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Gary Seronik article question.

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#1 scottmm2012

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Posted 05 July 2020 - 10:51 AM

I'm failing to grasp the instruction here.  Highlighted in bold red.

 

 

4. Baffling. I’ve seen Newtonians that are well baffled against stray light, and others that allow all kinds of unwanted illumination to pollute the view. The difference is usually due to knowing what effective baffling looks like. You don’t need to install an elaborate set of ring baffles like those found in refractors (in fact, I recommend against doing so), but you should make a modest effort to ensure that the only light reaching your eyepiece is coming from your primary mirror. The easiest way to confirm this is to put a collimation plug in the focuser, and have a look. If you can see the ground behind the primary mirror, or look past the top of the telescope tube, you have some work to do.

 



#2 sixela

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Posted 05 July 2020 - 10:53 AM

If you look through the focuser and you can see what's behind the scope just outside of the primary mirror (e.g. grass), then you need a baffle behind the primary.


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#3 ed_turco

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Posted 05 July 2020 - 11:06 AM

You might read my article (link below) as to how I addressed this problem.Cloudy Nights Article: May, 2015

"The Definitive Newtonian Reflector."
A discussion of the age-old question "Are Apos really better than Newtonian reflectors?"

 

http://www.cloudynig...reflector-r2983

Sky and Telescope articles:

"Making an Aplanatic Telescope." Nov. 1979: 473-7.
"Tripods from Crutches." Jan. 1996: 31.
"A Scavenger's 12-Inch Telescope". Apr. 1998: 96-97.

 

 

Ed Turco


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#4 TOMDEY

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Posted 05 July 2020 - 12:17 PM

The author addresses the two biggest sources of stray specular light getting to center field via the folding flat. The first is addressed by blackening/baffling the region surrounding (behind) the PM, the second by addressing ~behind~ the folding flat (SM), by lengthening the far side of the tube. For Truss Dobsonians, especially the extreme skeletal ones with short/eliminated mirror box or upper cylindrical cage --- stray light can be severe... both specular and scattered. This is why conservative black tube Newts are usually fine but ultralight truss Dobs are generally notorious.

 

An even more conservative test is to forgo the collimation plug and replace that peep hole with a larger hole at focus... sized to your biggest anticipated eyepiece field stop.

 

For some reason, many ads for gossamer ultralight scopes show no baffles whatsoever, with the optics just sitting out there floating in midair. I assume the instructions and options menu further explain and include baffles and shrouds.   Tom


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#5 gwlee

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Posted 05 July 2020 - 02:38 PM

For some reason, many ads for gossamer ultralight scopes show no baffles whatsoever, with the optics just sitting out there floating in midair. I assume the instructions and options menu further explain and include baffles and shrouds.   Tom

Ultralight scopes usually tradeoff many desirable characteristics to allow larger telescopes to be more somewhat more portable. These tradeoffs are acceptable for some, not for others. After experimenting with them, I am more inclined to tradeoff aperture when I want increased portability. 


Edited by gwlee, 05 July 2020 - 02:49 PM.

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#6 Jon Isaacs

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Posted 06 July 2020 - 04:08 AM

Stray light and the sensitivity of a scope to stray light depends on part how dark the skies are... Are you blocking only sky glow and star light or are there artificial light sources adding to the problem?

 

With my 16 inch Dobstuff, I eventually added a full circle baffle 18 inches tall around the primary, behind the mirror is not open around the edge.

 

Top side, it has a single ring but I added a full circle baffle with a baffle for the Focuser.

 

Under truly dark skies, such measures did not seem to be needed, Bortle 1-2. Otherwise.

 

Jon


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#7 sixela

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Posted 06 July 2020 - 10:08 AM

Mhhh -- even under Bortle 1-2 skies, things like a lot of light from a 22.7 mag/arcsec² sky background entering the eyepiece from the front of the scope can still generate quite some veiling glare, so you can't get away with just anything.


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#8 TOMDEY

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Posted 06 July 2020 - 10:29 AM

Mhhh -- even under Bortle 1-2 skies, things like a lot of light from a 22.7 mag/arcsec² sky background entering the eyepiece from the front of the scope can still generate quite some veiling glare, so you can't get away with just anything.

