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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Real life experiment with fan configurations...
      #4122861 - 10/17/10 08:16 PM

Since a year or so I've had a self-built truss Dob (a 16" f/4.5), and I was always wondering what fan set-up would actually work best at scrubbing the layer of warm air above the mirror...

Well, tonight I had an ideal opportunity to experiment. Temperatures were dropping like a brick and the outside temperature was a lot lower than where the scope was stored.

Jupiter was relatively high, with an Io transit in front of Jupiter roughly where the now-not-so-clear SEB used to be, so it was relatively easy to determine how good the views were (cues: how easy was it to see Io itself --not its shadow-- on Jupiter's disk, how round and how stable did the shadow appear, could you still see remnants of the SEB, could you still see a fairly faint blue festoon just reaching for Io, and how well delineated were two nice white storms and a barge in the NEB; there was also some detail closer to the North pole that helped a lot, but I need to brush up on knowing my nomenclature to name these.)

Observation without Paracorr, with 11mm TV Plössl in 2x Baader/Zeiss barlow, 324x. Obviously with the scope not colled properly on purpose (although fans had been running during the scope buildup routine and collimation, for roughly 10 minutes).

The scope's "official fan configuration": a mirror box top roughly 16cm above the mirror face, and 6 80mm fans on the sides (three at the bottom pushing air, three at the top pulling air). At the back there's a central 120mm fan that can be reconfigured to either push air or pull air. The side fans and back fan all have controllable speed.

Because of recent threads, I made a baffle that could be installed just on top of where the side fans are located (actually, the baffle used the part of the fans sticking out for support).

1) Side fans on, moderate speed (at full speed the mirror cools fast, but during observing running these at full speed gives an unstable view with momentary good images for very small intervals and mush at all other times).

1a) no back fan.
1b) back fan pulling
1c) back fan pushing

1b beats 1a beats 1c. If the side fans are switched off, 1c beats 1a, and 1b still beats 1c.

As the back fan pulling clearly wins, I decided to determine what side fan speed won. It turned out that very gentle side fan air movement won, at least with only the mirror box determining where the air comes from if you don't run the side fans.

2) Mauro da Lio baffle: 7cm over the mirror, with a hole just as large as the mirror (40cm); side fan orifices covered so that the airflow is forced to pass over the mirror's sides.

2a) fan pushing
2b) fan pulling

2b) beats absolutely everything else (and by quite a margin). Unlike the side fan arrangements, you don't get good "seeing" for brief moments and then mush, but a fairly stable view.

You did need more power fed to the fan to get the best results when it was pulling air.

Just to make sure I wasn't dreaming, I reverted to the other set-ups. The views had improved (the mirror had been cooling), but all the views with side fan arrangements still had more trouble with the mirror thermals than the baffle plus single fan pulling at the back.

Moving the baffle up (using some very crude spacers) made it less effective. Moving it down was impossible due to the fans still being installed.

So it looks my scope is going to become a lot more simple (thanks to Mauro da Lio).

Anyone close to Belgium interested in 6 good 80mm fans?


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backwoody
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Reged: 01/08/07

Loc: Idaho USA
Re: Real life experiment with fan configurations... new [Re: sixela]
      #4123165 - 10/17/10 10:35 PM

Sixela, thanks, great report. I've often wondered about the various fan options.

c/s,


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vliegnerd
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Reged: 01/26/09

Loc: Gouda, NL
Re: Real life experiment with fan configurations... new [Re: backwoody]
      #4123423 - 10/18/10 01:17 AM

Thanks, for the hard work, sixela.

That settles it for me. I will use a pulling fan and the "mauro da lio baffle" in front of the mirror in my 12" tube to truss conversion.


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Olivier Biot
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: Real life experiment with fan configurations... new [Re: vliegnerd]
      #4123504 - 10/18/10 03:05 AM

Thanks for reporting your test results Alexis!

I think I am set now with my fan configuration: 1 big fan at the back + a couple cheapies to the side for initial cooling.

Cheers!

