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Equipment Discussions >> Reflectors

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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Jon Isaacs]
      #5840566 - 05/04/13 06:33 PM

Quote:

That's why I wear gloves if I am interested in working at high magnifications, just touching the tube to track will cause a tube current.




That's one of the nice aspects to yesteryear's Cave-era tubed Newtonians... the "fiberglass" tubes they employed. My old 'Optical Craftsmen' 6" F8 is like that... not much thermal gets thru that stuff! Having an oversized tube of such material would be nice, as well- enabling the thermals inside to "chimney" up & out, hugging the walls, while still staying (mostly?) out of the light-path. Mine's tube is, unfortunately, *not* oversized in the least, but a rather tight fit to the optic. Still seems to work pretty well, tho.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob new [Re: Mike B]
      #5840575 - 05/04/13 06:48 PM

Quote:

Quote:

That's why I wear gloves if I am interested in working at high magnifications, just touching the tube to track will cause a tube current.




That's one of the nice aspects to yesteryear's Cave-era tubed Newtonians... the "fiberglass" tubes they employed. My old 'Optical Craftsmen' 6" F8 is like that... not much thermal gets thru that stuff! Having an oversized tube of such material would be nice, as well- enabling the thermals inside to "chimney" up & out, hugging the walls, while still staying (mostly?) out of the light-path. Mine's tube is, unfortunately, *not* oversized in the least, but a rather tight fit to the optic. Still seems to work pretty well, tho.




That's one of the nice aspects. But those tubes, they are not light. I think the OTA on my 12.5 inch F/6 Meade weighs right around 100lbs, the whole rig is close to 300 lbs.

Jon


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Jon Isaacs]
      #5840588 - 05/04/13 07:00 PM

Really? Heavier than today's comparable metal tube? My 6" is small enough that it doesn't seem too weighty to me... and i don't have a contemporary metal-tubed 6" handy to compare.


Who else & what else are today's higher-end tubers coming mounted in?... besides "Sonotube". Carbon-fiber?


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paulr57
member


Reged: 01/09/13

Loc: Asheboro, NC
Re: truss vs solid tube dob new [Re: paulr57]
      #5841326 - 05/05/13 09:23 AM

Now ya'll have overwhelmed me! In a good way though! Summer time in NC can be brutal at times and heat waves are everywhere. I just assumed it was the natural order of life especially when I fill my eyepiece with a full moon!

One thing I do as a matter of routine is I tend to wait til my target is very high in the sky so I don't have to fight some of the atmospheric waves I notice when viewing closer to the horizon.

I still consider myself a noob even though I got my first telescope 20 years ago! So please forgive me if I act like a wide eyed child in a candy store with a fistful of money!


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: paulr57]
      #5841426 - 05/05/13 10:49 AM

I don't recall you stating what scope you had- but if it's a Dob, then in warm weather it'll help greatly if you set it up on grass for your viewing session- rather than pavement, which typically retains much heat, & re-radiates it long into the nite. This plays major havoc on a Dob's views, since its mirror is slung so low to the ground, and all that warm air!

Beyond that, use a fan to help speed the mirror's reaching ambient temps, and try to avoid viewing directly over heat-radiating pavement, rooftops, & vents.

Keeps the wide-eyed child happier in me, too!


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob new [Re: Mike B]
      #5841622 - 05/05/13 12:54 PM

Quote:

Really? Heavier than today's comparable metal tube? My 6" is small enough that it doesn't seem too weighty to me... and i don't have a contemporary metal-tubed 6" handy to compare.


Who else & what else are today's higher-end tubers coming mounted in?... besides "Sonotube". Carbon-fiber?




Those big old scopes used thick walled phenolic tubes. The tube is about 0.40" wall, 15 inches in diameter, 75 inches long. I calculate the tube itself weighs over 70 lbs. Add in a full thickness 12.5 inch pyrex mirror, the cell and the rotating ring assembly, it's at least 100 lbs. The counter weights weigh 85lbs.

Jon


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Jon Isaacs]
      #5841807 - 05/05/13 02:51 PM

My old Meade 826c "Red-tuber" Newt, 8" F6, had the phenolic tube... was feather light that i remember. For the 12-16" tubers must be what you're talking about- tho i can't recall ever seeing one up-close.

