dreamregent
sage
Reged: 04/06/09
Posts: 454
Loc: Clearwater, FL
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I've settled on the way I'm building my OTA but I'm concerned about these thermal currents because I am installing some baffles. I've read that the currents migrate to the corners in a square tube so I am assuming it would be the same with my octagon. Do you guys think I would be ok if I cut notches in the corners of the baffles for the currents to go through?
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Building a f5.24 10" Dob
in an octagonal wood tube
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
I've settled on the way I'm building my OTA but I'm concerned about these thermal currents because I am installing some baffles. I've read that the currents migrate to the corners in a square tube so I am assuming it would be the same with my octagon. Do you guys think I would be ok if I cut notches in the corners of the baffles for the currents to go through?
The best strategy is to eliminate the thermals, then you don't have to worry about modifying your baffles. (In my opinion, tube baffles like these do virtually nothing to help suppress stray light in a Newtonian if you've done everything else properly, so my first suggestion would be the leave them out.)
I have the first part of a two part article on dealing with thermals up on my web site. I think you'll find it'll help you plan your thermal management strategy.
Good luck with the scope!
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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dreamregent
sage
Reged: 04/06/09
Posts: 454
Loc: Clearwater, FL
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I'm not really so concerned with the stray light issue. I'm putting in a few baffles for structural support and rigidity purposes. I was thinking 3 or 4 of them would be sufficient for that but maybe they're not necessary at all for this purpose???? I was only going to do it because I'm using 1/4" plywood for the ota.
BTW, Gary, thanks a lot for your website. I've been following along with your telescope project somewhat, though I haven't read the installments about tube currents, yet. However, I certainly will. Also, I'm using the plan you have on your website for the double-plate mirror cell. I went to the hardware store today to get what I needed to put it together. The only problem I had was making a little modification so I could move the mirror up and down a little bit but I think I've found a solution for that.
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Building a f5.24 10" Dob
in an octagonal wood tube
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
I'm not really so concerned with the stray light issue. I'm putting in a few baffles for structural support and rigidity purposes. I was thinking 3 or 4 of them would be sufficient for that but maybe they're not necessary at all for this purpose???? I was only going to do it because I'm using 1/4" plywood for the ota.
Were you planning a square tube? How long/what diameter? It seems to me that unless you're making something pretty unusual, you won't need the reinforcements except at the front and rear of the tube.
Quote:
BTW, Gary, thanks a lot for your website. I've been following along with your telescope project somewhat, though I haven't read the installments about tube currents, yet. However, I certainly will. Also, I'm using the plan you have on your website for the double-plate mirror cell. I went to the hardware store today to get what I needed to put it together. The only problem I had was making a little modification so I could move the mirror up and down a little bit but I think I've found a solution for that.
Good to hear.
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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The best strategy to eliminate tube currents is to eliminate the tube itself. This works especially well as scopes get larget than 10".
The attached images is a picture of my telescope seen at radius of curvature.
The combination of the stray light baffle and the illuminated field stop prevents light from opposite the focuser from illuminating the (eyepiece) field of view. The primary baffle prevents light from the ground from illuminating the FoV via the secondary. Not seen, the back of the upper ring assembly is also blackened so the primary cannot reflect stray light to the FoV, either.
This is a very dark scope in the illuminated FoV.
Edited by MitchAlsup (10/27/09 03:09 PM)
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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Here is an image showing the light path.
Notice that any stray light comming around the edge of the stray light baffle can only illuminate the FoV far off axis (mostly the insides of the eyepiece, itself.)
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dreamregent
sage
Reged: 04/06/09
Posts: 454
Loc: Clearwater, FL
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Quote:
Were you planning a square tube? How long/what diameter? It seems to me that unless you're making something pretty unusual, you won't need the reinforcements except at the front and rear of the tube.
I'm making an octagon out of 1/4" plywood. The ID will be 12" across at the flats and about 13" across the points. If I have 8 inches of tube past the focuser on the front end and keep the ends of the collimation bolts inside the back of the tube it will be about 56" long. More specific info about how I plan to build it can be found in another thread...see it HERE.
Quote:
The best strategy to eliminate tube currents is to eliminate the tube itself.
I totally get what you're saying but... Unfortunately, I've grown rather attached to the octagonal wood tube concept...
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Building a f5.24 10" Dob
in an octagonal wood tube
Edited by dreamregent (10/27/09 03:31 PM)
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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Quote:
Here is an image showing the light path.
Notice that any stray light comming around the edge of the stray light baffle can only illuminate the FoV far off axis (mostly the insides of the eyepiece, itself.)
