fan cooling for tube newtonians - Page 2

slice of heaven · #21 02-12-2005, 05:02 PM

Quote:

Originally Posted by Nightshift

I disagree Geoff, I have the 12" GSO and the volume of air passing out the top of the tube when the fan is running is very high, it clearly passes over the entire mirror and up the tube, if you have any doubt, try putting the cap on and removing the smaller aperture cap, the wind blows quite strong. Viva la GSO.

Change the configuration to a setup with a sealed bottom(as Starkler is suggesting) and the difference is quite noteable.

DaveP, the gs 12" fans are 75mm across the blades, might be a bit bigger than yours. Lately I've been using an 18v battery pack for the initial cooldown period, then reverting back to the 12v pack for normal operation after cooldown.

davidpretorius · #22 02-12-2005, 05:13 PM

thanks, mine is 56mm for the outer housing.

that is good, my hole is 57mm in the side of the tube, so i can always go bigger, but i can't go smaller!!!!

Quote:

Originally Posted by slice of heaven

Change the configuration to a setup with a sealed bottom(as Starkler is suggesting) and the difference is quite noteable.

DaveP, the gs 12" fans are 75mm across the blades, might be a bit bigger than yours. Lately I've been using an 18v battery pack for the initial cooldown period, then reverting back to the 12v pack for normal operation after cooldown.

slice of heaven · #23 02-12-2005, 05:20 PM

Quote:

Originally Posted by davidpretorius

thanks, mine is 56mm for the outer housing.

that is good, my hole is 57mm in the side of the tube, so i can always go bigger, but i can't go smaller!!!!

I've been thinking of looking for a bigger one still to try, or maybe coupling 2 or 3. The initial cooldown period is the time consuming part for a quick setup, hence the using of an 18v pack to crank it up.

asimov (John) · #24 02-12-2005, 05:32 PM

I usually stoke up a big pedestal fan & stick it at the back of the mirror after setup up. Turn it on flat out & go away for a couple of hours. It must help a bit? You can really feel the air coming out the focuser end dramatically!

Starkler (Geoff) · #25 02-12-2005, 06:45 PM

I wonder if a few finned heatsinks stuck to the exposed underside of the mirror with thermal goo would speed up cooling noticeably.

asimov (John) · #26 02-12-2005, 07:10 PM

Yep. Been thinking along those lines too recently.

davidpretorius · #27 02-12-2005, 07:34 PM

at the end of the day, what is our ultimate aims here?

for viewing: 1 to 2 degrees difference would be ok
for imaging: less than .5 degrees for the pro's
are we wanting the mirror to be able to cool at say 4 degrees a minute so that within 15 minutes you have caught up to ambient, do we want it in less than 15 minutes???

Starkler (Geoff) · #28 02-12-2005, 07:43 PM

Quote:

Originally Posted by davidpretorius

at the end of the day, what is our ultimate aims here?

No boundary layer !
I dont see why stricter thermal requirements should apply to imaging only.
At snake valley when the temperature was stable and seeing was good, it was good for everyone, imagers and visual observers alike. When it was bad, it was bad for everyone.

slice of heaven · #29 03-12-2005, 12:13 AM

Quote:

Originally Posted by Starkler

I dont see why stricter thermal requirements should apply to imaging only.

Neither do I, Your 1st thread opened my eyes to the possible improvement for visual viewing Dave. I,m now thinking Bird's peltier coolers might be the asset for visual as well,

davidpretorius · #30 03-12-2005, 04:40 PM

To get my brain straight.

Boundary layer is a layer of warmer air that adds to distortion and needs to be replaced with air that is ambient???

Starkler (Geoff) · #31 03-12-2005, 05:36 PM

Dave what we call the boundary layer is a layer of warmed air sitting on the face of a mirror which is warmer than ambient. This acts as a wavering lens and distorts the view.

There are two ways to attack this problem:
1: flush the boundary layer air out of the optical path.
2: make the mirror temp the same as ambient in the tube so there is no boundary layer.

davidpretorius · #32 03-12-2005, 05:41 PM

Quote:

Originally Posted by Starkler

1: flush the boundary layer air out of the optical path.
2: make the mirror temp the same as ambient in the tube so there is no boundary layer.

1: so optical viewing can still be done at say 1 degree if fan is left on with my current setup???

2: no temp diff = no layer

so at the end of the day, it would be great to be able to work out what temp difference create an unwanted layer.

maybe two identically collimated scopes, one cooled, one not in the same seeing conditions.

rmcpb (Rob) · #33 03-12-2005, 05:45 PM

From what I have read it seems to me that even 1 degree will influence the views dramatically but it seems to be mainly due to the boundary layer that a warmer mirror generates.

From this I think the diagram at the top of this thread is a beauty as the fan will cool the mirror over time BUT disrupt the boundary layer immediately. I'm changing the setup on my scope to be just like this and give it a go, can't do any harm but will probably improve the performance of the unbaffled fan I have at present.

Problem is its the first clear night for ages and my scope is in pieces and the mount is a disaster

Good thing I still have the binocs

Cheers