Dew straps

[multiweb (Marc)]

Hi guys, rainy days cause me to get busy somehow so I'm making some dew straps. I found some instructions here and here . I've been using nichrome wires since day one but it's sort of glued to the scopes and now I'd like to make the thing somehow portable. Has anyone tried to make their own? What's in the commercial ones? Same stuff?

[wasyoungonce (Brendan)]

I could never get Nichrome wire to join with anything..aka the joining wire. Needs to be riveted or spot welded to the joining material.

I tried DIY resistor heaters..too bulky and finish/fit was poor.

I gave up and buy the thick film resistive commercial dew heaters.

[multiweb (Marc)]

Yeah you can't solder nichrome but you can crimp. They work really well with electrical tape.

[multiweb (Marc)]

Stripped the wire off the ED80 last night and shaped it into a square wave pattern. I know the length of wire works nicely so I just reshaped it to fit the length of the dew starp which is approx.30cm long. Got some velcro from bunnings and rubber/foam seals + electrical tape. Works a treat. Might do a big one for the C11 according to the same instructions on the website.

[wasyoungonce (Brendan)]

Yeah I know we cannot solder Nichrome...makes it a PITA to connect. Maybe a job for a capacitive discharge spot welder. The other issue with Nichrome is the heat source is like a point source..it's not spread out like the thin film heaters.

Although this could be a good thing as it heats toast ok!

The other thing with Nichrome is I usually needed to double up a couple of loops of it to get the required resistance which again is nothing wrong just fiddly to use.

But as for +'s for Nichrome...you can custom make a heater to fit exactly where you want which makes for more efficient heaters.

[toc (Tim)]

Do you guys find that these Dew Straps affect the image quality? Ive read that for large SCT's at least, that dew straps can cause some thermal issues.

[mikerr (Michael)]

If you couple one of these into the tube....
maybe not

[multiweb (Marc)]

Originally Posted by toc

Do you guys find that these Dew Straps affect the image quality? Ive read that for large SCT's at least, that dew straps can cause some thermal issues.

I reckon dew affects the image quality more so I try to keep everything dry as much as I can. Doesn't have to be very hot. Just above the dew point.

[multiweb (Marc)]

Originally Posted by mikerr

If you couple one of these into the tube....Attachment 92985
maybe not

Too many tube currents I reckon.

[wasyoungonce (Brendan)]

Exactly as mark said. Either put up with dew or not. Obviously a dew shield helps but this will only work so far.

The straps are supposed to be mounted on the OTA tube, just behind the lens cell (not on the dew shield) as the energy they give off is re-radiated as IR heat energy from the OTA ...heating the optics slightly.

Although Kendrick have a range of heated dew shields...don't know how that works!

[Lester]

Hi Marc, I have a Kendrick strap on my 14" LX200 and a dew shield and I am not impressed with the results. If humidity rises above 75% the corrector will begin to get dew on it, even with the Kendrick set to heat to 5 degrees above ambient. So I am now using a hair dryer to dry the corrector inbetween images and then let it settle down before shooting again. I will be interested to see what you find works with your 11" SCT.

All the best.

[multiweb (Marc)]

Originally Posted by Lester

Hi Marc, I have a Kendrick strap on my 14" LX200 and a dew shield and I am not impressed with the results. If humidity rises above 75% the corrector will begin to get dew on it, even with the Kendrick set to heat to 5 degrees above ambient. So I am now using a hair dryer to dry the corrector inbetween images and then let it settle down before shooting again. I will be interested to see what you find works with your 11" SCT.

All the best.

Hi lester, on my C11 I have a big wire that is bonded with the corrector cell with some kind of compound. I pump 12V in it. It gets very hot. I can't keep my finger on it when it's running. I also have a dewshield which is flocked and the same length as the OTA. If the scope points right up I have to start warming before it gets cold otherwise once I'm dewed up it's too late. It won't keep up. Since I have changed my tube to a CF tube I have noticed that it is much easier to keep the corrector dew free and sometime I don't even bother running the heater. I think because the exposed ALU tube used to pass on the cold to the corrector cell then the corrector glass. The CF tube is always warm to the touch any hour of the night and as a result doesn't even get wet. I have also notice that on my CF triplet vs. the ALU ED80. Changing to CF was the best thing I ever did.