cooling fan suck or blow??

[lancerde]

Just a quick question, do the primary mirror cooling fans blow air onto the back of the mirror or suck the air away from it? I only ask as I have just tested the fan on my LB 10'' and the fan drew air from the the scope onto my hand.

[Tim]

Good question!

I usually set mine up to blow air against the mirror and up and out of the tube, so that dust isn't drawn into the tube, but that might be wrong

Tim

[jflowers]

12" LB Sucks too (I mean sucks air, its a great scope )

[todd8137]

My fs for the 12" dob blows never had any probs

[Moonshane]

I have always had mine blowing onto the back of the mirror and up the tube. if you are concerned about dust getting in you 1) probably should not have a newtonian! 2) can always add a filter before the fan.

[jimmyjamjoejoe]

Mine blows out the bottom. I remember seeing a review a while back where a guy tested both ways. There wasnt much in it as far as cooling time, but blowing out the bottom drew the boundary layer off of the mirror in a cleaner fashion meaning that views WHILST cooling were a bit better.

[JamesF]

I wondered about this when I put the fan into the box for my modded SPC900. I couldn't decide which way around might be better so looked at it from the point of view that either way around the air leaving the box would be warmer than ambient, and where would I prefer that to be? In my case that was out behind the box where it was heading away from the telescope, so that was the way I arranged the fan.

James

[cathalferris]

To get more effective cooling of the primary the airflow should be directed onto the mirror.

If the tube is correctly sized, there should be no real issue with slightly warmed air moving up the tube walls as this should be outside the lightpath.

The ideal scenario would be for one or two fans mounted on the side of the tube to push air across the surface of the primary (preferable from bottom to top in case a fan got loose) as well as a decent airflow fan with an adjustable speed pointed at the rear of the mirror.

The side-mounted fans are to break up the boundary layer at the front of the mirror and push this away, cooling the front as well. The rear fan should be on fast speed for the first while to speed up the temperature equilibrium, then kept at a slow speed throughout the night's observing to ensure that the mirror tracks the changes in temperature throughout the night. If the air in the tube is well mixed, there should be no temperature gradients that will distort the incoming light. A slow rise of warmed air from a mirror without a fan, will affect the eyepiece view more than a well mixed and fan-assisted tube of air.

Some of the best planetary scopes have been built with airflow as a crucial component of the design, with gaps in the tube just above the primary mirror to ensure that any boundary layer heated by the mirror escapes without entering the tube, using an oversized cork-lined tube to minimise heating or cooling of the air in the tube, and having well-placed and well-specced fans to break up boundary layers and ensure a mirror that tracks ambient temperatures throughout a session.

And remember to stand downwind of the scope if you can so that you'll not be a local source of hot air traversing your nice clean lightpath, and to breath out away from the scope if possible.

[JamesF]

To get more effective cooling of the primary the airflow should be directed onto the mirror..

Is that because you are then forcing cool air into contact with the mirror?

James

[lancerde]

Thank you for all the answers.

[jimmyjamjoejoe]

Either way you're going to be drawing air into the tube and onto the mirror. Whether it comes from the top or the bottom will have no realistic effect on air temperature at all, it's all about shedding the boundary layer cleanly.

Think of how it would look in a wind tunnel, blowing in from the bottom would force air smoothly across the bottom of the mirror at high pressure, around the sides, and then if there is no baffle above the mirror, it'l lose pressure and create turbulence right above the mirror and in the tube. I'd rather have the high pressure on the top of the mirror, and turbulence outside on the floor.

[lancerde]

Makes sense to me. Sucks it is

[todd8137]

Hi again I tested. 3 fans,now what I did was stick the fan on blowing,now if fed with 12v the fan blows at high speed miss the front of the fan a simple smoke test with some plastic pipe shows the air shots up the sides of the tube ,no good

Run at 9 volt and smoke tested the ,smoke was around the front of the mirror and the back sucking did nothing with a smoke test

It s the speed the fan blows at and hoe air circulates round the mirror

Pat

[cathalferris]

Is that because you are then forcing cool air into contact with the mirror?

Exactly. It's the best way to get the boundary layer off the rear of the mirror, otherwise you are just pulling air from around the mirror and this is a waste of energy as it is doing very little useful by comparison with blowing onto the mirror. Pushing the boundary layer off the rear of the mirror pulls heat from the centre and in turn the front of the mirror, and dumps it into the surrounding air. Sucking air out of the rear only moves air around the edge of the mirror cell, and doesn't disturb the boundary layers anywhere near as much, and fails to cool as well as the blow would.

Always blow on to the back of the mirror at a minimum. The slightly warmer air being blown around the rear of the cell has much less effect on the lightpath than a high temperature differential across the boundary layer on the front of a slowly cooling mirror, and will be relevant for a much shorter time than sucking out the bottom or no forced air..