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Equipment Discussions >> Reflectors

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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5599749 - 01/01/13 02:39 PM

Bob,

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train, as well as be a potential hazard to the primary if it isn't very tightly secured? And if the fan is secured tightly enough so that it won't fall, it might be a source of vibration.

For a 20" f/3 Dob, there should be plenty of access to ambient air, unless a shroud is around the UTA and mirror box. I wouldn't think you'd need to bring air "down the tube."

There is a very good chance I don't fully grasp what you're talking about here, so any further explanation - maybe pics? - would be appreciated! All my Dobs are solid tube, but maybe some of your ideas could be applied to them as well. And I do plan on acquiring about a 14" to 16" truss scope eventually.


Mike


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Dean Norris
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5599785 - 01/01/13 03:02 PM

Quote:



Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete




Wouldn't a dew shield on the end of the tube take care of that issue. I made a dew shield out of foam mat that I spray painted black. I built this to keep the dew off the secondary but I think it would also block thermals at this end of the tube.

Dean


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beatlejuice
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5599835 - 01/01/13 03:32 PM

Quote:

Recently, I did construct an annular baffle from foam core and pushed it up above the primary's surface. The row of holes that I drilled are situated between the surface of the primary and the bottom of the baffle.





Is the bottom of the foam baffle above the mirrors surface?
If so, what are the holes for?
I am trying to visualize this for my own scope, as I was orignally thinking of a solid ring baffle secured by my mirror clip screws with some kind of slightly flexible rubber molding glued to the inside of the tube such that it meets the top of the outside of the baffle to keep air loss up the sides of the tube to a minimum thus maximizing the flow across the mirror from my rear mounted fan.
Thanks

Eric


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5599949 - 01/01/13 04:53 PM

I am going to replace my larger fan and make an anular ring later this week. I have had a rainey and cloudy three weeks so I am itching to do some testing.

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Bob S.
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5599953 - 01/01/13 04:56 PM Attachment (21 downloads)

Quote:

Bob,

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train, as well as be a potential hazard to the primary if it isn't very tightly secured? And if the fan is secured tightly enough so that it won't fall, it might be a source of vibration.

For a 20" f/3 Dob, there should be plenty of access to ambient air, unless a shroud is around the UTA and mirror box. I wouldn't think you'd need to bring air "down the tube."

There is a very good chance I don't fully grasp what you're talking about here, so any further explanation - maybe pics? - would be appreciated! All my Dobs are solid tube, but maybe some of your ideas could be applied to them as well. And I do plan on acquiring about a 14" to 16" truss scope eventually.


Mike




Mike, Front fan is in the shadow of the 5" secondary and it held in place by .025 thick safety wire. The fan is a Sunon MagLev fan which is essentially vibration free. The rear fan is fully enclosed in the rear of the mirror box and is a MagLev fan suspended on Sorbothan and is essentially completely isolated from the scope.

Edited by Bob S. (01/01/13 07:29 PM)


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orion61

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Reged: 10/20/07

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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5599997 - 01/01/13 05:20 PM

I once had a 6" F10 Newtonian tube for Planetary, (wish I still had it) it had the tiniest secondary I've ever seen,
something like 2/3rds of an inch or so! Homebuilt 12th wave mirror.
but it suffered from tube currents in that LOOONG tube.
I simply bought a Radio Shack DC fan,hooked it up with rubber bands and Velcro taped the 9v battery to the tube with an on/off switch.
It worked superbly, no currents vibrations and the best Planetary views I have ever had. Easily as good as a Christian refractor had at the time!


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Mike B
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Thermal Issues and Fans Successes new [Re: orion61]
      #5600021 - 01/01/13 05:38 PM Attachment (16 downloads)

Quote:

It worked superbly, no currents vibrations and the best Planetary views I have ever had.




Same happens with this classic 1960's Optical Craftsmen 6" F8, fitted with a single rear-blower PC fan, suspended with rubber (innertube) straps + velcro. VERY sharp planetary views result, and commonly! Will also take silly-high mags, as well.


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5600089 - 01/01/13 06:24 PM Attachment (30 downloads)

Quote:

Actually, I'm using rubber bands now. They work well to isolate the optical train from vibration. But suspending the fan+baffle from rubber bands does not produce an air tight fit, of course.