Yep... specular is a killer and even scattered (singly or multiply) also hurts.

 

One tremendous advantage of a true domed observatory is that the lion's share of the sky is baffled from ever getting to the entire telescope. It's a giant advantage that is unappreciated --- until you build and use that dome.

 

The exact opposite is a minimalist truss scope right out in the open. --- that's a stray light magnet.    Tom


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#9 Starman1

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Posted 06 July 2020 - 04:49 PM

If you look through the focuser and you can see what's behind the scope just outside of the primary mirror (e.g. grass), then you need a baffle behind the primary.

That sounds like a good idea, but it seriously reduces the air flow around the primary.

Instead, I accomplished the same thing with a large black cloth on the ground under the scope.

I found a large black tablecloth (10' x 8') for the purpose at a 99 cent store.  Yes, I can see the ground under the scope in reflection in the secondary mirror, but the ground is jet black at night when you look down in past the primary mirror.

I also use a light baffle extension attached to the upper tube assembly opposite the focuser so no light can enter the bottom of the focuser from over the other side of the tube.

 

Light scatter suppression is accomplished by using:

--flat black secondary vanes

--flat black secondary holder

--flat black interior to UTA

--flat black interior surface to UTA rings.

--flat black pipe insulation on the poles**

--flat black interior to rocker box

--flat black interior to mirror box

--velvet on inside of UTA opposite focuser

--two circular baffles in mirror box

--velvet on inside of mirror box

--flat black top to mirror box

--flat black bottom to UTA lower ring

--flat back hole in focuser board where drawtube moves through the board

--flat black inside to drawtube

--flat black inside surface to focuser board that faces the secondary

--any/all nuts or bolts inside the OTA flat blackened--no shiny surface anywhere.

--black lycra shroud with matte side facing in

--upper tube light shield protruding 1" up from UTA**

 

Everything together yields excellent contrast.  A couple of those were my idea (**), the rest were Rob Teeter's.

 

As for Seronik's idea, light can enter the bottom of the focuser from more than the axial line, so a plug with a center hole is insufficient.

You need to remove anything in the focuser and look in the focuser at an angle.  If you can see past the top of the UTA, you need a light shield opposite the focuser.

 

Tom Dey is right about a domed observatory--I had a portable dome several years ago, but I sold it because I couldn't see the sky other than through the slit.

I was constantly walking in and out of the observatory just to see the sky.  A roll-off-roof observatory eliminates that issue, but also doesn't offer

the same degree of ambient light suppression.

it's true, though--optimizing light suppression in the scope is only part of the equation.  Optimizing light suppression in your environment is the rest:

Black cloth over the head and eyepiece, a hood, an observatory, or a light shield bucket around the focuser--whatever works.


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#10 TOMDEY

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Posted 06 July 2020 - 09:37 PM

Excellent list there, Don!

 

That sounds like a good idea, but it seriously reduces the air flow around the primary.

Instead, I accomplished the same thing with a large black cloth on the ground under the scope.

I found a large black tablecloth (10' x 8') for the purpose at a 99 cent store.  Yes, I can see the ground under the scope in reflection in the secondary mirror, but the ground is jet black at night when you look down in past the primary mirror.

I also use a light baffle extension attached to the upper tube assembly opposite the focuser so no light can enter the bottom of the focuser from over the other side of the tube.

 

Light scatter suppression is accomplished by using:

--flat black secondary vanes

--flat black secondary holder

--flat black interior to UTA

--flat black interior surface to UTA rings.

--flat black pipe insulation on the poles**

--flat black interior to rocker box

--flat black interior to mirror box

--velvet on inside of UTA opposite focuser

--two circular baffles in mirror box

--velvet on inside of mirror box

--flat black top to mirror box

--flat black bottom to UTA lower ring

--flat back hole in focuser board where drawtube moves through the board

--flat black inside to drawtube

--flat black inside surface to focuser board that faces the secondary

--any/all nuts or bolts inside the OTA flat blackened--no shiny surface anywhere.

--black lycra shroud with matte side facing in

--upper tube light shield protruding 1" up from UTA**

 

Everything together yields excellent contrast.  A couple of those were my idea (**), the rest were Rob Teeter's.

