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Thermal Issues and Fans Successes

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#201 azure1961p

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Posted 11 January 2013 - 01:37 PM

Yes mike I recommend a seperate 6v for each fan, its still tiny enough it's not cumbersome. They sit inside the rocker box. Well they used to. The 12v battery that's adjustable voltage sits on the grass. I tried one 6v and it was too taxing to run both and for lesser time too.

Pete

#202 demiles

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Posted 11 January 2013 - 03:46 PM

FYI, not all fans are not created equal. Some require as little as 4vdc to run others may need much more.

#203 Sarkikos

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Posted 11 January 2013 - 06:03 PM

Best to test before taking everything 50 miles to a dark site!

Mike

#204 rlmxracer

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Posted 11 January 2013 - 09:14 PM

I finally finished my larger fan and baffle install. It really moves air now. I have it sucking out of the bottom. The weather has turned bitter cold (for So Cal lol) it going well into the 30s by midnight so I am going to time how long it takes to get good views. The mirror will start at 70 degrees so thats a big drop to equalibrium. attached a few pics.

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#205 rlmxracer

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Posted 11 January 2013 - 09:15 PM

The Di Leo (I think)ring.

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#206 rlmxracer

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Posted 11 January 2013 - 09:28 PM

Ring installed after painting it flat black. I used 1/4" birch plywood for all the pieces.

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#207 azure1961p

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Posted 12 January 2013 - 09:41 AM

Well it's terrific but if you find like with my experiences your hot breath gets sucked down the tube and the secondary dews over its a simple fix by removing the fan, flipping it over the other way and reattaching. All else is intact.

Nice clean job'

Pete

#208 Sarkikos

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Posted 12 January 2013 - 10:07 AM

rlmxracer,

Did you construct the fan baffle from 1/4" plywood, also? How did you attach it to the OTA? I'd be concerned that a material like plywood would induce vibration into the optical system. I think I'd go with rubber or maybe foam core with Velcro attachments.

Mike

#209 rlmxracer

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Posted 12 January 2013 - 01:25 PM

rlmxracer,

Did you construct the fan baffle from 1/4" plywood, also? How did you attach it to the OTA? I'd be concerned that a material like plywood would induce vibration into the optical system. I think I'd go with rubber or maybe foam core with Velcro attachments.

Mike

I had the first night out with the new set up. The fan I used is a very low noise unit rated at 19db. The standard 80mm fan was 28db. I ran a bead of black silicone around the outer edge of the mirror cell where the wood rests on it. I have it held on by the "locking" screws which are also isolated with rubber gromets. Last night the seeing was just avg but I did use 208x on Jupiter with no notable vibration. One night is not enough to fully acertain how well this set up works but it seemed to cut my time to get good views by more that half. Before I did this I made a cardboard version of the ring and baffle for my stock 80mm fan. It cooled faster than no baffle but still took over an hour for the views to get better. That was with a temp delta of 72f indoors to 50f outdoors. Last night it was cold for So cal down into low 40s. After 20min of placing the scope outside with the 120mm fan running I had very clean views of Jupiter at 208x so I think its a step in the right direction.

I live in the inland empire of So cal a very dry place 99% of the time. I have only had a dew problem once in the last year and that was after 3 straight days of rain.

#210 azure1961p

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Posted 12 January 2013 - 04:08 PM

Glad the 120mm worked . That's what works for my 8" though if you go the boundary layer fan, 80mm is perfect - again for my 8". I never did the cool down versus time test. I still give it a full hour. It'd be great if I could cut this down like you have.

Pete

#211 Scanning4Comets

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Posted 12 January 2013 - 05:50 PM

Picked up the D cell holders today and wired everything up. Tested it and the fan runs and I'm good to go. I just have to re-solder the bottom left terminal because it is coming off.

Cheers,

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#212 azure1961p

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Posted 12 January 2013 - 06:16 PM

Looks serious!!!

Pete

#213 nevy

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Posted 12 January 2013 - 10:21 PM

Does anyone know if the Duracell procell ( the black & red ones that's sold in bulk on eBay) are better or worse than the copper ( gold) normal Duracell batteries ( D size) for running fans in the eight batterie holder power pack.

#214 Mark Peterman

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Posted 12 January 2013 - 11:47 PM

So what is the verdict???

