Suck or Blow?

[BlueAstra (Graham)]

Anyone know definitively whether fans behind a Newtonian mirror should suck air down the tube onto the mirror surface or blow up the tube onto the rear of the mirror?

[ourkind (Carlos)]

My guess is 'Blow' I would hate to suck dust down the tube and onto the surface of the primary mirror.

Let see what those in the know have to say.

[iceman (Mike)]

Blow.
You want to get the ambient air from outside and blow it on the back of the mirror.

It helps cool it down to ambient temperature and helps to stop dew forming on the mirror.

[GrahamL]

Definately ,100% it needs to Suck .

[Barrykgerdes]

Blow is definitly the way to go.

Suck will just bring the outside air/moisture into the tube.

Blow can even process and filter the air before it is introduced to the mirror.

Barry

[AstralTraveller (David)]

Obviously a topic to start religious wars . I'd start by looking at what makers like Obsession or SDM do. I imagine they have done their homework. Digging around in the recesses of my memory I think the answer is 'both'. Blowing does a better job of cooling because the air is forced more directly onto the mirror but the air coming around the mirror generates turbulence in the tube. Sucking is less effective but the air flow in front of the mirror is smoother. So blow for initial cooling (until the mirror is just above ambient air temperature) and then suck to maintain the relative temperature as the air cools during the night. There is also a school of thought that you put the fan in the side of the tube just above the mirror with an air inlet opposite and draw air across the front surface.

Of course in the UK you just leave the scope inside and curse the clouds (he says looking out the window at the rain).

[multiweb (Marc)]

I've seen both. Sucking on a Dal–Kirkham, blowing on big dobs, sucking or blowing in small newt, blowing on SCTs.

I guess there are a number of options depending on the scope you are using and if you are trying to get the optics in temperature quickly or just wanting to get rid of the boundary layer on your primary surface. What is more important is knowing the flow of the air in your scope rather than blowing or sucking. It differs with every design and type.

re: moist air and dust, doesn't matter. One way or the other you'll cope plenty of both through the front aperture over time.

Some useful reading here.

[erick (Eric)]

my dob don't suck. Blow!

[asimov (John)]

By sucking air down the OTA you set up a cyclonic effect: The column of air in the OTA starts to spin (nature of a fan) & induces some not nice viewing/imaging 'opportunities'..Don't do it

[Shark Bait (Stu)]

This thread has inspired me to fit a fan to the underside of my primary mirror. I have set it up to blow onto the pyrex surface. The SW sales pitch claims that a cooling fan isn't required for pyrex but a number of threads on this topic has convinced me to see for myself.

The wires are a little untidy at the moment. I will tidy them up when I have had a chance to confirm that it is worth keeping.

This unit was purchased from a local computer store and is running off my 12V portable power supply. All components were picked up for just under $20.

[BlueAstra (Graham)]

Thanks for all the replies so far. I didn't realise what a complicated topic this is! The manufacturer of my scope installed fans that suck (down the tube, not rubbish fans!), and with posts I'd seen about fans blowing air onto the rear of mirror I wondered if this was correct.

One other thing, in daylight when you look down the tube it looks quite dark, but when you switch the fans on you can see a fair amount of light leakage onto the lower inner surface of the tube at the edge of the mirror. Need a baffle or filter to stop the light leakage?

[Peter Ward]

http://www.youtube.com/watch?v=ZXOAc5yt218

[bartman (Bart)]

Suck from the tube...... is my non professional view. My thoughts are that if it blows up the tube it creates eddies above the mirror. If the fan draws air from the tube and away from the mirror ...no turbulence.....
Just my thoughts.....
Bartman

[Shark Bait (Stu)]

Suck or Blow?

Austin Powers might have a different take on this.

[GrahamL]

..

[tlgerdes (Trevor)]

For sucking to work, the whole rear of the newt needs to be enclosed with the only escape via the fan, else the fan will just suck air from next to fan outside the tube and have absolutely no effect.

You can test this by putting a fan in front of your face blowing awau from you and feel the wind rushing past your ears as it suck the air past you into the nearby room.

Now do the same test above with a 1m cardboard tube place against the rear of the fan and put your face up close to the tube.