 

As for Seronik's idea, light can enter the bottom of the focuser from more than the axial line, so a plug with a center hole is insufficient.

You need to remove anything in the focuser and look in the focuser at an angle.  If you can see past the top of the UTA, you need a light shield opposite the focuser.

 

Tom Dey is right about a domed observatory--I had a portable dome several years ago, but I sold it because I couldn't see the sky other than through the slit.

I was constantly walking in and out of the observatory just to see the sky.  A roll-off-roof observatory eliminates that issue, but also doesn't offer

the same degree of ambient light suppression.

it's true, though--optimizing light suppression in the scope is only part of the equation.  Optimizing light suppression in your environment is the rest:

Black cloth over the head and eyepiece, a hood, an observatory, or a light shield bucket around the focuser--whatever works.

I especially like the top of mirror box (oft neglected) and extension protruding upward from UTA. I also figured that one out and added it to my 29" way back when. It's just aluminum flashing, cheap, weighs close to zero, Rustoleum flat black on the inside. Yikes! I never blackened the bottom of the UTA bottom ring... I think I noticed it, but never got around to it... the way things often go, for most of us...

 

But yes to your counsel there... addressing all the ~big, medium, and little~ things adds up to a HUGE improvement in stray light control / improved contrast.    Tom

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#11 Asbytec

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Posted 07 July 2020 - 09:00 AM

"You need to remove anything in the focuser and look in the focuser at an angle. If you can see past the top of the UTA, you need a light shield opposite the focuser."

Don, I took the opposite approach to almost the same end. I looked over the upper end of the (solid) tube, past the spider, and into the bottom of the focuser. Put a flashlight beam (or probe with a laser pointer) along that visual line looking at the grazing angle at the lower end of the focuser. Out of curiosity, though I could not see the flashlight itself from the eyepiece end, I could see some light inside the focuser.

Yea, of course the flashlight is very bright, but it revealed light might make its way up the focuser even though it has some micro ribbing. I was going to add a light shield, anyway, this just showed me why I probably need it to significantly increase the grazing angle. I just found the perspective from outside the optical tube looking into the focuser interesting. Because, if we can look into the focuser, light has a path to bounce in. Especially at shallower grazing angles, and even if you cannot see the sky from the eyepiece end. I could not see the sky, but added the shield anyway.

On external light, I had a unique set up where I could hang tarps (on a stone wall topped with a barred fence) completely around my driveway to a height of 8 feet. It was essentially an observatory open to most of the sky. No way to see a single ambient light anywhere. Thankfully many were shielded to the sky, anyway. Not because the neighbors were conscious about dark skies, but because the (rain) shields were cheap. So, yea, it was a private dark spot to observe from. That is, until my wife turned on the porch light to see if I snuck away. :)

Edited by Asbytec, 07 July 2020 - 09:10 AM.

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#12 Oberon

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Posted 08 July 2020 - 05:44 AM

Yep... specular is a killer and even scattered (singly or multiply) also hurts.

 

One tremendous advantage of a true domed observatory is that the lion's share of the sky is baffled from ever getting to the entire telescope. It's a giant advantage that is unappreciated --- until you build and use that dome.

 

The exact opposite is a minimalist truss scope right out in the open. --- that's a stray light magnet.    Tom

This is true. And you don’t get near as well dark adapted under the open night sky as you can inside a dome with a relatively small aperture.

 

Still...domes tend to stuff the seeing...so choose your compromise!



#13 TOMDEY

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Posted 08 July 2020 - 07:38 AM

This is true. And you don’t get near as well dark adapted under the open night sky as you can inside a dome with a relatively small aperture.

 

Still...domes tend to stuff the seeing...so choose your compromise!

Yeah... Starting a half-hour before sunset, I ~Blow Out~ the entire dome and scope with giant box fans for at least an hour before observing, and a total of two hours before ~Serious Observing~ ... at which point the seeing often has become --- spectacular! The regional elevation, local elevation, flow cavity underneath, grass, bushes, and trees... also seem to help a lot.  Tom

 

~click on~ >>>

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