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Astrotrac power and dew control


jgs001

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I used my Astrotrac for the first time on batteries last night...in a field near Brighton with a photographic group. All started well, and then about 20 minutes in, it started to make some very odd noises, and started to rewind. Only the rewind didn't work well either. In fact, it sounded like the screw drive mechanism had jammed... after faffing with it for a little while, and getting nowhere, fast (much faster than Meat Loaf did in his song), I decided to change the batteries (AA's)... this resolved all my problems which surprised me, rather pleasantly, as the thought of my AT having got damaged after only 4 or 5 uses was rather infuriating.

Anyway... onto the questions..

1) What's the best option for power in the field ?

2) The conditions were rather soggy, and the front of my macro lens misted up, at home, I'd normally use a hairdryer in my back garden, but obviously, that wasn't going to work in the middle of a field. I'm guessing something like an Astrozap dew tape, but what's the best size ? Just measure the outer diameter of the lens and get one to match (or a little larger ? This will need power too, so needs to be taken into consideration with 1)

I'm guessing something like the skywatcher power tank (17Ah) may suffice for power for both for quite some considerable time, given the light load placed by the two (perhaps three, if I get/use an astromiser external power supply on my camera), but can anyone confirm this expectation, and that 17Ah would be sufficient for quite some time ?

I know when I was using my NexStar (it'll run on 8xAA's) a Maplins powertank (not sure on the rating) lasted hours.

What are your thoughts please ?

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For the dew heater you'll definitely want to use a PWM controller if operating on battery; basically you set the power output to the minimum you need for the conditions and the controller will pulse the current on and off, saving a lot of battery power during the 'off' part of the cycle.  If you run the heater without a controller it will use a lot more juice.  You can buy a commercial one (they are all PWM as far as I am aware) or make a DIY one for a lot less cash

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

You can also make the heater tapes DIY for a lot less than buying them if you are so inclined:

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

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Hi, hope you don't mind me jumping in and asking a question about the PWM controllers.

I'm also in the process of putting together a completely mobile battery operated setup including the dew control.

I have a spare ST80 dew strap which I will be using for now on my camera lenses, looking at the 6-12v PWM controllers to use 4xAA or a single 9V battery, haven't decided yet. What sort of ampage should I be looking at as there is quite a few different ones.

Cheers.

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Hi, hope you don't mind me jumping in and asking a question about the PWM controllers.

I'm also in the process of putting together a completely mobile battery operated setup including the dew control.

I have a spare ST80 dew strap which I will be using for now on my camera lenses, looking at the 6-12v PWM controllers to use 4xAA or a single 9V battery, haven't decided yet. What sort of ampage should I be looking at as there is quite a few different ones.

Cheers.

All the calculations are in here:

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

Whilst it explains how to build your own from scratch, it will help you also.  Measure the resistance of your spare strap and its approximate length.  About halfway down the article there is a link to a spreadsheet and if you skip to "Step 3" you can input your measurements (resistance, length and input voltage) to find out the power output.

You are aiming for something in the region of 0.3W per cm of length (so a 10cm band should ideally product 3W).  The straps are just made from resistive elements and they don't really care about the input voltage as such. The lower the input voltage, the less power output.  A bit more or a bit less than 0.3W per cm will be OK but too low and it won't be very effective and too high = toasted equipment :)

Depending on the PWM controller, it will tolerate some range of input voltages - you'll have to check the specific model to find out what works.  The cheap eBay LED dimmers work for 12-24V, much lower than 12V and they may not work.

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