Post a pic of your fan / baffle setup Mike! Here is mine. it is a 12 Volt computer fan fixed to a light flat black, (on the inside it is flat black), thick cardboard baffle / seal. I get zero vibrations and have three speeds on the fan. I set it cranked at full speed for cool-down and leave it on low when I observe or until the 8 "AA" batteries expire. I need a new 12 Volt source so the fan can stay on longer, but I don't know what other alternatives there are for a small 12 Volt battery!


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Scanning4Comets
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600090 - 01/01/13 06:24 PM Attachment (22 downloads)

Pic #2 showing the fan mounted to the thick cardboard baffle which I can leave on or remove. Note the small, beige three-speed switch at the top left.

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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Dean Norris]
      #5600096 - 01/01/13 06:26 PM

Dean,

Quote:

Quote:



Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete




Wouldn't a dew shield on the end of the tube take care of that issue. I made a dew shield out of foam mat that I spray painted black. I built this to keep the dew off the secondary but I think it would also block thermals at this end of the tube.

Dean




I agree. Once in a while the secondaries in my 8" and 10" Dobs would begin to fog. But this ended immediately when I put dew shields on the scopes. They cut glare and keep the dew off the optics. FWIW, I never notice any effects from my body heat or breath in the optical train of my Dobs. It's a nonissue.

IME & IMO, the best dew/light shields are homemade from black foam sheeting fully-flocked on the inside with vertical strips of ProtoStar. PVC and metal dew shields are inferior by comparison.

Mike


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600122 - 01/01/13 06:43 PM

Quote:

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train...




Bob's solution is one way, in fact probably very effective. In the case I was describing, one would seal the bottom of the tube and exhaust air out the back of the tube. This arrangement sets up a low pressure area behind the mirror and draws air into it, hopefully carrying the boundary layer with it. The lip along the edge of the tube just above the primary serves to act as a vacuum aimed across the mirror's surface instead of pulling air directly down the tube wall. The idea is, this descending air will pile onto the mirror's surface and get sucked off the edge of the mirror.

I think Pete tried this. The challenge seems to be creating a sufficient volume of air moving in the tube. That's a lot of CFM to move (in either direction) from the back. In one review, a guy had fans strong enough to feel a slight breeze from the open end of the OTA. If it sucks into and down the tube, you run the risk of pulling your breath down the tube. If it blows up the tube, you run the risk of drawing in contaminants from the ground below the primary. Bob's system just forces air onto the surface to the same effect. It seems pretty effective at mixing and moving warm air. If it is mixed well it probably does not need to be evacuated from the tube.

With nothing to do, I watched some videos on laminar flow. One interesting thing is how a side blowing fan sets that up. We've all seen images of bow waves forming in supersonic flight. Well, a side blowing fan that strikes the mirror's edge will do the same thing. A weak bow shock will form over the mirror and depart the mirror's surface. This sets up a laminar flow that starts at the surface then arches above the mirror. Below that laminar bow wave and above the mirror is a turbulent zone much like an aircraft wing near stall.

I bet someone could do their Dissertation on this complex subject.

To me, again, it's all about moving air through pressure and the path of least resistance. Bob's set up creates high pressure on the mirror's surface, so air is driven into the surrounding low pressure near the tube wall. Sucking air out of the back of the tube (low pressure behind the primary) does the same thing but probably needs to move more air volume inside the tube. Blowing air up the tube (high pressure behind the primary) does, too. That high pressure air will make it's way to the lower pressure area at the upper tube and eventually out the top. Driving air inside the tube has the added benefit of cleaning out the entire optical path, if you can move that much air (even gently.)

Hitting the mirrors surface seems to require less air movement and to be more efficient. But the air probably won't evacuate the tube without a path for it to follow (like an exhaust port.) But, that may be just fine.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5600127 - 01/01/13 06:45 PM

Eric,

Quote:

Quote:

Recently, I did construct an annular baffle from foam core and pushed it up above the primary's surface. The row of holes that I drilled are situated between the surface of the primary and the bottom of the baffle.




Is the bottom of the foam baffle above the mirrors surface?
If so, what are the holes for?
I am trying to visualize this for my own scope, as I was orignally thinking of a solid ring baffle secured by my mirror clip screws with some kind of slightly flexible rubber molding glued to the inside of the tube such that it meets the top of the outside of the baffle to keep air loss up the sides of the tube to a minimum thus maximizing the flow across the mirror from my rear mounted fan.




