My primary and secondary are getting horribly dewed up lately with the scope pointing up for long periods while imaging Jupiter.
With my 10" scope, I was able to leave the fan running while imaging, which prevented dew from forming (and no vibrations or image degredation) - but with the 12", the fan is mounted differently (directly to the mirror cell) and it's introducing vibrations and degrading the image quality, which means I have to turn the fan off when I start imaging.. therefore, dew starts forming, I lose transparency, image quality degrades.
Anthony (bird) has solved this by cutting "trap doors" at the bottom end near the mirror, and one under the focuser for access to the secondary. When he starts dewing up, he can open the door and use the hairdryer to dry up the dew and reduce the fogging up.
So I want to do the same.
How should I do this? What's the best method and tools?
Mike,
If you haven't already tried it, use a dew/ light shield attached to the end of the tube. I found this had a 100% success in eliminating dew, try this before you start cutting up your "baby"
I would be attacking the problem a little differently.
I would be getting some "dew heater straps" and attaching these to the inside of the tube with velcro, just above the primary mirror level. This will prevent the air above the mirror reaching "dew point". I would start by using 2 small straps designed for 1.25" eyepieces and placing these on opposite sides of the tube.
In terms of this having an effect on image quality? I don't think it will.
Mike, I made the 3 trapdoors on my scope very simply - each door is made by cuting out a square of aluminium from the OTA, and then reattaching it with cheap hinges from bunnings. The closing mechanism is just velcro, glued onto the door and OTA with 5minute araldite. Works fine.
Make sure the door opens so that you can direct warm air across the reflective surface of the mirror, you don't want to be warming the glass directly, just evaporating dew off its front. This has a negligible effect on the mirror temp.
You need doors so that you can keep them closed to stop dew from getting in. If it has to come all the way down the OTA then its effect is reduced. Just cutting open holes will likely make the dew problem worse.
When using the hairdryer it's nice to have the doors open all round so that the warm air can escape directly without having to travel up the OTA. Reduces the lost time waiting for the air in the tube to settle down afterwards. You point the hairdryer so that warm air comes in one open door, travels across the mirror and then blows out one of the other open doors.
I use these doors all the time, but my OTA is 1.8m long so I really have no other choice :-)
I wouldn't advocate heating straps, as they will introduce thermal effects into the air in the tube that you don't want.
Below the focuser I have another, larger opening, covered by a sliding door. I can slide this open to see the face of my diagonal without taking the wheel and camera out of the focusser, and I can point a hairdryer in there to clear any dew.
A sliding door was needed so as not to get caught in the cables etc around this part of the scope, and also the wheel takes up a fair bit of space.
ps I'm also considering adding a dewshield, made from some light plastic. I have a lot of trouble with dew on my secondary cause its so close to the top end of the OTA.
With the correct material it won't add any weight (maybe 100 grams?).
I wouldn't advocate heating straps, as they will introduce thermal effects into the air in the tube that you don't want.
cheers, Bird
Anthony,
I don't necessarily agree with this. We are talking a minimum amount of ambient heat being introduced. In addition, on this scope which does not have active cooling, it will reduce the temperature differential between the air above the mirror and the mirror surface, thus reducing boundary layer effects.
I don't think you can "guess" at what might happen in this situation. You may be right, but I think it would be a case of "give it a try". I certainly don't like cutting holes in telescope tubes unless it's absolutely necessary.
Me neither John. I have a thin piece of nichrome around my secondary, and haven't yet had any problems with the primary. I do take on board Bird's thoughts about tube currents with the dew heaters, but the same perhaps applies with the hair dryer??
I too would opt for the foam dew shield, at least first, before hack and cry.
Gary
P.S. Sorry Bird.
get a cheap angle grinder from bunnings ($15 - $20) and get a a flexovit ultra thin cut off wheel 1mm thick 102mm wide. you should quite happily cut your sections out initially by a plunge cut and then moving it across etc.
I hope to have mike peltier cooler finished this weekend, and have noticed the effect of any heat (however small) on high mag work.
I believe any "constantly on" heat source would play havoc. a short blast of heat and air (hair dryer) buggas the seeing for about 10 minutes, but clears the mirror and then everything equalises out again
If vibration from the rear cooling fan is a problem..and the fan is the same as the ones fitted in computers..how about mounting the fan to the mirror cell via a medium density foam rubber spacer...I think Bunnings or clark rubber carry a selection of the stuff..just cut out a ring and epoxy the fan to it..test the level of vibration by powering the fan with a 9 volt battery..place it ring side down onto a table..place your ear onto the table and listen...if the vibes are minimal...then stick it to the mirror cell..i've even seen a few scopes with a flexable automotive manifold-to-aircleaner heater hose attached to the mirror cell and a fan pumping air through it!..
Good luck!
Thanks for the feedback guys. I appreciate JB's thoughts and "try it and see" is definitely a good approach, however i'm not opposed to cutting a hole or 3 in my tube. This scope is designed to be a high mag planetary workhorse, and if a few holes means it can do that job better, then that's what i'll do.
The 10" fan didn't vibrate because it was mounted differently, on a piece of aluminum held on with velcro over the top of the collimation bolts (with appropriate holes for the bolts to stick through). The design of this 12" with the fan attached to the mirror cell is the definite cause of the vibration, and I have contemplated mounting it how I did with the 10", but as DP said, there's active cooling coming my way soon so i've held off with mounting the fan differently for now.
hey mike, two thoughts.....i live in a very humid area and dew is a major problem. it sometimes forms in just minutes. especially after the temp starts dropping. i use the dew heating strips but they are attached outside the tube just above the mirror. it works pretty well except in the worst of conditions.i can't notice any thermals but i am not imaging. also could you possibly mount the fan/fans in a way that they still blow into the tube yet are attached to a separate holder? not touching the tube or mount.
This is the exact technique I use with my flextube OTA rather than having a dew shield or OTA shroud. Quick blast with the hair dryer works wonders on the secondary when imaging. Trap door OTAs would be very useful in the UK where we have serious dew issues. Personally I find dew shields bad news for thermal activity.
My fan is mounted on a 'spider' of very light ( 1 kg maybe ) fishing nylon below the cell. I also only run it at 5 volts if required. ( It has a two way switch for 5 or 12 volts).
I have yet to be able to detect any vibration. Easy, cheap, effective.
Cutting blow holes in my OTA
