Refractor dew control WITHOUT heat - it is possible!
Hi all,
Following on from my experiments and success with using fans to control dew formation on SCT's and Maks, I got to thinking "well, why not refractors too?".
I guess the mental block here is fracs come with a ready made lens shade/dewshield, which for some reason most of us feel this cap is mandatory to use and cannot be removed under any circumstances.
Well, say's who this is the case!?
Why must the original lens shade be used?
And with many lens shades being painted black on both the inside and outside, it is PREDETERMINED that it will be a dew magnet.
And again here we go sticking heating straps on the whole thing, pumping huge amounts of precious electrical energy into a metal tube which only transmits a small amount of this energy into the glass, and most is lost into the heat sink that is the metal tube.
There is an alternative that uses bugger all energy in comparision
A heat free dew control solution.
The manufacturer supplied lens shade is the main problem here. So, first step is to overcome this mental block that IT must be used all the time. Being made of metal, and that at least one surface is painted black, these lens shades add to the dew problem.
So, pull it off, put it in the scope's case, and make a new one. One that you can cut into.
Second, once the new shade is formed, cut a flap into the side, approx 100mm in front of the objective, to accommodate a 40mm X 40mm 12V fan. The flap is not totally cut out, but one side is left to hold the fan in place. The way the fan sits in this slot the air it blows into the lens shade will create a vortex of air that will dry off any dew that's formed on the objective, and keeps it from forming, in the exact same manner as I described in my earlier piece - SCT & Mak total dew control without heat - a solution!.
I made this new lens shade out of Coreflute. I lined the inside of it with black felt, and it stays in place with Velcro spots and a Velcro strap. Thing weighs significantly less than the original metal lens shade too. Every second cell of the outside surface of the Coreflute is cut so that the thing will easily wrap around the objective.
I also added a small PWM unit to regulate the revs of the fan, low or high, as I feel is needed on any given night.
If the night is not a dewy one the original lens shade can be left on, or the lighter weight one can be used with the fan removed and the flap closed.
No heat, less power, a lighter weight, and a dew free objective.
Thanks for reading,
Alex.
Last edited by mental4astro; 12-04-2020 at 08:36 AM.
Reason: typo
The Coreflute wrap I made for the 120mm refractor seen in the pics below, it cost me ZERO!
I used an old election Coreflute poster that I collected after the last Fed election. I collected a few different sized posters as these Coreflute boards are pretty handy things. And these ones are a good size for refractors, with the bigger posters being enough for a 6" refractor.
Even if you need to buy a new sheet, this stuff is not expensive. A small sheet of 3mm thick from Bunnings is less than $10, and can make up to 4 refractor lens shades.
A 40mm 12V fan draws from 0.08A to 0.1A, and less using a PWM. How does this compare to your heating straps?
Fans have been used with Newts and other reflectors for a long time now. I have not heard of vibration issues, nor have I seen or felt any with my scopes - BUT not to say others have not had this problem. If you are concerned, cheap fans use ball bearings. Better ones are brushless. The smoothest are Maglev fans. The fan does need to be secured appropriately though. You don't see fans being flimsily held in place. The lens shade won't be held in place by Pixie dust or it will fall off. The mass and design of the scope & mount also very effectively attenuates potential fan vibrations. If vibrations come into play, it's a situation of a poor fan or something touching the blades, not that fans are a problem.
Last edited by mental4astro; 12-04-2020 at 09:32 AM.
Reason: more info - thanks Lee.
Vibrations from fans can definitely cause problems, the fan I added to the rear of my newt was causing blurred images and star elongation.
That said, I think you're definitely onto it here, Alex. I too made a dew shield out of corflute and adhesive felt and I was thinking about modding my newt to add a fan to protect the secondary from dewing. Adding heat to a secondary seems like a terrible idea to me, I'm not sure why so many people do it.
Vibrations from fans can definitely cause problems, the fan I added to the rear of my newt was causing blurred images and star elongation.
That said, I think you're definitely onto it here, Alex. I too made a dew shield out of corflute and adhesive felt and I was thinking about modding my newt to add a fan to protect the secondary from dewing. Adding heat to a secondary seems like a terrible idea to me, I'm not sure why so many people do it.
I used a small scroll fan on my 10" f5 open strut Newt that I built about five years ago. It was a laptop type flat scroll fan that I bought off eBay for a few bucks. Attached it to a strut and aimed the airflow to blow across the surface of the secondary (not directly on it, just across it). I had a cheap PWM to drive it and kept it pretty low. It worked fine.
