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Equipment Discussions >> Reflectors

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dscarpa
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Reged: 03/15/08

Loc: San Diego Ca.
Re: truss vs solid tube dob new [Re: paulr57]
      #5839053 - 05/03/13 09:01 PM

One things for sure with my made extra heavy because it's an F/5 11" Teeter STS on the way I just best keep lifting those weights! David

Edited by dscarpa (05/03/13 09:10 PM)


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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: Daniel Mounsey]
      #5839411 - 05/04/13 12:40 AM

Thermal control is only one of several reasons a tube is superior for visual performance. Nevertheless it's good to see what people look like in reality.

http://www.youtube.com/watch?v=lSFwH0BVd3Q&NR=1
http://www.youtube.com/watch?v=sRnWCvxawrc&feature=related
http://www.youtube.com/watch?v=J7Xgbf3eZlM


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5839608 - 05/04/13 06:07 AM

Quote:

Thermal control is only one of several reasons a tube is superior for visual performance. Nevertheless it's good to see what people look like in reality.

http://www.youtube.com/watch?v=lSFwH0BVd3Q&NR=1
http://www.youtube.com/watch?v=sRnWCvxawrc&feature=related
http://www.youtube.com/watch?v=J7Xgbf3eZlM




Similar views are possible just looking at the defocused image of a star in a telescope...

Jon


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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: Jon Isaacs]
      #5839749 - 05/04/13 09:11 AM

That's true Jon however, a binoviewer with a barlow and no eyepieces is far superior if you really want to conduct the schlieren test in its full splendor. Using an eyepiece is like looking under water without any goggles and it's a distorted picture. I've shown this to a number of observers and they were stunned at the clarity of so many disturbances in the optical path. You can clearly see little boundary waves above the surface of the primary when they exist. You can also differentiate jet streams, seeing, body and ground currents once you know what to look for. It's very dynamic.

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Jon Isaacs
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Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5839891 - 05/04/13 10:58 AM

Quote:

That's true Jon however, a binoviewer with a barlow and no eyepieces is far superior if you really want to conduct the schlieren test in its full splendor. Using an eyepiece is like looking under water without any goggles and it's a distorted picture. I've shown this to a number of observers and they were stunned at the clarity of so many disturbances in the optical path. You can clearly see little boundary waves above the surface of the primary when they exist. You can also differentiate jet streams, seeing, body and ground currents once you know what to look for. It's very dynamic.




Interesting... A work we use schlieren for imaging shockwaves, it uses two 80mm F/11.3 Meade/Mizars telescopes and a high speed camera.



Jon


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mountain monk
Carpal Tunnel
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Reged: 11/06/09

Loc: Grand Teton National Park
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5839895 - 05/04/13 11:00 AM

Daniel,

Wow. And when you consider the trillions of things that live in the air---viruses, bacteria, seeds, spores (one puffball mushroom releases seven trillion of them), fungi, algae (some live in clouds), pieces of (or whole) mosses, caterpillars, aphids, spiders (some live at the top of Everest and their entire diet comes from junk in the air), flies, moths, beetles, mites...---well, it's amazing we can see anything with our scopes. If I set up next to our creek, I find mayflies and caddis flies on my mirror. Hummmm.... A certain tenacity is required for this hobby.

Dark skies.

Jack


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5839990 - 05/04/13 12:00 PM

Quote:

Thermal control is only one of several reasons a tube is superior for visual performance. Nevertheless it's good to see what people look like in reality.

http://www.youtube.com/watch?v=lSFwH0BVd3Q&NR=1
http://www.youtube.com/watch?v=sRnWCvxawrc&feature=related
http://www.youtube.com/watch?v=J7Xgbf3eZlM






Those vids mean nothing. The only time I stick my hand in the optical psth is when I'm messing with the secondary.

There are no vids showing body heat from outside the optical path effecting the layers of air within the optical train.


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5840080 - 05/04/13 12:54 PM

Quote:

It's very dynamic.




It is! In my post (somewhere above) where i described "smokey thermals", i was doing very similar to what Daniel described (no BVer, only "mono"), and it was quite the show- almost mesmerizing!

Even so, i had to make major efforts to have my body or breath induce anything visible in the "bright platter" of defocused starlight. I will continue to fiddle with this in the future- particularly after the scope has reached ambient temps... it is perhaps then that subtle influences could be better detected?

But my suspicion is that we're talking "subtle" here, at most, and then probably with steeper temp gradients between observer (98.6*) and the air within the lite-path of the scope.


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Pinbout
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Reged: 02/22/10

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Re: truss vs solid tube dob new [Re: Mike B]
      #5840115 - 05/04/13 01:21 PM

Quote:

i had to make major efforts to have my body or breath induce anything visible in the "bright platter" of defocused starlight. I will continue to fiddle with this in the future- particularly after the scope has reached ambient temps... it is perhaps then that subtle influences could be better detected?






even if you had a fan sucking the air into the front, when I did my smoke test I had a hard time getting the smoke into the layers of air being pulled. most of the time the smoke was repelled.


