Controlling Dew

[gonzostar]

I have a 8" sCT and have a dew cap. However is it really worth going for the dew heaters and if so what to people recommend?

[Cjg]

Had a 5" SCT and a dew shield, was never enough on damp nights; even leaving the scope outside at home, not having heating on in the car on the journey to the obsy; the SCT's are dew magnets.

Fortunately at Seething we have powerpoints on the pads and hair driers with long leads, but it just delays the inevitable.

The dew tape and controller arrived today from FLO for my new scope, it's a pain as more to carry / faff about with, but this year I plan to make the most of the dark and not pack up early because it's damp. We get few enough nights as it is in the UK, so for the sake of £100 /£150, less if you're clever with electrics?

Chris

[gonzostar]

Thanks for your reply. I am certainly no electrician! What kind of controller did you go for? Does the hairdryer treatment really work?

Dean

[Cjg]

Hi Dean,

The hair drier dries off the dew, buying about 30 minutes observing before needing to repeat. So spent yet more...

I purchased the dew tape

http://www.firstlightoptics.com/dew-prevention/astrozap-dew-heater-tapes.html And the mini controller

http://www.firstlightoptics.com/dew-prevention/hitecastro-single-channel-dual-port-dew-controller.html

You'll need a power supply too, earlier this year I purchased a tracer battery about £109. Though cheaper / heavier options are available in the form of Sealed Lead Acid batteries.

There are heated versions of the dew shield for your scope, and I did read (perhaps in a sky at night mag) about someone covering their OTA in foil too.

Hopefully you'll get replies from others too to this thread.

Good luck,

Chris

[acey]

You could also try a longer dewshield. The longer it is, the more effective it will be.

[cjdawson]

Rather than going into it all again.

Here's project that I'm working on to replace the power to my scope.

http://stargazerslounge.com/topic/250514-power-to-my-scope/

It includes a dual channel dew heater.

Here's another post that I recently made on this topic, which I think you'll find worth reading.

http://stargazerslounge.com/topic/164646-dew-shields-do-they-work/page-3

In summary, I say yes, have a dew heater available if you can.  It's the difference between a long night, and being cut short.

[gonzostar]

Thankyou for your replies I have being convinced that i do require one as we do tend to have few clear nights in this country, Do they drain the batteries from the tracer quickly? I have the skywatcher powerpack However not to impressed by this battery, 

Acey how long is your dew shield? I have the standard celestron one

[cjdawson]

Everything that I've read about Dewshields, bascially the best length is 50% longer than the diameter of the tube.  So if as you have an 8" scope, your dew shield shield be 12" long, shorter won't be as effective, longer will probably be mostly a waste and add extra weight with little gain.

My experience is that when running at full power my dew heater pulls uses 2.3 Amp.  so with a 17Ah battery for the heater along, you should be able to run at full power for about 7 hours.  However, you'll not get all 17Ah, so it's probably more like 5 hours.   But that's when running at full power.   If use the combination of shield and heater (as I do) you will only likely need to run at about 1/3 power, or even lower.  so you'll only be pulling .76 of an Amp, which means a possilbe 22 hours max running time (say 18 hours), or more likely all night   Even with 2 heaters running at .76 Amp, you'll still get 11 hours (or more likely 9 hours) run time.

Why am I saying that you'll likely get less hours than the theorical max?  There a few reasons, 1. this is back of envelope maths, 2. Batteries tend to be inefficient at night when they get cold (they leak power) 3. you should never fully discharge a battery as it can damage it.

I personally have taken to going for complete over kill and using a 70Ah battery.  Rather than having a seperate battery for the scope, heater, guider, skyfi, anything else.  I've chosen the big single battery approach - means there only a need to charge one battery to be ready for observing.

With my setup, I've worked out that in reality, I've got about 40Ah of power to play with during an observing session.  That means there will still be 30Ah of power that can leak, etc.   With 40Ah, but running all my kit, which in total should normally use about 2.3Amp (see here for the breakdown) I'm able to get 17 Hours of use from my entire setup.  The thertical max power scenario is that I'd get about 10 Hours (70Ah / 7Amp).

[gonzostar]

Thanks for your detailed response it is very helpful. Do you charge up after every use? My concern is running out of power during a session. It is still taking me a while to do all the polar alignment stuff! 

[cjdawson]

yep.  I do charge every time.  Having said that, when I go away for a weekend, I'm sure that I have enough power to run everything.  That said, I also have 2 70Ah batteries, so there is no chance of me having power problems on a weekend away.

Longer than that and yes, I do like to charge up ready for a session.   It's also good practice as I am using caravan batteries (lead acid) and they prefer to be kept with a full charge.

[Jokehoba]

Why am I saying that you'll likely get less hours than the theorical max?  There a few reasons, 1. this is back of envelope maths, 2. Batteries tend to be inefficient at night when they get cold (they leak power) 3. you should never fully discharge a battery as it can damage it.

Perhaps a daft suggestion, re point 2 above, but what if you added another heating strap to keep the battery warm? That way you might be able to get away with a lower capacity/lighter battery?

[cjdawson]

That's not a daft suggestion, actually what I'm planning on doing is making a battery box out of a large toolbox, and adding a few things to it to make it useful.  Things like 12v sockets, usb sockets, amp meter, volt meter and maybe a solar charge controller.  Not sure of the details of the project yet.  I'm refusing to think about it until I've got the other projects that I have on the go finished.

[acey]

Thankyou for your replies I have being convinced that i do require one as we do tend to have few clear nights in this country, Do they drain the batteries from the tracer quickly? I have the skywatcher powerpack However not to impressed by this battery, 

Acey how long is your dew shield? I have the standard celestron one

I have a dob and use a homemade camping-mat dewshield. Optics dew from being directly exposed to a clear sky in cold conditions. To prevent dewing (the old fashioned non-powered way) you want to minimise direct exposure to the sky. You do this by having the optics in a long tube so they see no more sky than they absolutely need to. The tube of a reflector serves this purpose for the primary mirror; a further shield is needed to protect the secondary. On an SCT you would want a "tube" projecting as far as is convenient and practicable. And don't leave the scope pointing at the sky if you aren't actually using it. Instead point it at ground or wall, or cap/cover the optics (a thin cloth over the end of the dewshield would be sufficient).

[gonzostar]

Thankyou for that suggestion I will give it a go