't know if it was removed to help cooling, prevent dew/ice, or to save costs - but i've taken an old black (used to be at least) and strapped it on behind the primary. I image right next to a road, and i have cars lighting up my scope very strongly every time they pass by, as well as i have a few street lamps right next to me.

I'm hoping this will help prevent any outside light to enter the tube, while still breathe enough to not cause any problems inside the tube.

What i'm worried about is, will fabric just get wet fast, and stay wet, causing much more problems then it solves?

[Tinker1947]

The secondart mirror requires its own dew heater, the one you have if it get to warm will just create thermals out of the tube, the main mirror needs air movement to cool down, your piece of cloth will suck up the dew and add weight to the tube and unbalance it, once it has soaked up enough dew in the winter will turn to a block of ice, i would remove it and the heater round the top, get a dew shield something like the one in the following image....

[Jannis]

Thanks. I didn't think about a seperate heater for the secondary. Unsure how i'll make it though, without blocking the optical path in any way?

For the main heater - i've had to use this instead of a simple dew shield like you have in the pic. While the dew shield did help a bit, it didn't help much/any if i was imaging stright up. After i've put on the heater, i've had a lot less dew problems then with just the dew shield. It's only recently i've had some issues on very moist nights.

I haven't been able to try both at the sam time yet, but i'm planning to use just that now - wich is part of why i moved the heater a bit futher in, so i can have the shield in front of it. The cat kind of ate up my dew shield shortly after i made the heater, so need to makie a new one...

I'm hoping like this i'll only need minimal heat, and maybe no heat at all when imaging lower in the sky.

Cooldown isn't an issue as this will be standing outside permanent, so minimal sudden temperature changes.

You're probably correct about the fabric just gathering moisture and freezing though. So i guess i'll remove that again.

Anything else i can do to prevent light fron entering from teh back, or shuold i just try to deal with it?

[brantuk]

I would remove everything you've stuck on to start with. Then I'd put a fan on the back of the primary mirror to keep ambient air flowing over it so dew won't form (and it will cool quicker). Then I'd stick a dew heat pad on the back of the secondary using silicon glue - trail the wire along a spider vane and out through a hole in the side to a mounted jack socket. Optionally I would also use an eyepiece dew band and/or a finder dew band. You'll need a dew controller with either 2 or 4 channels depending how many of the above you use. I would also make a dew shield out of dark coloured camping mat (which will help prevent stray light entering the tube from the sides).

Bear in mind that if the dew is heavy then that's usually a sign to pack up and finish for the night cos no amount of dew control is going to work - some nights are like that I'm afraid. Hth

[Jannis]

Thanks for the tip on that secondary heater.

I'll take of the fabric at the back at least. The heater i'll leave on for now i think as it add very little weight and i'll have it as a backup untill i've found another solution that works/works better. My problem is that i live close to water, so it's very moist here almost constantly, and almost every single night it dews up very fast if i forget to put on the heaters.

A fan have been on my mind, but wouldn't that fan also cause the secondary to dew up faster - or would it actually slow down secondary dewing up as well? And what about vibrations? I'm guessing i'd need a super smooth fan to make it work properly, and not just any standard computer fan for this task?

[Oily]

I made a dew heater like this for my secondary,it wraps around the base of the mirror,works brilliantly

[brantuk]

Ahhh... now I see your problem - I never get the scope out near water if I can avoid it because of the mist problem from the likes of lakes, rivers, ponds, etc. That level of mist is impossible to combat with most any form of dew controller that I know of. It's a sure fire way to end a session early as soon as the mist rolls in.

The idea of a fan is to blow ambient air on the primary to keep it at ambient temperature and keep air moving over the surface. This prevents dew from forming - but if it's sucking in mist then I've no idea how that will affect the mirror. I've never heard of fans causing a vibration problem on imaging scopes - they certainly don't cause a problem for the observing scopes that I've used.

Perhaps someone who images near water will chip in to your thread - but my feeling is that you need to get away from it.