Primary Mirror Fan

[Skyhawk (Justin)]

Hi Guys

Just wondering for a newtonian telescope, is it best to have the fan behind the primary mirror extracting air out of the OTA or pushing air in?
Thanks
Justin

[Allan]

It's best to have the fan blowing air onto the back of the mirror, as this is the quickest way to cool it down. The air should flow around the edge of the mirror and exit out the front of the OTA. This is how the fans work most efficiently.

If you try to suck the air into the front of the tube, not only will it take longer to cool the primary down, I think you will find you will end up with a very dirty mirror very quickly.

[Allan_L (Allan)]

Hi Justin,
Most people have the fan pushing air onto the primary mirror as far as I know.
Thats what mine does anyway.

I have never heard of anyone having their fan running in reverse, but it is an interesting idea.
As I said, the main aim is to fan outside ambient air onto the primary, so as to bring it to ambient temperature as quickly as possible.

Others may have different opinions?

Allan

[Skyhawk (Justin)]

Thanks Allan

[AstralTraveller (David)]

Whether it's better to suck or blow is an old chestnut and I don't know if there is a definitive answer (though doubtless I will be told that there is!). I think Allan is right about cooling rates but the counter-argument to his other points is that (1) the air coming in the top of the tube is likely to be cleaner then the air near the ground and (2) drawing air down the tube creates less turbulence in the column of air inside the tube. So perhaps it's better to blow to get the mirror down to temperature after the scope has come out of a warm car or room and then suck to maintain the best temperature (about 1-2 degrees above air temp) as the air temperature slowly falls.

The third alternative is to blow air horizontally across the mirror's front surface. Of course it's mechanically more difficult but apparently it works well.

[Skyhawk (Justin)]

Hi David

Those were my thoughts, push air onto the mirror until it's cool and then reverse the fan, from what I've read around the web the most logical is to push the cold air onto the mirror but yes, it also pushes the "dirty" air into the tube. Extracting the warm air out the tube would be better but not as efficient....
Cross flow would be the best but I'm not about to drill huge holes in the side of my OTA.