Olivier


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cadiz - Spain
Re: Real life experiment with fan configurations... new [Re: Olivier Biot]
      #4123523 - 10/18/10 03:58 AM

Sixela,is this design isn't it?. And then the side fans don't help very much. The most effective would be a good rear 120 mm fan pushing air and a deflector ring
http://autocostruttori.blogspot.com/2009/08/yet-another-method-for-cooling-mirrors.html


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: Olivier Biot]
      #4123549 - 10/18/10 04:47 AM

Quote:

Thanks for reporting your test results Alexis!

I think I am set now with my fan configuration: 1 big fan at the back + a couple cheapies to the side for initial cooling.




The problem with the cheapies on the side is that when they're not working the big fan will pull air through their blades (instead of from between the mirror and the baffle, from the top of the mirror).

Unless of course you put them on top of the baffle, but then they simply don't do much, at least not if they're not well above the baffle angled down (what could work for initial cooling is blowing from the corners of the top of the mirror box, with fans angled downward).

Looking a the airflow with just the "deflector" baffle, though, I doubt all these top fans are actually necessary for anything.

I'm getting rid of mine.


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: VĂ­ctor MartĂ­nez]
      #4123552 - 10/18/10 04:48 AM

Yes, that's the design, but pulling air works better than pushing air.

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magic612
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Reged: 09/30/08

Loc: S. of Chicago's light dome
Re: Real life experiment with fan configurations... new [Re: sixela]
      #4123815 - 10/18/10 09:19 AM

Interesting results. I wonder if this would have a similar effect on a tube reflector as well.

One of the benefits of a refractor is that the lens is up quite high above thermal issues occuring on the ground - rocks, pavement, concrete, body heat, etc. So pulling air from the same general location as where a refractor lens would be (from the top of the tube down to the mirror) I would think might create better thermal consistency. The other thing this potentially does for a reflector is stabilize the air flow into the tube; pushing air towards the back of a mirror and then up and around it would creates turbulence. The airflow coming towards the face of the mirror should eliminate a good deal of that.

Of course, it also means mirrors might need to be cleaned more often, and the secondary may need a heater to avoid dew problems. But if it stabilizes the image, then this would be a worthwhile idea.

Thanks for sharing your results, sixela.


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: magic612]
      #4123829 - 10/18/10 09:27 AM

Quote:

and the secondary may need a heater to avoid dew problems.




Actually, I found that if you use a shroud (which forces the air to come from the top of the UTA) it also helps in preventing dew just as much as a set-up that blows air into the tube (and it keeps the spider vanes close to ambient, too). Granted, my sceondary isn't that prone to dewing either, not being installed in a shroud: I've found that a shrouded secondary holder and the vanes act as a very efficient heat sink, so my secondary is just glued with three dabs of silicone to the (plastic) secondary holder.


I expect it works even better in tube scopes (those are radially symmetric).


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Fireball
professor emeritus


Reged: 03/24/06

Re: Real life experiment with fan configurations... new [Re: sixela]
      #4123868 - 10/18/10 09:45 AM

Sixela, I assume your dob has a square layout of the rocker box similar to Mauro's, hasn't it?.
Do you think it will be any different when you have a round one (e.g. like the Lightbridge)?
I was thinking about introducing it to my scope but still do not know what the optimum distance to the mirror will be.
BTW, I use a shroud and have one fan sucking the air out of the tube. The back of the tube is isolated with an acrylic glass very similar to the one Sky Captain is using.


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: Fireball]
      #4123960 - 10/18/10 10:24 AM

Quote:

Sixela, I assume your dob has a square layout of the rocker box similar to Mauro's, hasn't it?.




Yes, but that's actually worse than a round one.

Quote:


Do you think it will be any different when you have a round one (e.g. like the Lightbridge)?





This thread suggest that it works. Note that he actually blows air from the back; I tested that and at least in a square mirror box with a baffle at the top it doesn't work as well (the air over the mirror is evidently a lot more turbulent), though it might actually indeed work well in a Lightbridge plus one ring baffle (the Lightbridge has no baffle at the top of the mirror box and has a cylindrical mirror box).