Use a fan to blow or suck on the rear of the mirror?

How many CFM is needed (blowing or sucking) to effectively scrub the front of the mirror? (I assume this is a function of the size of the primary.)

With an enclosed rear cell (blowing or sucking) is a fan scrubbing the front of the mirror still necessary?

If mounting a fan to scrub the front of the mirror, is it best to mount the fan under the primary blowing up or on the side of the primary, blowing across?

This is all just blowing my mind.

#215 Scanning4Comets

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Posted 13 January 2013 - 08:34 AM

Just finished getting the battery pack all fixed up. I re-soldered the one wire about to come off and I found an old plastic magazine rack that had been lying around for a while and used a hack saw to cut it to size and sanded the edges so the 8 D cell battery holder fits in it.

The whole contraption fits inside of the dob stand. Now I can run my fan for a much longer time blowing air onto my 10" / 1" thick primary.

Cheers,

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#216 Scanning4Comets

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Posted 13 January 2013 - 10:08 AM

Here is the 3 speed fan attached to thick cardboard which is held to the back of my mirror cell with elastic bands. I run the fan at full speed for cool down and cut the speed back to the slowest speed when observing. Looks crude right now, but it works. I might make a new backing with different plugs because I am using clips right now like the kind you have for a 9 volt battery.

I found a small CP fan yesterday and I considering cutting a hole in the side of my telescope tube to mount it there and have air blow across the face of the 10" primary.

I'll see how it goes first with just the fan on the back first before I get to cutting a hole in the side of the tube.

Cheers,

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#217 Datapanic

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Posted 13 January 2013 - 11:27 AM

Here's what I came up with for the Horsetrail Cave. The fan is mounted on 1/8" plexiglass which is sandwiched between two foam rings. A DC power jack and on/off switch complete the setup. The foam dampens any vibrations and the whole thing can be flipped around to change direction of air flow. I could wire the switch to reverse voltage on the fan, but the fan blades are designed to go in one direction only.

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#218 azure1961p

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Posted 13 January 2013 - 02:08 PM

Just remember Markus that 30% of the 80mm fan, or 100mm for a ten inch needs to be below the mirrors reflecting surface so the full wash of the blades is blowing on the mirror. If you have the fan just above the mirrors siurface it literally does no good at all. It ll look like you r partly cooling the side of the glass and you are but the benefit is the stram of air is flooding the boundary layer and brushing it off.

Pete

#219 Scanning4Comets

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Posted 13 January 2013 - 03:49 PM

So if I made a small hole for a boundary layer fan, you're saying I would need it at the bottom?

Cheers,

#220 Sarkikos

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Posted 13 January 2013 - 05:04 PM

Pete,

Just remember Markus that 30% of the 80mm fan, or 100mm for a ten inch needs to be below the mirrors reflecting surface so the full wash of the blades is blowing on the mirror.


Are you saying that an 80mm fan should be used for an 8" mirror to scrub the boundary layer, but a 100m fan should be used for a 10"? Where did you get this idea? Is it really necessary to use a larger fan for the 10"?

Mike

#221 tnranger

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Posted 13 January 2013 - 05:39 PM

Not trying to highjack this thread, but I'm trying to understand it all.

I have a XT10i and 2 scavenged computer fans. One fan's mounting holes line up exactly with the fan holes on the mirror mount. The other fan is slightly bigger (~8.5-9cm). I do not want to cut holes in the OTA for side fans.

Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing? Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount? Should the fan blow or draw?
Thanks for any help.

#222 Starman1

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Posted 13 January 2013 - 05:45 PM

1. Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing?
2. Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount?
3. Should the fan blow or draw?
Thanks for any help.

1. Yes, it will.
2. Probably, but it depends on the conditions of use, too.
3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.

#223 rlmxracer

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Posted 13 January 2013 - 05:58 PM

I felt the mounting the fan on the mirror mount was too close to the mirror an would impeade the air flow reguardless of fan direction.

#224 tnranger

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Posted 13 January 2013 - 06:06 PM

That makes sense. A ceiling fan moves more air if it's away from the ceiling instead of flush mounted.

Thanks guys!

#225 Scanning4Comets

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Posted 13 January 2013 - 07:34 PM

This thread seems to be chock full of mass confusion. :confused:






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