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

Loc: Los Angeles
Re: truss vs solid tube dob new [Re: Pinbout]
      #5840196 - 05/04/13 02:22 PM

I run a small fan all the time that results in positive air pressure in the tube, exhausting at the top. Unless I put my hand in the optical path or breathe into the scope, I see no evidence of body heat in even an out-of-focus star image.

I 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.


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Starman1]
      #5840211 - 05/04/13 02:31 PM

Quote:

a group of people standing around and jawing about 10' in front of my scope




They're fine... just tell 'em to shut-up & stop breathing so heavily...



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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: Jon Isaacs]
      #5840268 - 05/04/13 03:07 PM

Jon,

That's a nice setup.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob new [Re: Daniel Mounsey]
      #5840335 - 05/04/13 03:54 PM

Quote:

Jon,

That's a nice setup.




Thanks:

We use it with a mini-shock tube and the Imacon 200 Highspeed camera. The Imacon takes 16 frames and is capable of exposures of 5 nanoseconds with frame rates up to 200,000,000/sec though we normally operate in the 100 nano-second regime.

One thing to be aware of is that Schlieren imaging is qualitative rather than quantitative and shows the gradient in the index of refraction, you see the defocused component, you don't see the focused component. In that sense, the existence of "schliere" is only an indication that there is a defocused component but it does not indicate the level.

Jon


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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: Jon Isaacs]
      #5840407 - 05/04/13 04:43 PM

Jon,
Thanks for sharing.


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Starman81
Carpal Tunnel
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Reged: 03/06/08

Loc: Metro Detroit, MI, USA
Re: truss vs solid tube dob new [Re: Jon Isaacs]
      #5840425 - 05/04/13 04:54 PM

Quote:

Quote:

That's true Jon however, a binoviewer with a barlow and no eyepieces is far superior if you really want to conduct the schlieren test in its full splendor. Using an eyepiece is like looking under water without any goggles and it's a distorted picture. I've shown this to a number of observers and they were stunned at the clarity of so many disturbances in the optical path. You can clearly see little boundary waves above the surface of the primary when they exist. You can also differentiate jet streams, seeing, body and ground currents once you know what to look for. It's very dynamic.




Interesting... A work we use schlieren for imaging shockwaves, it uses two 80mm F/11.3 Meade/Mizars telescopes and a high speed camera.



Jon




You're using telescopes... at work??! Lucky duck!


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob new [Re: Starman81]
      #5840459 - 05/04/13 05:15 PM

Yeah, but that's not even a 'shoot-out' they've got rigged... more of a stare-down.

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: truss vs solid tube dob new [Re: Starman1]
      #5840464 - 05/04/13 05:20 PM

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.


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JimP
Pooh-Bah


Reged: 04/22/03

Loc: South Carolina
Re: truss vs solid tube dob new [Re: Starman81]
      #5840466 - 05/04/13 05:21 PM

I was imaging Jupiter one night (10" scope at F/45) and my daughter walked into my observatory and passed in front of my scope (but below the optical path). The amount of image distortion which occurred from her body heat rising in front of the optical path was unbelievable. The image of Jupiter changed from a fairly sharp clean image to a pulsating blob of heat waves. It was quite dramatic.

I would prefer a solid tube based on how beautiful the old Cave reflectors looked. I think a solid tube on a metal mount would be far more professional looking. I am not referring to anything being made today. I must admit I have seen no professional scopes made of plywood. If, portability is required then a truss tube may certainly be necessary.

JimP


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Mike B
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: truss vs solid tube dob [Re: JimP]
      #5840497 - 05/04/13 05:42 PM

Quote:

I must admit I have seen no professional scopes made of plywood



What exactly constitutes a "professional scope"?
Mine's made by someone who makes (okay, *made*) Dobs for a living, and he was paid for the job, by its purchase.

Or, if one's scope or it's accomplishments are "published", does that make it professional?

I think a case could be made that "Obsession" & other Dobs made ("professionally") by those who receive a price for the build are "professional scopes".

I merely ask out of curiosity.

Quote:

The image of Jupiter changed from a fairly sharp clean image to a pulsating blob of heat waves. It was quite dramatic.



Do you recall what the temps/conditions were that nite? I'd be curious as to temp "gradients". The air being still may've also enabled such a constrained vertical flow of heat plumes... as might've the enclosure of your observatory.

Don's example might've been less prominent had it been one li'l girl at 10-feet, versus a whole gaggle of adults.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: truss vs solid tube dob [Re: Mike B]
      #5840546 - 05/04/13 06:21 PM

Quote:

What exactly constitutes a "professional scope"?




To my mind, professional implies a scientific research telescope, something a "professional" astronomer would use.

I doubt there are any that are made of plywood but then in the last 50 years, I would have to think there have been no new "professional" Newtonians and almost all new scopes are built using truss structures.

Regarding the effect of body heat. On s stable night with a fully cooled scope, it is easy to see the effect of momentarily touching a metal tubed scope. 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.

Jon


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