DIY dew control help please

[Gasmac]

Hi all,

I'm a proud owner of a new SW 250px flex tube and I am in the process of modding it out.

I live on top a hill in the north west and although it affords me good views (though the lights from the heinz factory are annoying) I get a lot of dew.

I have built a light shield and will be getting some more foam/neoprene for a secondary dew shield.

I know this will afford me a little more time, but come the winter it will not last long, so I am looking at the cheaper options for electrifying my scope.

I have a few questions.

1) is it worth putting dew shield on the primary and secondary or just the secondary? (in addition to finderscope and telrad)

2) For the guys who have done this before what resistance wire/resistor ladder did you use?

3) What size of battery would be required say if I was expecting to stay out for 4-5 hours?

4) I have seen a in-built resistor dew control mod for the tel rad. Has anyone tried a version of this with the telrad connected to a leisure battery by a phono wire as well as the reticle  to sace on batteries?

I'm taking a trip to Maplins and I am generating a shopping list, any advice or suggestion would be much appreciated.

Graham

[Jim Smith]

When my new dob arrives I'm going to see how I get on with the 12V-hairdryer-powered-from-powertank method.  My plan is to zap the dew when/if it condenses on the mirrors.  If this doesn't do the trick, I will see what you decide to do.

[Jim Smith]

Oh, I forgot...I had a telrad once and it was for ever dewing-up, so I think you're right to plan to heat it somehow.

[Owmuchonomy]

Hair dryer +1. I use my daughters cylindrical type which blows 360 degrees (that's not deg C!).

[IanL]

The hairdryer method is good in an emergency, but it is better to avoid dewing in the first place.  Once you get dewing, the gap between hairdryer applications reduces rapidly as it is only clearing the surface of the mirror or lens which then cools again more quickly than when you first set up.  Using a dew heater you can keep the whole mirror or lens just above the dew point indefinitely.  Take a look at these for a guide to building what you need cheaply:

http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html

You'll get more time out of your battery if you use a PWM controller as allows you to reduce the power output to the minimum required, and draws no current during the 'off' part of the PWM cycle:

http://www.blackwaterskies.co.uk/2013/05/a-cheap-multi-channel-dew-heater.html

A rough guide to battery life is to take the total draw on the battery in Amps (i.e. everything you have plugged in) and the amp-hour rating of the battery:

discharge time (hours) = amp-hour rating / total draw

This isn't strictly accurate since the voltage of the battery may drop as it discharges and if you are using a pwm controller it can be hard to figure out the net current draw.  It will give you an idea of the time available though.

[LeeB]

I make my own dew heaters they are simple to make and work great, and cost around £8 each to make with pwm controiler. You can make a small one for secondaries and another for telrads or finders, a larger one for the main mirror and a also a heated eyepiece box. all for less that £30. Mine all run from 12v and as I store my scopes in an outbuilding they can be left on low all the time to stop condensation in the mornings if its a rapid temp rise. I will post a how to later in the week if anyone is interested.

[Mav359]

I make my own dew heaters they are simple to make and work great, and cost around £8 each to make with pwm controiler. You can make a small one for secondaries and another for telrads or finders, a larger one for the main mirror and a also a heated eyepiece box. all for less that £30. Mine all run from 12v and as I store my scopes in an outbuilding they can be left on low all the time to stop condensation in the mornings if its a rapid temp rise. I will post a how to later in the week if anyone is interested.

I'm VERY interested in your 'How To' if your going to do one.

[Gasmac]

I've been looking at heating my telrad. Am i right that using a 12v dc supply i will need a 150ohm resistor? Also should i be getting a 1 watt version?

I'm also tempted to shorten the whole thing, getting rid of the battery and getting a 12v to 3v step down to power the reticule from the same source. Has anyone done this or have any advice?

[NEM]

I've been looking at heating my telrad. Am i right that using a 12v dc supply i will need a 150ohm resistor? Also should i be getting a 1 watt version?

I can't say if 150 Ohms at 12V will produce enough heat to keep the Telrad clear as I've not done it myself.  However, power equals voltage squared divided by resistance, which is 12 * 12 / 150, so yes, for 150 Ohms you'll need a 1 Watt resistor.

[Joe_L]

Using a dew heater you can keep the whole mirror or lens just above the dew point indefinitely. 

How do you judge the setting on a PWM controller to keep it just above the dew point? If you under-drive it and start to get dew I imagine it's too late and will take a long time to correct, if ever.

Joe

[IanL]

How do you judge the setting on a PWM controller to keep it just above the dew point? If you under-drive it and start to get dew I imagine it's too late and will take a long time to correct, if ever.

Joe

As explained in one of the posts to which I linked above, this is a hard problem to solve through automation.

- Working manually, You can use a TemperHum device (search for them on eBay or similar) to measure the temperature and relative humidity and thus calculate the dew point.  Most of the capture packages will link to these cheap USB devices and display the results (e.g. BackYardEOS, APT, etc.).  Once you know that you would have some idea as to whether you need to set the dew controller to low, medium or high.  The main problem I've found is that the one I have is not at all accurate and there is no easy way to calibrate it (short of buying a much more expensive calibrated device or a wet/dry bulb thermometer set up).

- You could go further; it is possible to use an Arduino or similar to measure the dew point (using a similar device to above), plus a temperature probe for your lens or mirror and finally to drive a PWM board.  You can then establish a feedback mechanism that tracks the dew point and lens/mirror temperature and adjusts the power of the heater band as needed.  Search the DIY forum and there are several threads explaining this kind of thing.  Again I suspect one would run up against the limits of the measuring devices which are pretty inaccurate unless you spend a lot of money on scientific-grade kit.

At the end of the day it comes down to experience - most nights I can walk outside and tell whether it is going to be a good night or a bad one for dewing and start out with the right setting on the controller. Gently applying one's hand to the outside of the band or the metal of the scope / dew shield where you have the band will give you a clue as the night progresses.  If it is even slightly warm to the touch you should turn the power down - if it feels as cold as the rest of the unheated tube or damp (dew will form on the outside before the lens/mirror), turn it up.  It should just feel like cool metal but neither warm nor cold.

If you do get dewing (and don't have a hairdryer handy), whacking the power on full for a few minutes is usually pretty effective at clearing it, provided you catch it early enough.

[Gina]

I've produced a design using an Arduino with sensors and PWM dew heater control.  This uses a DHT22 (temp & humidity) and one DS18B20 (temp) per dew heater.  These are accurate to half a degree C and should be quite adequate.  I have used a similar system to avoid dew on the sensor of a cooled DSLR camera in the past with good results.