What sort of dew prevention do people employ?

[lineout]

Hi all,

I'm going down to The Snowy Mountains for Easter and am looking at some dew prevention accessories. I was just wondering what other people use and which is better, a shield or strap, or both?

I've got an 8" Meade LX-90 SCT.

Cheers

Rene

[gbeal]

I don't have an SCT, but I would opt for both. If you can, use a dew strap of some sort, and fit the dew shield over the top of that.
If you are scrooge like me you will try some of that thin (maybe 5mm thick) insulated foam that campers evidently sleep/lie on (too thin for this old coot), and cut it to fit as a wrap around for the end of the tube. This insulation will keep the heat from the dew strap in, instead of radiating out into the ambient air.
I made my own nichrome wire dew straps a while ago when I had the Maksutovs, but a commercial dew strap is as good or better.

[RB]

Like Gary said, I'd recommend using both.
The shield also helps if there's any stray light about.
But really a dew strap is essential to keep the dew at bay and prevent it from building up.

[Robby]

Dewnot dew straps are the best and excellent value for money when compared to other commercial straps.
I also use the dewbuster controller, which is also excellent, after dabbling with homemade dew heaters, which were hard to control the "amount" of heat.
Cheers

[Omaroo]

Photo 1) On the C8 - Here's my home-made heater "ring" - the red doovie just inside the flange. I run it off a home-made pulse-width modulated controller.

Photo 2) I also put the dew shield on which keeps the slightly warmer air near the face of the corrector plate.

Photo 3) I also have a home-made heater strip (off the same controller) fro my red-dot finder.

Photo 4) Another ring I made for my 12"

Photo 5) My home-made PWM controller

I've never suffered loss of observing time due to dew with these things running.

[gary]

Hi Rene,

As others have mentioned, when the dew is heavy, you will definitely need both.
At the beginning of any night, even if you don't think you will need a shield
and the dew heaters on, put them on as luck will normally dictate that at some
point during the night you will need them. ;) Otherwise after a while you start to
wonder why you can't see anything until you check the correcter plate and find it totally
dewed up. :doh:

You will also definitely need a strap for the eyepiece as well and if you use a finder
scope, for that as well.

Enjoy your trip and I hope the weather, seeing and transparency goes your way.

Best Regards

Gary

[Geoff45]

You can also use a 12v hair dryer from boat supplies (eg here (http://www.whitworths.com.au/main_itemdetail.asp?item=40000&search123=hair+dryer&intAbsolutePage=1)) as a supplement.

[gary]

Use either something like inch-wide elasticized strapping sown into a loop or Velcro
OneWrap strap to hold the piece of camper mat in place around the dew strap and
circumference of the corrector plate end of the SCT.

Best regards

Gary
Mt. Kuring-Gai

[lineout]

Thanks for all the info guys.......it's been very helpful. It sounds like I need both a shield and dew strap.

Rene

[Starkler]

+1

It is certainly much easier to prevent dew than to remove it once it becomes a problem.

[gbeal]

What you really need is to ask Robby for a picture of his dewshield, LOL.

[sheeny]

Yes, I concur. I have an 8" SCT and operate in sub-alpine conditions. You will need both! It gets very frustrating otherwise. I also frequently use a 12V hair dryer to give the heater and dew shield the occasional boost when required! I find the dew/frost can start to appear from your secondary out even with the dew shield and heater on, so at the first sign of loss of contrast, check your corrector.

The only other advice I have is to avoid rain (it's just heavy dew really!!!:P)

Al.

[Sentinel]

I can recommend a 4 Channel Dew Heater controller from Thousand Oaks.

Allows adjustment of 4 different heaters and has a 12V port as well. I run the Argo Navis of the dedicated 12V port.

Very handy.

http://www.thousandoaksoptical.com/www/Products/dew.html

I think Star Optics are the agents.

There may be some other more locally made controllers around.

[Starkler]

I have the same one and have heaters/straps for the eyepiece, finderscope and telrad. Im still happily observing when others are cursing the dew and have given up :D

I also use the spare port for the argo navis.

[gbeal]

And one last thing I didn't mention before. Removing dew seems to be like pushing it uphill with a match. Use the dew heater before you get dew, and this coupled with the thermal dew shield should keep you happy all night.