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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: truss vs solid tube dob new [Re: Pinbout]
      #5841818 - 05/05/13 02:58 PM

Quote:

Quote:

have, however, seen heat rising from a group of people standing around and jawing about 10' in front of my scope, even though they weren't blocking the light.




that's cool but that would be expected if they were under the optical axis.

This typical discussion usually centers around the person hovering over the focuser and how their body heat/breath effects the air in the optical axis.




Danny,

Certainly this is in regard to the concern of the optical axis and perhaps several other factors on the pros and cons of tubes and trusses. I would like to reiterate though, that a person does not have to place their hand in or stand under a telescope just to disturb the images. People standing several feet away can still cause image degradation and what Don is talking about is very valid.

At Charlton Flats, the seeing can drop to .5 arc seconds and if a person is standing upwind, anywhere near the front or side of a telescope, observers will sometimes politely ask them to step somewhere else to remove doubt. It can make a difference and it certainly matters from world class observing platforms like Charlton, it's all relative. You can even see the pulsations of an airy disc from a telescopes stepper motors in some cases.

Take a match stick, light it and then blow it out. The remaining smoke may mix and disperse in several directions, or in one direction. An observer may not feel much of anything, but that does not mean the air carrying the thermals is standing still. Everything in the air from you and around the telescope is moving and that's all it can take to degrade a world class image.

The better the seeing location, the higher the expectation is by the observer who appreciates that difference. I didn't drive 60 miles to a semi light polluted sky like Charlton in search of darkness. I went in search of world class seeing and transparency to see things very few observers get to experience in a lifetime. A tube still has several other advantages as well, it isn't just this.

Human beings are a tremendous source of heat plumes, especially standing right next to the optical entrance of a telescope. If the average body temperate is 98 degrees F. and your ambient air temperature is 60 degrees F. that's an incredible difference.

A primary optic just 2 degrees F. above ambient can exhibit some boundary effects. By comparison, a human with a Delta T of 38 degrees F. is staggering and cannot be ignored. It's all about the Delta T. It is for this reason that professional observatories who give public tours require that all people be removed from the dome by a certain hour. Sensitive detectors can easily be disrupted.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5841913 - 05/05/13 03:53 PM

Quote:

I didn't drive 60 miles to a semi light polluted sky like Charlton in search of darkness. I went in search of world class seeing and transparency to see things very few observers get to experience in a lifetime.



Is there a particular spot/location where you and others setup their telescopes at Charlton Flats?


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dscarpa
Post Laureate
*****

Reged: 03/15/08

Loc: San Diego Ca.
Re: truss vs solid tube dob new [Re: paulr57]
      #5841970 - 05/05/13 04:21 PM

Teeter's tube dobs are felt lined, hope that helps with body heat issues. I've seen body heat plumes with my cats under very good conditions if my hand is by a gap in the dew shield and seem to recall something similar with my long long ago 6" F/8 newt with excellent optics. David

Edited by dscarpa (05/06/13 07:06 PM)


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5842007 - 05/05/13 04:44 PM

Quote:

Human beings are a tremendous source of heat plumes, especially standing right next to the optical entrance of a telescope.




That's why I eat apples while I'm observing...if you get too close and mess up the air you better duck...

but seriously, the plumes don't mix very welln as seen in your previous links, and most of the heat comes off the top of the head, ~70%, away from the optical axis.

I didn't put my shroud on last night and saturn was low enough that while I was sitting my feet were under the open truss, I could see it was effecting the air, but I didn't bother to put the shroud on cause the seeing wasn't good enough to begin with. I still got nice colored belts on the disc and could see a b c rings, 5 moons. just couldn't push the mag as I usually would.



Edited by Pinbout (05/05/13 06:02 PM)


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5842008 - 05/05/13 04:46 PM

Daniel,

Yes, the deltaT is a huge difference, but the outside of the garments I'm wearing are not 98.6 degrees--they are considerably colder because they insulate my body from the cold. The only parts of my body giving off large amounts of heat, typically, are my hands and face, and they are too small in area to have much effect unless I hold a hand in the light path.