Guess that puts a premium on having well blackened and well baffled eyepieces, doesn't it?
That's realy ingenious. How much much ambient light do you have in your neigborhhood? Enough to see not to trip over the scope in the dark? Enough to change an eyepiece in the dark? Enough to read the label on the side of the eyepiece in the dark?
Gene Baraff
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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Quote:
Quote:
Here is an image showing the light path.
Notice that any stray light comming around the edge of the stray light baffle can only illuminate the FoV far off axis (mostly the insides of the eyepiece, itself.)
That's realy ingenious. How much much ambient light do you have in your neigborhhood? Enough to see not to trip over the scope in the dark? Enough to change an eyepiece in the dark? Enough to read the label on the side of the eyepiece in the dark?
Now to be fair, I think the idea came from the group that contains Mel Bartels back when Ultralights were just getting started (circa 1997-98).
Basically as long as there is not light illuminating the stray light baffle it is very dark.
The outside of the (aluminum) truss tubes are not painted and are easily seen. In addition, anything not in the optical path is seen as furnature grade wood, or aluminum or in its original condition. Everything in the optical path is either a mirror, or painted flat black.
There is plenty of light to change eyepieces even in places as dark as Ft. Davis Texas (once you (me actualy) are fully dark adapted.
I don't read my eyepieces in the dark, I feel them. 31mm, 20mm, 13mm, 9mm, and 7mm Naglers all feel significaltly different. 31 is the widest, 20mm is the heaviest (T2), 13mm is similar to the 20mm but with a 1.25" skirt, the 9 is light with a 2" skirt, and the 7 is 1.25" skirtless. Impossible to misrepresent, and easy to feel.
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1434
Loc: Highland Park, CA
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Mitch:
That's a gorgeous scope. 10"?
I'm thinking my next Newt is going to have to be a truss-tube.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1434
Loc: Highland Park, CA
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[OT:]
Gary, what's the maximum dimension for carry-on (Carrion?) luggage? Do you know of anyone who's tried to make something larger than 8" for a travelscope?
[/OT]
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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Quote:
Mitch:
That's a gorgeous scope. 10"?
It is actually a 20"-er.
It will actually fit in the back of a C4 Corvette {it is a nightmare to actually perform this fitting}.
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GaryS
super member
Reged: 10/30/06
Posts: 191
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Quote:
The best strategy to eliminate tube currents is to eliminate the tube itself.
Except that you still have the thermal boundary layer to contend with, which is a more pernicious problem. The *best* strategy is to get rid of the source of both, namely, a primary mirror that is warmer than the ambient air temperature.
As I note in the article on thermals on my web page, the phrase "tube currents" describes the symptom, not the disease.
Regards,
Gary
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Sky & Telescope Contributing Editor
SkyNews Columnist & Blogger
www.GarySeronik.com
A place for stargazing enthusiasts.
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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Quote:
Quote:
The best strategy to eliminate tube currents is to eliminate the tube itself.
Except that you still have the thermal boundary layer to contend with, which is a more pernicious problem.
Agreed, except that particular effect is not a "tube current".
And you must have missed the thread where I use a hurricane fan to cool the mirror.
Edited by MitchAlsup (10/27/09 08:47 PM)
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tim53
Pooh-Bah
Reged: 12/17/04
Posts: 1434
Loc: Highland Park, CA
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Okay, now I can't resist...
I sorta joked in my thread on the cats forum about moving the dew heater strap on my 9.25" from the corrector end to the primary end, and thereby heating up the whole interior of the scope instead of just the corrector (and the air at the "top" of the tube near it).
The performance will probably pull a good vacuum (i.e., "suck"), but it'll be so easy to do, that it's kind of dumb not to find out whether introducing a boundary layer around the corrector and outside the tube is worse than having one around the primary.
Probably a dumb idea, though, no matter how you look at it!
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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dreamregent
sage
Reged: 04/06/09
Posts: 454
Loc: Clearwater, FL
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I have made a basic 3-D model of what I plan to build. The tube is 56 inches long. A picture of it is below. I am estimating that by the time I attach the hardware and install the mirror cell the whole thing will weigh 40 pounds or so...that includes the cradle clamped around the tube. I know that the wood for the OTA itself will weigh only 12.5 pounds but the rest is an estimate. The bearings are 10 inches in diameter...should I have larger ones? Also, with something built along this line, do you guys think I'll have to worry about the baffle issue for internal bracing and rigidity?
--------------------
Building a f5.24 10" Dob
in an octagonal wood tube
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