Larger newt secondary dew heaters

[Stonius (Markus)]

I finally managed to get out last night - first clear night where I was actually free for a while.


I was determined to get some video of the planets, but it turned out to be one of 'those' nights where the astronomy Gods looked down and whispered 'not tonight, my child'.



Who is the god of astronomy anyway? I want to know who to pray to, or at least who to swear at!



Since I'd had to travel an hour to get there, I managed to use my ingenuity to make the best of a bad situation, using various redundant systems, but eventually called it at about 12 because the dew was just so bad and I was not having fun.


Anyway, I came here not to gripe (ok, to gripe a little bit - poor me!) but to ask about secondary dew heaters for larger telescopes as that was the real killer.


There seem to be two kinds - a little split circle that goes around the top of the secondary holder, and a collar that covers the 45 degree holder (which I lean towards). My secondary holder is 23cm around and about 73mm in diameter.



The thing I don't get is that heat rises, right? And all these methods sit above the secondary, so how do they help at all? Surely they must mess with the wavefront, at least a little? I also have a small fan set up to blow across the secondary as an experiment, but that was one of the things that didn't work last night (don't get me started). Would constant air-flow over the secondary be enough, or is it going to dew up regardless?



And how do you wrangle the cables? Doesn't it give you one spider vane that is fatter and a bit wobbly compared to the others? Does the diffraction have any effect on the image?


And lastly, power? These things can chew up almost an amp. I'm trying to keep things as light as possible everything attaches to the telescope and rotates with it, but at that rate I'll have to bring out the IHDSB (Incredibly Heavy Deep Cycle Battery) and climb down the ladder to drag it around it every time the telescope slews more than a few degrees in Az which doesn't sound like much fun. Who would ave thought an ultra-light would involve so much heavy lifting?


I guess now we have 2 weeks of clouds forecast in which to find a solution.


Thanks for any help.

[mura_gadi (Steve)]

How large is your dew shield?

Was an old school rule that your dew shield should extend forward to the equivalent of the size of the mirror.

[gaseous (Patrick)]

Markus, I had the same issue as you - it was hard/impossible to find a decent secondary dew heater for my 20" Skywatcher, so I ended up making my own for less than $15. I bought one of those USB powered heating pads off Ebay. If you strip the fabric apart, there's quite a decent length of plastic coated heating wire inside. I used silicone to spot-fix this wire evenly across the back of the secondary, then ran the cable along a spider vane, fixed with a long strip of black electrical tape. This makes a negligible difference to the thickness of the spider vane - certainly nothing that would make a perceivable diffraction spike issue at the eyepiece. The cable plugs into a small thumb-sized controller, the connection for which thankfully sits just outside the telescope frame. This then connects to your power source via the USB connection. Depending on the heating pad you buy, you may or may not need a usb extension cable to reach your power source. I've bought a couple of the heating pads (I also made a dew heater for my 6-slot filter slider), and they normally have a few different heat settings - even the lowest one seems to be sufficient to keep the dew away up here in Brisbane. I only run the heater and the GOTO off a Jayco Powertech battery, and while it drains a bit more power (the battery indicator level actually goes down one bar, rather than none at all), it still seems sufficient to get a couple of full nights out of it at least.

[astronut (John)]

Hi Markus,
Is your secondary mirror glued to a post or is it in a holder?
My secondary mirror is in a holder, and I have used THIS dew heater for 16 years without any problems. It works efficiently and reliably. Uses very little power.

I mounted the heater to the back of the mirror, and made a hole in the holder to pass the power cables through.
If you would like to see my results, I'm happy to put up some images.

[Stonius (Markus)]

Thanks for the suggestions, guys.


I ended up going with one of these from Testar https://www.testar.com.au/collection...ondary-mirrors


Hopefully it should do the job. I've now fixed the issue I had with the fans so I'll have that as well, and I'm making it so I have a 12v astronomy hairdrier in my kit too. Three redundant systems should be enough, I hope!


I'm intending to run the cable along a spider vane and keep it locked in place with a fold of gaffer tape over the top.


The tricky thing is the dew controller. I don't like the idea of it heating things more than necessary, so I'm going to ave a crack at making one of the projects from around the web. Dew heaters seem insanely expensive for what they are. $300 for a couple of pots attached to some mosfets seems excessive when there is no readout or ambient temperature / humidity sensing.


Thanks for the idea about cannibalising the heating pads, Patrick I may try that with some other projects I'm considering. And it's good to know it doesn't affect the diffraction. I'd be curious to see how you attach a dew control system, to the filter slider. I imagine that will be an issue that I will have to solve at some point too so I'd love to see what you're done there.



And thanks John. I know I said I prefer the wrap-around ones. That was before I googled a bit more and realised that the elliptical ones are glued to the back o the secondary itself, duh! A much better solution if you can manage the wires efficiently.


Cheers
Markus

[astronut (John)]

Markus, just a little more information on the secondary heater installed on my scope.

[Stonius (Markus)]

Originally Posted by astronut

Markus, just a little more information on the secondary heater installed on my scope.

That actually looks like quite a good system you have there. So for roughly $65AUD you get the heater *and the built-in controller - and it all runs off a 9v battery? That's a lot of functionality for a little price. I like the fact that it's not just a 'dumb' PWM controller, but runs the dew point calculation and compares ambient to mirror temperature and adjusts accordingly.


Nice! Thanks for the info.


Markus