I experimented with a thin cardboard baffle ring screwed down directly to the tops of the mirror clips. But I could never get a good seal where the ring contacts the OTA. IME, rings directly on the clips are better to cover edge errors on the primary rather than to direct air flow.

For my baffle ring, I cut out a circle of black foam core just the right diameter to have a snug fit inside the OTA. I used the thicker type, so it would be sturdy and have a snug fit against the OTA. I cut out the central area a little larger than diameter of the primary. Then I cut a ring of ProtoStar whose central opening is just large enough to expose the entire surface of the primary mirror. I glued this ProtoStar ring on the top of the foam core ring. Finally I pushed the baffle up into the bottom of the OTA.

I drilled the lateral series of holes in the OTA so they will be at the edge of the primary surface. The baffle is positioned so that it is above these holes. The holes are between the baffle and the surface of the primary.

The idea is that the baffle will help direct the flow of air around and to the vent holes, helped by the fan(s) below the primary and the fan at the opposite side of the OTA (when I eventually place one there).

Mike


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5600149 - 01/01/13 07:04 PM

Norme,

Quote:

Bob's solution is one way, in fact probably very effective. In the case I was describing, one would seal the bottom of the tube and exhaust air out the back of the tube. This arrangement sets up a low pressure area behind the mirror and draws air into it, hopefully carrying the boundary layer with it. The lip along the edge of the tube just above the primary serves to act as a vacuum aimed across the mirror's surface instead of pulling air directly down the tube wall. The idea is, this descending air will pile onto the mirror's surface and get sucked off the edge of the mirror.




I think that we should have a fan below the primary blowing ambient air onto the bottom of the mirror. The dual goals are to adjust the temperature of the mirror to ambient and to eliminate/diminish the boundary layer on the surface of the mirror. A fan blowing air onto the bottom of the mirror, then pushing the air around the sides of the mirror and finally over the surface of the mirror to vent out holes immediately above the mirror seems like an elegant, multi-function solution to me. If the bottom of the OTA is sealed except for the intake of the fan, so much the better ... as long as no vibrations are induced. Now if another fan is placed at a hole at the lower edge of the primary's surface, to blow in air over the surface of the primary and out the vent holes at the upper edge of the mirror, the synergistic effect should be excellent.

Mike


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Bob S.
Carpal Tunnel
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600167 - 01/01/13 07:17 PM Attachment (31 downloads)

The rear sucking fan is regulated by a potentiometer that allows it to move air fairly slowly at low speeds and move a lot of air at high speeds. The rear of the mirror cell is enclosed and the air cascades over the annulus and around the edge of the primary and out the back. Here is a picture of the rear fan imbedded in the mirror cell with Sorbothane. The fan is a MagLev fan and there is no noticeable vibration at any speed. The front blowing fan seems to create what is likely some semblance of laminar flow as the images definetly are improved with both fans running simultaneously. I guess I would have to do smoke tests to actually see what is going on?

Edited by Bob S. (01/01/13 07:32 PM)


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Sarkikos
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Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5600244 - 01/01/13 08:03 PM

Bob,

Quote:

Front fan is in the shadow of the 5" secondary and it held in place by .025 thick safety wire. The fan is a Sunon MagLev fan which is essentially vibration free. The rear fan is fully enclosed in the rear of the mirror box and is a MagLev fan suspended on Sorbothan and is essentially completely isolated from the scope.




OK, so the way I understand this is that both front and rear fans are in the vicinity of the mirror box. The front fan is far enough above the surface of the primary that it will still be effective in pulling air down the "tube," but close enough to the primary that it can hide within the shadow of the secondary. The suspension wires for the front fan would be another source of diffraction for the optical train, besides the spider vanes or wires that secure the secondary.

A good solution for large Dobs, maybe not so good for smaller scopes - especially solid tubes - but something to think about.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: orion61]
      #5600251 - 01/01/13 08:08 PM

Hi Larry,

Quote:

I once had a 6" F10 Newtonian tube for Planetary, (wish I still had it) it had the tiniest secondary I've ever seen,
something like 2/3rds of an inch or so! Homebuilt 12th wave mirror.
but it suffered from tube currents in that LOOONG tube.
I simply bought a Radio Shack DC fan,hooked it up with rubber bands and Velcro taped the 9v battery to the tube with an on/off switch.
It worked superbly, no currents vibrations and the best Planetary views I have ever had. Easily as good as a Christian refractor had at the time!