I used a second one as a primary boundary layer fan. Plenty of uses for those little scroll fans.
I have used a dew shield which wraps around the objective and scope dewshield and its made of that same stuff you put up against your windscreen in summer.
If you install before sundown it works well. It keeps the objective a few degrees warmer than ambient.
I have used a dew shield which wraps around the objective and scope dewshield and its made of that same stuff you put up against your windscreen in summer.
If you install before sundown it works well. It keeps the objective a few degrees warmer than ambient.
Greg.
This is the same principle that is behind insulating SCT's and Maks that I discussed in my other thread!
I thought it could be used with refractors too, so there you go There is a cross over
Great thinking Alex. While I can just grab another scope when mine dews up, avoiding it altogether is definitely better. Would you advise against insulating the whole tube like you do with cat's, and if so, why? Keeping the entire OTA slightly warmer, and you could do away with the fan perhaps? Or even combine dew heater and full tube insulation? Anyway good thread this.
Alex
Very ingenious, love home made accessories
I made a cooling fan for my DSLR to reduce the sensor temp by 5 to 7 degrees using a 60mm high velocity axial fan ( not to reduce dark current but to prolong the life of the sensor during long exposures)
Works a treat and does not affect guiding at all , I’ve guided with and without it on the same night and no discernible change in guiding error on DEC or RA
Here’s some photos
Mirko, no reason why you wouldn't also insulate a refractor. While a refractor does not have a primary mirror or baffle tube, it will also benefit from insulating as it will also forego the need of a cooling period. As with SCT's and Maks, insulation will only slow down the dewing up of the optics, not stop it. That's were fans come in to stop its formation. That's the whole premise of the use of fans, to get away from using heat.
We need to change our mindset about the need to use heat. You DON'T NEED HEAT. You shouldn't.
Last night was a very dewy one for me here in Sydney. I set up my 9" Mak at sunset, with its wrap. After two hours I started to notice dew forming on the mount and dampening the wrap. So I installed the fan and flicked it on, with all the juice it could take. By the time I finished up at 4:30am, EVERYTHING was soaking wet, including the wrap, but the corrector was nice and dry What did fog up was my eyepieces, but that's easy enough to deal with. With only the insulating wrap, the corrector would have started dewing up after a couple of hours, and I would have had to give it a shot from a hair drier every little while over and over again, if not put an end to the session altogether.
Bob, the thing to look out for with fans is the amount of air they they push through. That fan that your put a link to, it depends on the dew conditions of any given night and the scope. In light conditions, that fan may be enough for refractors and SCT's & Maks. But in heavy dew conditions, like it was for me last night, that small fan may still be ok for a small refractor, but it won't be enough for an SCT or Mak or possibly larger refractors.
The PWM unit gives me the versatility with my Mak and refractors, and the conditions of whatever each night throws up.
I've been following the scope insulation threads with interest and I plan to add insulation to my C11. It already has a carbon fiber tube so with a layer of coreflute it should stay nice and warm. I have a solid, flocked dew shield which I also thought to insulate but your fan idea is causing a rethink.
Anyway, about refractors, sort of. Attached are some photos of my Quest 15x80 binoculars with their jackets and dew shields. The jackets are wall insulation and the caps are pie tray grade aluminium reinforced at one end with a slice of cardboard tube and at the other with a section of a tin can (pre ring-pulls). The inside is painted with some heavy black goop which, given my old bloke's employment, was probably used in the automotive trade. This has kept the binos dew-free since 1984. Is there nothing new under the sun? I don't know how I came up with the idea but I doubt it is mine. I used to read S&T so perhaps there was something in Gleamings from ATMs (any time from the late 60's to the early 80's).
In a fit of isolation inspired engineering I have made a crude copy of the fine work shown by Alex in another post.
Using a 30mm fan much like this one, running off a wall wart,
I attached it to the rail on my SCT so it blows across the lens hood
Of a piggybacked lens.
With the fan being so tiny I did not notice any vibration.
The wall wart is a 9v and my fan is 12v so it is running slower.
I also put a layer of foam rubber either side of the bracket
I made, sandwiching it so that it provides a sort of shock absorber
Between the fan and the body of the SCT.
Last night was not particularly humid but after about 2hours the SCT corrector
Was starting to get foggy where the lens was not.
I'm calling that a success and thanks Alex for the great idea.
Next phase is to turn the fan around so it blows across the corrector.
PS it worked well but my photos were crap so not showing here.
There is a cross over 
