Heating and cooling..

[MarcuSmythe]

I am getting ahead of myself here because of clouds.....but I was wondering about heating and cooling. As far as I can work out from the excellent posts/stickies on this fine site; I need to use a fan to cool the primary and use dew straps+shroud to heat the secondary to avoid dew? SW 200PDS. Just seems a bit counterintuitive.....or is it 1 for visual and another for AP?

 

Thanks

Marcus

[Cornelius Varley]

The cooling is required to produce a column of thermally stable air above the primary mirror (cold air is more stable than warm), the dew strap or shroud helps to keep the secondary mirror just above the dew point. Helps both visual and astro photography.

[Guest]

You definitely have the jist of it.  The fan is definitely needed if you are moving the scope out from the house.  If you keep it in a unheated shed or garage you wont need to run it very much if at all, this will depend on the size of the scope.

As to heater bands just make sure to run them just enough to keep dew feom forming.  I run multiple bands and the one for the OTA is usually around 30 percent power.  On really heavy nights or winter i will be at 50%.  I run the one on the finder at 50% all the time and the one for the eyepiece the same as the OTA give or take.  If you start stacking eyepieces in barlows or coma correctors you will have to play around with the band location and power settings. 

[Mr Spock]

It's never made any sense to me, spending ages getting the scope cool only to heat it up again. Any heat source at all will create heat currents, no matter how slight. 

[Stu]

2 hours ago, Mr Spock said:

It's never made any sense to me, spending ages getting the scope cool only to heat it up again. Any heat source at all will create heat currents, no matter how slight. 

There are times when it is unavoidable, the alternative being dripping wet corrector plate. I’ve even had a Mak dew up on the inside of the corrector and the primary.... A lot depends on your location and how bad the dew is, it seems to vary a lot.

Obviously dew strips should be on the lowest setting just to keep the corrector above the dew point.

[Guest]

3 hours ago, Mr Spock said:

It's never made any sense to me, spending ages getting the scope cool only to heat it up again. Any heat source at all will create heat currents, no matter how slight. 

Try observing in my conditions, it will make perfect sense.  Its all about balance.  Use just enough heat to keep stuff clear.  As far currents, while I have never really looked for it, I have yet to notice any.