Where exactly did you place the fan? Below the primary and blowing air onto its bottom? Or in a hole at the side of the OTA?

Mike


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600264 - 01/01/13 08:19 PM

Just a note about suspending a fan in front of the mirror:

The "umbral" shadow of the secondary is always somewhat smaller than the secondary because of off-axis light shining on the mirror from "around the edge" of the secondary (off axis light).

The longer the tube, the less size the "umbra" of the secondary shadow has for the same off-axis angle.
[in practice, long tubed scopes don't have as large an off-axis angle, so the umbral shadow of the secondary is still nearly the same size as the secondary itself]

You can work it out with simple trig by figuring out 1/2 of the maximum field size of the scope as the upper acute angle, the distance from secondary to primary surfaces as one side of the triangle and the width of the penumbra of the secondary shadow as the bottom of the triangle. Since you know one side, and the angle, you can figure the width of the penumbra
[remember, the penumbra is actually larger than the size of the secondary because of the shadow from the opposite side angle, but for purposes of this, we can figure the penumbra to be a thin ring inside the diameter of the secondary].

So long as the fan is smaller than the umbra of the secondary shadow, no possible diffraction can occur from it. And the wires can be really thin.....


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beatlejuice
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600269 - 01/01/13 08:21 PM

Quote:

The idea is that the baffle will help direct the flow of air around and to the vent holes, helped by the fan(s) below the primary and the fan at the opposite side of the OTA (when I eventually place one there).




So the implication is that allowing the air to leave the bottom of the scope is superior for eliminating the boundary layer than letting it drift out the front of the scope?

Eric


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Thermal Issues and Fans Successes [Re: Sarkikos]
      #5600280 - 01/01/13 08:29 PM

Quote:

A fan blowing air onto the bottom of the mirror, then pushing the air around the sides of the mirror and finally over the surface of the mirror to vent out holes immediately above the mirror seems like an elegant, multi-function solution to me.

Now if another fan is placed at a hole at the lower edge of the primary's surface, to blow in air over the surface of the primary and out the vent holes at the upper edge of the mirror, the synergistic effect should be excellent.

Mike




Mike, that set up could work provided you could move a sufficient volume of air around the mirror to disperse the layer. As Bob mentioned, some smoke tests might help. One guy tested this "lip" system on youtube, but his results seemed to barely move any air. The smoke just kind of lingered above the mirror. He might could have used more CFM.

One thing to take into account is volume. The volume of fast moving air directly coming off the fan must spread itself around the entire rear surface, move in all direction with enough CFM to actually be felt in the huge volume of space above the primary. If the air flow is too weak, you will not get disruption toward the center of the primary (thus a cross blowing fan would help.)

Absolutely, if you can get the dual benefit of cooling and layer scrubbing with a single system, that's the way to go. Blowing cold air directly on the primary is probably most efficient, but the mere act of removing the boundary layer (blanket) will allow for some degree of cooling, too. I like Bob's set up because it kills two birds with one stone AND evacuates all that mixed up air out the back. (Even though I am adverse to adding fans or additional spider vanes into the light path.)

Again, there are so many ways to do this. The trick is to find the most efficient use of batteries. Another good design seems to be 3 high volume fans blowing from the rear and out the front. It moves so much air you can feel a slight breeze out the top of the tube. It evacuates the entire volume of air inside the tube and replaces is with cooler ambient air while it cools the primary. And the low pressure force might be sufficient to draw off the boundary layer, too. You can observe crisp images through gale force winds provided it is laminar in nature and a constant density.


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Thermal Issues and Fans Successes [Re: beatlejuice]
      #5600292 - 01/01/13 08:36 PM

Eric,

My physical intuition - later to be affirmed or refuted by the facts - is that it might be best to vent the warm air out of the system as close as possible to the source of the heat, that is, the primary mirror. So I think that in a solid tube Dob, probably the best place to vent the warm air from the primary is immediately at the uppermost edge of the surface of the primary.

I go back to my thought about high-end SCTs and Maks. They have a row of vent holes directly around the edge of the correcting lens. Might be a good idea in all solid-tube scopes of all types if there were vent holes around the rim of the objective, correcting lens or primary mirror.

It's really easy to drill a series of holes along the high edge of the primary mirror in Dobs. Depending on results in my 8" Dob, I'll be doing that for my 10" and 5" Dobs next.


Mike


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