Dew problem

[Imd]

Hi All,

I have been having problems with few recently on my secondary. I have a skywatcher 200p reflector. I font use a dew shield or heat straps. I am wondering whether the reason for my problem is that I place the camera on topside of the scope and so the secondary is pointing mostly up at the sky. I'd do this as I fear the camera may fall out of the focuser if I place the camera bottom side.

Do you think this is the reason why i am having problems with dew?

How do you you position your camera?

Thanks

Ian

[Imd]

Retyped dew  to predictive text errors

Hi All,
I have been having problems with dew recently on my secondary. I have a skywatcher 200p reflector. I don't use a dew shield or heat straps. I am wondering whether the reason for my problem is that I place the camera on topside of the scope and so the secondary is pointing mostly up at the sky. I'd do this as I fear the camera may fall out of the focuser if I place the camera bottom side.

Do you think this is the reason why I am having problems with dew?
How do you position your camera?

Thanks
Ian

[andrew s]

Ian, I doubt it is the position. I had dew on my secondary the other day for the first time in years (mine points along the Dec axis). I made a small heater with 8x100ohm resistors wired to give 50 ohms  which gives about 3W at 12 V. This seems to work ok.

Regards Andrew

[Zakalwe]

If the camera is so loose that it might fall out, then it's going to cause issues with flexure.

I do know what you mean though...I'm paranoid about £2k of camera/filterwheel/flters falling out if something gets loose. I have used a neck lanyard to make a safety strap to tie the camera back to a mounting point on the 'scope (I use a 'frac so the camera is always pointing to the ground).

[Imd]

Thanks Andrew and Zak,

The camera is not loose I just get paranoid too. I may need some dew heaters but I think I will try creating a dew shield first because its cheap, there are less wires and it will make my scope look like a right beast. 

I did once read that when mirrors are point up they are more likely to catch the dew.

Ian

[andrew s]

Ian you are right that if your main mirror is pointing at the sky, which it will be, then the mirrors will be hot with respect to the sky and so radiate energy and cool. If the mirrors cools below the dew point then dew will form. A dew shield helps by reducing the amount of fresh air that can get to the mirrors. The secondary well cool irrespective of if it is up down or sideways.

Regards Andrew

[mareprah]

Hey Ian.
The position has no or almost no effect on the dew. I struggeled with dew on my secondary mirror as well. Since the secondary on ma 10" quattro is huge, I was wondering, if to fit it with 8 resistors or just 4? I went for 4, and the result is excelent (they are 1W).
Now Andrew mentioned he has 8 resistors with full resistance of 50ohms. This gives you 2,88W of power, which is (at least for me) too much!!! The resistors get hot over 1,5W on my secondary, and I don't want to heat it more than is neceseary.

To give you a rough idea. If you'll be running your resistors on 12V, the formula is:

P=U^2/R   (R being the total resistance of all resistors, since they "have" to be wired one after another - don't know the english word for it).

So, what I made is this. I've got 4 resistors with total resistance of 42,2ohms, and I'm running them on my universal 500mA adapter. From that, I can get 0,52W; 0,92W; 1,48W or 2,05W....

At this point, I must say I haven't yet tested it below -2°C.

If you want your heater to run on about 1,5W with 12V, use 8 12ohm resistors (can be the normal 0,25W ones) or 4 24ohm resistors (must be 0,5W or higher).

For any other info, send me a PM, I'll be glad to help you, send you a foto of my heater etc... It's not that big of a deal, but you should be a bit carefull...Not that anything scary can happen, but I don't know what can go wrong if you overheat your sec....So the best way to avoid that is to stay as low on power as you can...

[mareprah]

One more thing about the position of the camera. I keep mine facing the mount, its easier to balance the scope that way...

[andrew s]

Yes I use 50ohm but I have a controller that switches the current on and off so I can set whatever level of heating up to a max of about 3 watts

Regards Andrew

[mareprah]

Aha, thats better:) Didn't want to panic or anything, but I never even tried 3W since 2,5 almost burned my fingers...couldn't hold the resistor. Still, i have to wait for the winter to come, to give you more info, I presume 0,5W will really be too little for like -15°C