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Olivier Biot
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: Real life experiment with fan configurations... new [Re: sixela]
      #4124165 - 10/18/10 11:52 AM

For my 14" I plan on using one 120mm box fan. The unit I purchased is silent (20.5Db) and has a 44.7cfm air flow at 12VDC. If I install a baffle ~5cm above the top of the primary, I understand this should work for my setup, right?

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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: Olivier Biot]
      #4124337 - 10/18/10 01:06 PM

Yup. Mine is capable of 73 cfm, but I'm only driving it at a speed that generates roughly 40 cfm. You do need to baffle well so that all the airflow is useful airflow, though.


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Olivier Biot
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Reged: 04/25/05

Loc: 51°N (Belgium)
Re: Real life experiment with fan configurations... new [Re: sixela]
      #4124569 - 10/18/10 02:47 PM

Okay

I just cut a 120mm x 120mm square hole in the mirror cell board.

I was planning to use rubber spacers mounted in small O bolts in the 4 corners of the hole to secure the box fan and a grid to the back of the cell. The rubber spacers serve as shock absorbers.


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: Olivier Biot]
      #4125213 - 10/18/10 07:18 PM

I use a Noiseblocker multiframe fan - rubbers already integrated in the corners of the fan.

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cheapersleeper
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Reged: 01/22/10

Loc: Sachse TX
Re: Real life experiment with fan configurations... new [Re: sixela]
      #4126055 - 10/19/10 03:09 AM

Do you know anyone with a tubed scope that you can try that on?

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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... new [Re: cheapersleeper]
      #4126131 - 10/19/10 05:51 AM

I'll let others decide, but you could always ask SteveG (thread details above) to reverse fan flow direction and report results for a second smoke test.

I already did ask, but SteveG must've missed it.


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jtpowers
more clever in the first place
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Reged: 11/03/05

Loc: Cambridge, MA
Re: Real life experiment with fan configurations... [Re: sixela]
      #4126278 - 10/19/10 08:35 AM

Thanks for well reasoned report, Sixela. With a mirror as large as yours perhaps it's a small detail, but the exactly-mirror-sized baffle will cause some degree of vignetting. Won't it allow only strictly parallel lines to hit the edge of the mirror.

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RossSackett
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Reged: 08/17/07

Loc: Memphis, TN
Re: Real life experiment with fan configurations... [Re: jtpowers]
      #4126361 - 10/19/10 09:22 AM

This looks like a nice clear result: a back pulling fan does a better job of sweeping up convecting air in the light path, which in retrospect seems obvious since the back pushing fan is delivering a lot of warmed air from the back and sides of the mirror to right where we don't want it to be.

But what about initial cooldown? My guess is that the higher-velocity blast from a back-pusher would remove heat from the mirror much faster than the gentler pull of the pull-fan at the same cfm.

If this is the case, we might want a back-pusher for launch and a back-puller for cruise. How about a fan mounted on magnets that we can easily reverse?

Ross


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sixela
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Real life experiment with fan configurations... [Re: RossSackett]
      #4126608 - 10/19/10 11:06 AM

Quote:


But what about initial cooldown?




My experience is that you sometimes need a stronger fan, but once you've generated enough airflow (very little, actually, less than what is necessary to "scrub the boundary layer") it makes little difference. All you want is to refresh the air around the mirror, and how fast you do it has little influence as long as it's fast enough (that's also the results than Bryan Greer arrived at in his S&T articles).

Quote:

My guess is that the higher-velocity blast from a back-pusher would remove heat from the mirror much faster than the gentler pull of the pull-fan at the same cfm.




Once you have baffles, the same cfm will probably do the same thing, but you do need a stronger fan to arrive at the same real cfm (those mirror boxes do have quite a bit of impedance as far as moving air is concerned, especially when you direct the airflow via baffles).

In my case, pushing and pulling is the difference between a Noiseblocker M12-S2 and an M12-S3HS.

Quote:

How about a fan mounted on magnets that we can easily reverse?




As you can see from the test, mine can actually be reversed (it's held in a hole just big enough for it to be held through the pressure of the Noiseblocker vibration suppression rubbers in the corners, so you can just pull it out, reverse it, and stick it back in). But I don't think I will ever reverse it, since the airflow at 12V is enough to make pulling displace quite some air...


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