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Suck or Blow?

BlueAstra

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jimmyjamjoejoe

jimmyjamjoejoe

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The difference is negligible. I saw a study a while back that concluded sucking air down the tube clears the boundary layer much more cleanly, allowing good views before the mirror was cooled, whereas blowing onto the back of the mirror cooled it down slightly quicker, but with messy tube currents.

So, swings and roundabouts really. Generally, if i'm using a fan it's because i want to view sooner rather than later, so i go with option one. :)

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Carbon Brush

Carbon Brush

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I suppose that if the fan blows dust from the air onto the back of the mirror, that has to be better than encouraging it to fall onto the front.

If you include a filter in the fan housing the mirror rear stays clean.

Apart from that uneducated and ill informed thought, dunno!

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ollypenrice

ollypenrice

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Neither. The mighty Ralf, creator of the 40th wave watercooled Newt and the Binosaur, and many other optical wonders, insists that you should suck (one side) and blow (the other) across the mirror to destroy the boundary layer.

Well, you did ask!! :grin:

Olly

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BlueAstra

BlueAstra

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Fantastic! I have three fans at 120deg behind the mirror. Do I arrange 2xSuck+1xBlow or 2xBlow+1xSuck? Or I could make a electronics unit to have each fan alternate between suck and blow with timings such that it creates a rotary effect. :smiley:

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Psychobilly

Psychobilly

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Fantastic! I have three fans at 120deg behind the mirror. Do I arrange 2xSuck+1xBlow or 2xBlow+1xSuck? Or I could make a electronics unit to have each fan alternate between suck and blow with timings such that it creates a rotary effect. :smiley:

The problem with modern brushless fans is they won't reverse direction by simply swapping the polarity of the power connections over... what you want is some good old fashioned DC brushed ones...

Peter...

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BlueAstra

BlueAstra

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Well, the reason I asked was that mine suck air down the tube as fitted by the manf. and I wasn't sure it was correct as I'd seen many comments from people with fans blowing up the tube onto the rear of the mirror. I also think I'm getting a layer of dust on the mirror, but this may be my imagination. I thought there would be a definitive answer, but it seems 50:50 at the moment!

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ollypenrice

ollypenrice

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Yes, I agree with Chris. Ralf's watercooled mirror is only watercooled to bring it to ambient, then it stops. Three other things; hot air rises so go with the flow, the outlet is bigger at the top and the bottom of the mirror is less precious so can risk being blown on. (Grit etc?) These are only conjectures, though.

Olly

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Tinker1947

Tinker1947

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I did consider a fan, but will sometime soon, attach a metal plate to the 4 screw holes and then adjust the balance of the scope by adding a magnet and metal plates to adjust the weight between my Pentax XW30 and the ATIK 428, the difference being 10 ozes

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ollypenrice

ollypenrice

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The most effective way to deal with thermal management in a Newtonian reflector was clearly demonstrated in this "Sky & Telescope" article in 2002: http://www.crossflow...cle_01-2002.pdf .

Yes, so Ralf was right. But his watercooling of the mirror takes it down to just below ambient as he starts his crossflow fans. He then turns off the watercooling. However he ground the mirror without the usual allowance for thermal distortion, knowing that he wasn't going to get any. It seems odd that the crossflow idea has't gained ground.

Olly

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jimmyjamjoejoe

jimmyjamjoejoe

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I don't like the idea of sucking air down towards the mirrors front face. Doesn't it have a tendency to suck all the rubbish down to he mirror as well that's in the air?

I don't honestly think it's an issue. If something is airborne, it's very light, and a fan is going to pick it up and move it around whether from the top or the bottom of the tube. It's not like airborne dust will hit the back of the mirror, then give up and go home. There is airflow around the edges of the mirror etc, so dust is gonna get in one way or another.

In nearly a year of using my fan, the mirror still looks almost pristine, even with a light shone down the tube.

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BlueAstra

BlueAstra

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Yes, so Ralf was right. But his watercooling of the mirror takes it down to just below ambient as he starts his crossflow fans. He then turns off the watercooling. However he ground the mirror without the usual allowance for thermal distortion, knowing that he wasn't going to get any. It seems odd that the crossflow idea has't gained ground.

Olly

Driil big holes in my nice carbon fibre tube :eek:

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ollypenrice

ollypenrice

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Driil big holes in my nice carbon fibre tube :eek:

Drill nice big holes in your nice CF tube. Remember that CF is great for not expanding or contracting but not great for losing heat. Or put a crosslflow push-pull fan system in so your (nice) holes won't show...

Courage!

Olly

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BlueAstra

BlueAstra

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One thing I have noticed with the rear fans in daylight - when the fans are stationary it looks quite dark when looking down the tube at the mirror. However, when the fans are on you can see a fair amount of light spill around the inner base of the tube near the fans. One more reason to add air filters - they will also cut down on light spillage.

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