Dew shield and Hair dryer

[David Johns]

I have a new 11 inch SCT (CPC) and have been troubled with heavy dewing. Could anyone suggest suitable material (and where to buy) for making a 'flexible' dew shield for an 11 inch SCT? I have phoned/visited quite a few craft/art/hardware stores but apart from cartrigge paper they could not help. Some stores do have suitably thick plastic sheets but not in the size I require (at least 100 cm wide).

Also, to help with my dew problem I have just purchased a 12 volt hair dryer for $20 but there are not specification regarding the current it draws. I assume that since the dryer operates from a car cigarette lighter it would raw at least 5 amps (is that right ?). However, I want to purchase an elctrical power source (from 240 to 12V) but am not sure what amperage I need (I don't want to end up overheating the power source). I have a feeling that the power source will be a lot more expensive than the hair dryer. I would welcome any advice.

Many thanks

David

[Dennis]

Hi David

I made a dew shield for my C9.25 from closed cell foam - the kind of stuff used in camping mats for sleeping on. Their closed cell construction prevents them from soaking up moisture like a sponge, and it also appears to make them tough and tear resistant. Good foam/rubber outlets, such as Clark’s may also stock the closed cell mat too. Something in the 5 to 10mm thickness range would be ideal.

I chose the strategy of preventing dew rather than clearing it off after it had already formed. To this end, I use the Kendrick dew heater system (http://www.kendrickastro.com/astro/dewremover.html). Basically it is their standard controller unit with dew heater straps that plug into it, and then they fit around the SCT corrector, eyepiece(s) and finder scope to prevent dew build up in the first place.

Cheers

Dennis

[RAJAH235]

Hi David,
I doubt very much that you'll be able to buy a suitable 240V to 12Volt 'plug pack' to run your hair dryer as most take upwards of 8 to 12 AMPS.
Some even more.
You're better off with a high capacity battery, say 17AHr n ^^?, if out viewing for a long time.
You probably won't use the hair dryer much but....
And 'ditto' to what Dennis has said re; Clark Rubber & The Kendrick System.
Regs, L.

[erick]

David

Wattage of hairdryer (on the side or on the packaging?) divided by 12 volts gives the amps required.

Expect the hairdryer to be around 120 Watts (note that a 240V hairdryer will be more like 1500-1800 watts!!). Therefore 120/12 = 10 Amps.

I have one and have connected to my SLA 12 volt battery (Jaycar's 18 Ahr I think it is) and it chews the amps out of it! I'm a bit worried the discharge rate won't do the battery any good. :sadeyes:

And if you have a 240V hairdryer around for comparison, you'll quickly see the huge difference in heating between the two. But, any heating is better than none!

Eric

[acropolite]

Altronics has a 15amp power supply (http://www.altronics.com.au/index.asp?area=item&id=M8260)variable from 11-16volt for $149 which should (see disclaimer in ad re incandescant loads) run the 12 volt hairdryer, however a cheaper option would be to buy a 240v hair dryer for those times you want to use 240V power or simly use a battery and charger.

[erick]

That's what I did - you'll get an el-cheapo 240 Volt hairdryer for less that $20. Just make sure it is one with an easy to use, nice chunky on/off control. Mine looked fine in the shop, but in the cold with gloves, it was getting a bit fiddly to slide it on/off.

[David Johns]

Many thanks for the advice. It looks like I need to get a large power supply or a car battery....or sneek my better-halfs 240v version. I do like to option of preventing dewing with a heater rather than curing it once it has formed ... so I now have to make a decision (oh, no!).

Many thanks again

David