Anti-dew suggestions 12" imaging newt

[rustigsmed]

Hi IIS'ers!

Melbourne seemed to have skip Autumn and head straight to winter this year, so I really need to start thinking of a dew control system for my imaging newt. I'm going to get a usb hub and try to tame my wild cabling so why not extend the project for some heating solutions.

My current method is the high tech hair dryer technique directed at the secondary between subs, on the tube and camera and it worked well, the last thing I needed was more cables going everywhere but I think by sorting it with a USB hub can deal with it.

Looking for suggestions/links/tips/hints I admit I really don't have that much of a clue of what I need. Tube heater Strap? Secondary mirror heater? Camera? Do I need to bother with the primary? Some kind of control unit?

the scope is 12" as mentioned from memory the secondary is 88mm. I don't have an observatory but I set up everytime and use mains power.

Regards,

Russ :thumbsup:

[gregbradley]

Using a hair drier can ruin the figure on your lens for a long time. So if you start getting blurry images it may be from doing that.

Dew control seems to be done in several ways:

1. Dew Strap heaters - I started a thread on this myself about 3 weeks ago. Basically there are Kendrick, Dew-Not, AstroZap. And several controllers. I picked Dew-Not and DewBuster control unit with sensors.

For me it was more about power consumption as I use a generator and I know anything that heats uses lots of electricity. The Dew-Not uses about half the Kendrick although Kendrick have a film based dew heater which may use less. I notice dew strap companies offer heating solutions to heating the secondary mirror.

The controller has air sensors so it only cuts in the dew heater once the temp of the scope/mirror gets close to the air ambient temp. Hopefully again this saves power. You have mains power so not really relevant in your case unless you later want to go to a dark site and use a battery in which case it is relevant.

2. Insulation on the tube. Flocking the inside of the tube, putting an insulation wrap around the tube especially around the tube where the mirror is means the mirror is always a bit warmer than ambient and with falling air temps you should delay dew until the temp drop slows and the mirror catches up to ambient and you get dew.

3. Fans. Fans pulling air through the scope can reduce the onset of dew. The fan needs to suck air out of the tube flowing air over the mirror.

4. A dewshield.

Per the AstroPhysics data sheet that came with my scope Roland does not advise the use of hair driers but if you absolutely have to then run it on cold air and if no go then only the slightest of heat played around using the back of your hand to gauge the temp. He says using a drier will ruin the figure of the optics for a long time until it cools down again.

Greg.

[peter_4059]

Greg,

I don't follow your rationale on the power consumption comment. Are you saying Dew-Not are capable of putting the same amount of heating into the OTA while using half the power of a Kendrick heater? The only way I can imagine that would be the case is if the insulation on the Dew-Not is twice as effective (which I doubt).

My EQ6 and Kendrick controller both running draws 0.47A whereas my heater strap for the SN10 is rated at 2A - ie the heater strap can use an order of magnitude more power than the controller. Although the strap is rated at 2A, it would only draw this current if you ran it at 100% output on the controller (which I never do).

I have a Kendrick controller that is capable of sensing ambient temperature, dew point and also the optics temperature. The controller keeps the optics a fixed delta above dew point. It switches the heat on and off to do this and never exceeds 50% output.

I suspect the Dew-Not strap is simply just rated at a lower power, so to keep the optic at a certain temperature, you will need to run a higher output on your controller and the actual power consumption will be pretty much the same - you just have less headroom in your system.

I've attempted a side by side comparison of the power ratings of the two systems.

Here's the data:
Kendrick http://www.kendrickastro.com/dew_premierheaters.html
Dew-Not http://www.dew-not.com/specifications.htm

Peter

[gregbradley]

Its more resistive wire heaters versus resistive film. The film consumes less power and spreads the heat more evenly than a wire.

http://www.dew-not.com/kendrick-comparison.htm

As far as power consumption goes I the unit I was looking at was the 16 inch strap and as I recall it was rated to just under 5 amps which at 240V = 1200W which would make my generator rev rather than power everything at idle with less noise and less fuel consumption.

Kendrick also have brought out a film heater strap as well.

The Kendrick controller is more expensive. I am sure its fine. I was told they are a bit complex. I also want to keep my system as simple as possible as the tiring thing in this pursuit is something not working properly that night and chasing it down!

This is the review that alerted me to power usage which I wasn't even considering as an issue but could see it may well be using a generator:

http://www.astromart.com/articles/article.asp?article_id=105

Also bear in mind this is merely as a result of my internet research when looking at dew removal recently not from practical experience as I have never used a dew strap. So if your experience is otherwise I bow to your practical experience over internet reports as I would with anything. The internet is a source of data both good and bad. Like anything in life you have to be able to evaluate the data and match against your own observation and experience and decide if its valid or not. I am sure I don't have to tell you how much rubbish data there is on the internet with vested interest and personal bias.

Greg.

[DavidTrap]

5amp at 12v equals 60watts

DT

[peter_4059]

Greg,

In the case of a dew heater, I believe all of the electrical power is converted into heat regardless of whether it is wire or thin film resistive device. The only difference will be how much of that heat is transferred to the place you want it. If the heater strap is not well insulated, some of the heat will pass to the surrounding air (convection) however if the contact between the strap and the OTA is good, the heat transfer rate from the strap to metal (conduction) will be much greater.

Kendrick also has a webpage discussing power consumption, heat distribution and capability of heater straps with a lower power rating:

http://www.kendrickastro.com/dew_flyheaters.html

I don't believe there will be any advantage in terms of power saving if you are running the strap in a temperature controlled mode.

[rustigsmed]

thanks for the information guys.

generally the main problem is with the secondary, do people think a dew shield plus a heater to the secondary be required or does a dew shield by itself (and perhaps with flocking/insulation) do the job well enough? it would be nice to avoid electronics if it is at all possible to do so.


Cheers,


Russ

[peter_4059]

I would try a shield first. If that doesn't work you can get a 9volt powered self regulating secondary heater. I installed a Kendrick secondary heater on my 10" newt and that worked well. I didn't find the flocking had much effect on dew avoidance.

[gregbradley]

Oh yes that's true. I was thinking of the 240 volt input to the transformer. So its not that much of a drain then.



That may well be the case.

Thanks for that. My site does go down to 2C at the lowest but generally 5C.

If the Dew-Not does not work out I guess its a Kendrick one. Its the controller that's the more expensive item anyway.

Greg.