Cooling fan question!

[matteboy]

Hi Everyone

It's been a while since I've posted but happy days are here!

I got a new toy a couple of weeks ago from Altair Astro, an 8" newt and it has a cooling fan on it. Forgetting it was there I haven't used it yet but was wondering:

Is it just used to cool the 'scope down quicker and do you turn it off after a while or leave it running while observing?

Will it help or hinder with any dew build up?

I must say how pleased I am with my new addition and although I've only had one half decent observing session so far I'm sure I'll get to master the mount (HEQ5) eventually.  It's taken a lot of learning and massive brainache to get this far but it is so worth it when you clock a moon shadow on Jupiter and watch the moon come out from in front of the planet live with your own eyes!

Looking forward to your replies and thank you in advance!

[Bizibilder]

The fan simply blows air through the scope.  It will help with cool down but may hinder if it blows more humid air over the optics as dew is more likely to form, especially if they are cooler than the dew point.  I believe the purpose of the fans is to use them occasionally to "blow away" tube currents and the like to improve the "view".

[Laurie61]

The main idea is to help bring the mirror temp down to match air temp, as Roger says they can also be used for clearing tube currents. I have a cheap thermometer with temp sensor which I have fitted to the mirror. I keep the scope outside but if night time temp's are falling away quickly the fan can be used to help the mirror keep up. I usually turn it off when the mirror is about +1c above air temp, this helps to stop primary mirror dewing on damp nights.    

[matteboy]

Thanks for the replies guys, another little bit figured out