In your defense, however, when I had a steel-tubed 6" f/5, sitting with my legs under the scope caused heat plumes to appear in the star images.
Covering my legs with a blanket stopped the heat plumes.
I rebuilt the scope with a fiberglass tube and the sensitivity to placement of my legs or hands disappeared. These new tube liners, or even flocking the inside of the tube, stand a good chance of cutting down on this sensitivity, even with steel tubes.

Curiously, I don't notice any heat plumes from my body when I use a shroud on my dob, and ice or dew forms on the top side of the shroud. The shroud doesn't seem to hold heat or conduct it away from your hand. Part of that is the material, and part of it is the much greater clearance between the light path and the shroud, compared to a tubed scope. I wonder if shrouds made from space blanket material would be even better.


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cpr1
professor emeritus
*****

Reged: 03/24/12

Loc: Louisiana
Re: truss vs solid tube dob new [Re: Starman1]
      #5842125 - 05/05/13 05:42 PM

Has anyone tried to sew in a mylar sheet or some type of similar insulation. Like the thin reflective stuff they use under metal roofs. It works really well to insulate roofs. At least that is what they put on metal shops around here.

It would have to be sew into the shroud somehow with a flat black backing.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob new [Re: Achernar]
      #5842378 - 05/05/13 08:19 PM

Quote:

When you reach 15 or 16-inches, a truss is really the only practical option if you have to transport it. As for thermal issues, I don't see any disavantages at all, indeed my mirror cools off quite rapidly and I have an Obssession pattern mirror cell. I have a variable speed fan that I leave running all the time at low speed, without vibration problems and turning it off or on does not seem to affect the views. It's no worse or better than my 10-inch solid tube, also with a cooling fan.

Taras




So here's an honest question for you... Is your scope truly "rock solid" thermally stable? When the seeing is excellent, are you seeing nice clean, "refractor-like", in-focus, diffraction rings at say 800x?

Jon


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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: truss vs solid tube dob new [Re: Starman1]
      #5842383 - 05/05/13 08:27 PM

Don,

Next time we observe, I'll bring a few tools to do some tests with your dob. I did try numerous experiments using insulated gloves, coats and what not, but the heat just kept pouring out, but that's not to say your points are not valid.


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Daniel Mounsey
Vendor (Woodland Hills)
*****

Reged: 06/12/02

Re: truss vs solid tube dob new [Re: Alph]
      #5842389 - 05/05/13 08:30 PM

Quote:

Quote:

I didn't drive 60 miles to a semi light polluted sky like Charlton in search of darkness. I went in search of world class seeing and transparency to see things very few observers get to experience in a lifetime.



Is there a particular spot/location where you and others setup their telescopes at Charlton Flats?




There is but it's currently closed for a while until political matters are solved with Angeles National Forest.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5934504 - 06/22/13 12:04 PM

Hey Mr. Mounsey

check it out

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5933664/page...


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johnnyha
Postmaster
*****

Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: truss vs solid tube dob new [Re: Pinbout]
      #5934992 - 06/22/13 04:57 PM

My best thermal results have been blowing a 12" RV box fan, sitting on a chair, directly into the UTA of my almost-horizontal, shrouded 15" Classic Obsession down towards the mirror for cooldown... so cooling "from the top"... then while viewing having the box fan sitting on the chair opposite the scope from me at low speed, gently blowing my body heat directly away from the scope. I also have a three-speed brushless fan under the mirror that exhausts outwards that I sometimes keep on low speed while viewing.

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Achernar
Postmaster
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Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: truss vs solid tube dob new [Re: Jon Isaacs]
      #5935499 - 06/22/13 10:42 PM

I never have seen a night like that, that is where I would get a steady view at 800X. However, I can say that unless the temperature just keeps falling rapidly all night, I haven't noticed problems with tube currents in the 15-inch because it doesn't keep pace with temperature changes. I have a fully open tailgate and mirror cell with a cooling fan, and the cover on the rear of the mirror box is Ripstop with a large hole in it for air to flow through. The shroud is Ripstop also. I see a clear diffraction pattern on both sides of focus, one that shows mild over or under correction. In focus I have noted one faint diffraction ring around a star. The highest magnification I have used so far is 575X, the seeing where I live is just not good enough really to go much more than 400X. So yes, it appears to be quite thermally stable in that the telescope does keep pace with the temperature on most nights.

Taras


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