[Babalyon 5]

I made my own heaters and control box, all in 12volts. I have one for my corrector(18 watts), under the dew shield, one for my eyepiece(5 watts) and one on each end of my finder(combined about 3.5 watts). Its not a battle to clear the dew, but it is good sense to turn them on if you know the dew point is going to be low. Im still observing with no dramas while everyone else is screaming about leaving the hair dryer at home!! :lol:

[Babalyon 5]

All you need is a variable potentiometer that can handle amp draw. I used dimmer switches from Ford Lasers as they have a little light and a scrolling thumb wheel and can handle the amp draw up to say, 15 amps DC.:eyepop:

[higginsdj]

I use 2 homemade nichrome straps - one for the 14" and one for the 4" guidescope. The 14"version is powered by up to 5 amps (in 8 steps) and a dew shield and stops everything but fog while the scope is pointed at or near zenith.

I live in Canberra and my equipment had been frosted over (hard ice) and the SCT corrector has still been clear and the scope still hit and imaged every target.

[AstralTraveller]

There is another consideration with dewing. The other night, while waiting for the clouds to clear :mad2:) a few of us were discussing the issue of dewing in newtonians. Someone whos name I've forgotten (sorry mate - too many new names) reckoned that you have far less trouble if the tube is silver or white. It's to do with the loss of heat - black is a very good radiator of heat while white is very poor. So the light coloured scope will stay a bit warmer. I also read the same thing in S&T years ago.

This seems to tally with my experiences. My newt is white and is pretty dew-resistant (the decent length of tube in front of the secondry helps too). But the black refractor gets a layer of dew over it at the drop of a hat.

The S&T article I read also gave me the idea for a modification I made to some 15x80 binos I use in an observing chair. These dew up almost anytime. Cardboard dew caps helped a bit. I made new caps out of heavy aluminium foil, reenforced at each end and painted matt black on the inside. Then I wrapped the body in that silvered flexible material they use in house insulation. The improvement was dramatic. Now they only fog on very bad nights if I look too close to the zenith :thumbsup: and for then I have a 12V drier.

I know this approach won't stop a SCT from dewing and you will need a heater. But others may also find they can control their dew problems passively.

[OneOfOne]

I always put my dew shield on, even in the summer, and particularly at public viewings as it helps to keep the fingers from little kids off the corrector! I find the shield will keep dew off for maybe a couple of hours if you are looking away from the zenith, but if you point the scope high it will dew up in minutes. I remember one night I was looking at something and swung around to something on the other side. In the process the scope had to swing high for a few seconds before pointing down again. When I looked in the eyepiece I couldn't see a thing. The corrector had dewed up in the time it took to slew over :( One of the next things on my list is a heater...

[asterisk]

I know a lot of people make their own - I tried with limited success. Astrozap make a heated dew shield for a variety of telescopes

http://www.astrozap.com/pages/dew-heat.htm

I always put one of thes on my SCT - even "dry" nights end up "wet" very quickly.

Thousand Oaks have a good little dew controller if you don't want to make your own:

http://www.myastroshop.com.au/products/details.asp?id=MAS-299

If you are prepared to wait (10-14 days), Smartastronomy in New York have good prices on heater straps which will plug into the Thousand Oaks unit:

http://yhst-1469288697390.stores.yahoo.net/dewheaters.html

[Krusty]

I ended up making my own dew heater with cheap parts from Dick Smith Electronics. I purchased a PWM kit http://www.dse.com.au/cgi-bin/dse.storefront/481406b50ad2f5b2273fc0a87f9c0710/Product/View/K3070
because I was too lazy to design my own. And for $15, who could be bothered. I then purchased some nichrome resistance wire http://www.dse.com.au/cgi-bin/dse.storefront/481406b50ad2f5b2273fc0a87f9c0710/Product/View/W3205
for an extra $4.

I built the kit and installed it in a pvc box so that the potentiometer adjustment could be accessed from the outside. I made up a length of wire with a dc socket on the end of it to go from the PWM controller to the telescope.

For my 5" SCT telescope, I wrapped insulation tape around the very end of the tube. I then wrapped a few lengths of resistance wire on top of the insulation tape, making sure the wire was not touching anything but the insulation tape. I then wrapped more insulation tape over top of the resistance wire to hold it in place. All in all, the resistance wire measured 22 ohms, so would draw just over 0.5A. I soldered a dc plug to the end of the resistance wire to plug into the wire coming from the controller.

I use this along with a dew shield. So far this has worked like a charm. Before
making this, my scope used to dew up every time I used it.

Wade.