Dew Prevention - Secondary Mirror

[lukebl]

Hi folks,

It's very humid here, and I'm looking for something more effective than a dewshield to prevent my Secondary mirror dewing up. Are there any inexpensive dew heaters which can be fitted to a Newtonian secondary mirror? The only one I can find is the Kendrick Split Newtonian Secondary Heater at around £70, which is darn steep for a little bit of electronics. 

[Mark at Beaufort]

Luke I made an extra large dewshield from neoprene for my 12" Dob. In saying that I did get a little bit of dew on the secondary before the end of the night. Last week there are a great deal of dew  and I cleared it with my 12 volt hair dryer. Its not easy to direct the dryer down the tube to get it in place but it did work and of course its a cheaper option.

[lukebl]

Thanks, Mark. I've got the extra-long dewshield and hair drier. I'm just after something to keep the dew off during a long unattended imaging session when I'm tucked up in bed! On very humid nights, the dewshield isn't sufficient.

[brantuk]

If you fancy a bit of DIY you could try something like this:

http://www.deepskywatch.com/Articles/newtonian-dew-heater.html

There are several sites with diy solutions - or you could query the diy section here on the forum - there's plenty of threads around. I agree - £70 is a bit steep for this sorta thing.

[Stub Mandrel]

I went hunting for and found an old RS 12 ohm 10 watt ceramic resistor yesterday. Its round with a hole in it and smaller in diameter than the support for my secondary, but wasn't very flat.  I was able to VERY SLOWLY turn the back of it flat with a carbide tipped tool. A test shows that 12V (which is 12 watts) gets it quite warm quite fast, so reducing the current should make a neat dew heater. I just have to figure out a secure clamp (I don't want it landing on my mirror) and to lead two wires along the support vanes.

 

[faulksy]

i just made like a ladder system with resistors works a treat, you still need a controller though

330ohm 1/4 watt resistors, around 15p each from maplins e.t.c.

[astronymonkey]

I have a kendrick secondary heater which I must admit is one of the best mods ive done to my scope. Ive never had a problem with dew since fitting it and its one of the oval ones which fits inside the secondary holder. My secondary is large but I dont remember it consting anywhere near £70 although with the pound being weak it the moment that doesnt suprise me.

One of the main things to remember is to get good contact with the glass in order to allow the heat to transmit as a poor contact will make this difficult so you have to run on a higher power which may add heat currents to the scope. Im not confident that my wiring skills would be good enough for the DIY option but many folk have had success and saved a small fortune.

cheers

 

[Merlin]

A problem with many imported closed-tube Newtonians  that they make the tubes too short. At the bottom end the mirror-adjusting screws are sticking out, when they should be recessed by about an inch. at the top end, there isn't enough tube length to prevent dewing of the flat and, also, keeping out stray light from the  sides.

[Stub Mandrel]

4 hours ago, Merlin said:

A problem with many imported closed-tube Newtonians  that they make the tubes too short. At the bottom end the mirror-adjusting screws are sticking out, when they should be recessed by about an inch. at the top end, there isn't enough tube length to prevent dewing of the flat and, also, keeping out stray light from the  sides.

I think my footlong(TM) dew-shield solves that :-)