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Reflective truss poles, why?

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#1 25585

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Posted 10 October 2019 - 01:26 PM

While solid tube OTA users seem scrutinous over the flocking or blacking of their OTA's interior, truss & collapsible scopes are accepted with shiny poles in carbon fibre, gloss black or silvery aluminium. Its a puzzlement to me & there seem to be different mindsets.

 

Given that any skeleton element will be compromising exterior light exclusion unless very well shrouded, what about inside reflections? Why are shrouds not made to go inside the truss poles, and why are shiny poles accepted?



#2 junomike

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Posted 10 October 2019 - 01:31 PM

A shroud on the inside (of poles) would have nothing to keep it out of the light path

Not sure on shiny poles although a Newt's ability to "baffle" is not one of it's strong points.


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#3 Dan Crowson

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Posted 10 October 2019 - 02:04 PM

I wonder about this as well. I have one of the 'cheap' truss RCs that I use for imaging. I don't use a shroud because there are no lights around the observatory(s) and it just seems to limit my imaging nights due to wind. 99% of the time, everything works well but if the moon is at a certain again, I will get a reflection off one of the very shiny carbon fiber trusses. I was thinking about putting some black gaffers tape on them but was worried about it falling off an causing an issue.

Dan


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#4 Mike G.

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Posted 10 October 2019 - 03:17 PM

my LB truss pole dob has black painted poles. more like matt black, not gloss.   I don't really worry about reflection since they are inside the shroud.  still, I had heard about the poles being a source of heat radiation in the light path so I bought some polyolefin heat shrink tubing and covered them with the black heat shrink.  they're pretty dull now and you really can't even see them looking down the tube when the shroud is on and it's dark.  typical seeing usually limits me to 150-175x so even if there was light bouncing around or convection currents, I probably wouldn't see it.


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#5 Keith Rivich

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Posted 10 October 2019 - 04:35 PM

A shroud on the insides of the poles will sag into the light path unless a support system is put in place. Most use either black foam, shrink wrap or paint to knock off the shine. However, just like solid tube newts, blacking out the interior offers minimal benefits under dark skies. 


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#6 MitchAlsup

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Posted 10 October 2019 - 07:27 PM

A shroud on the insides of the poles will sag into the light path unless a support system is put in place. 

This depends entirely on the diameter of the mirror and the diameter of the inside of the truss poles.

 

For example I have a 13" F/3 mirror and the inside diameter of the poles is 19"; thus there is plenty of space.

 

However, just like solid tube newts, blacking out the interior offers minimal benefits under dark skies. 

Even in the dark skies of Ft. Davis (one of the darkest sites in CONUS) the ground reflects the light from the milky way off the sandy ground and can negatively impact contrast.

 

That said, I use flat black spray paint on the insides of the truss poles, along with anything else visible from where the EP would go:: (The opposite side of the upper assembly gets properly darkened, the spider vanes get blackened, the topside of the bottom assembly gets blackened, the bottom side of the upper assembly gets blackened.) Anything you can see from the Focal plane that is not black needs to be black.


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#7 Keith Rivich

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Posted 10 October 2019 - 09:49 PM

This depends entirely on the diameter of the mirror and the diameter of the inside of the truss poles.

 

For example I have a 13" F/3 mirror and the inside diameter of the poles is 19"; thus there is plenty of space.

 

Even in the dark skies of Ft. Davis (one of the darkest sites in CONUS) the ground reflects the light from the milky way off the sandy ground and can negatively impact contrast.

 

That said, I use flat black spray paint on the insides of the truss poles, along with anything else visible from where the EP would go:: (The opposite side of the upper assembly gets properly darkened, the spider vanes get blackened, the topside of the bottom assembly gets blackened, the bottom side of the upper assembly gets blackened.) Anything you can see from the Focal plane that is not black needs to be black.

I have a friend with a 20" f/4 and he put his shroud inside the tubes. He had to fabricate a spring assist to keep tension on the shroud as it would sag into the light path as the night went on. Dew and all. It worked but I still see no observational benefit. Visually it looked nice!

 

I agree with you on reflections and all but I still know, from experience, that it does not make that big of a difference. A few tenth's of a magnitude, maybe. I observe near the Prude several times a year (headed there in a few weeks as a matter of fact) and when the winds kick up I take the shroud off the  25" for stability. Honestly, I do not see a real difference in the views. At my local observing site I do see a difference...again, not huge, but a difference. More sky glow. And dew. 

 

I agree with you on blackening all surfaces. On paper it is the right thing to do. But like boundary layer fans the benefit is probably quite small. 


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#8 Oberon

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Posted 11 October 2019 - 07:28 AM

Professional telescopes often have gloss white truss poles.


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#9 Mike Spooner

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Posted 11 October 2019 - 08:47 AM

Solid tube owners may be worried about the tube flocking but some of it may be overkill, IMHO. Across from the focuser and perhaps around the primary are a couple of important areas. I like having the cell support behind the mirror made to block light from showing around the mirror from the back especially for solar viewing with solar film filters. There may be some benefit to flocking the deep interior of the tube but i thought there had been studies where the grazing angles of reflected stray light didn't really cause much problem. With truss tubes, the steeply circular profile should reflect stray light in random directions such that it has minimal impact on the image. That being said, I do try to blacken anything directly seen through the focuser. 

Since this is conjecture and anecdotal, YMMV!

 

Best,

Mike Spooner


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#10 Keith Rivich

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Posted 11 October 2019 - 12:08 PM

Solid tube owners may be worried about the tube flocking but some of it may be overkill, IMHO. Across from the focuser and perhaps around the primary are a couple of important areas. I like having the cell support behind the mirror made to block light from showing around the mirror from the back especially for solar viewing with solar film filters. There may be some benefit to flocking the deep interior of the tube but i thought there had been studies where the grazing angles of reflected stray light didn't really cause much problem. With truss tubes, the steeply circular profile should reflect stray light in random directions such that it has minimal impact on the image. That being said, I do try to blacken anything directly seen through the focuser. 

Since this is conjecture and anecdotal, YMMV!

 

Best,

Mike Spooner

 

The one item I "sense" the most benefit from is a shroud extension opposite the focuser. I use a blackened piece of old cage shroud material sticking about a foot or so above the upper cage. This provides a shadow from starlight for the eyepiece. 


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#11 Starman1

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Posted 11 October 2019 - 01:08 PM

While solid tube OTA users seem scrutinous over the flocking or blacking of their OTA's interior, truss & collapsible scopes are accepted with shiny poles in carbon fibre, gloss black or silvery aluminium. Its a puzzlement to me & there seem to be different mindsets.

 

Given that any skeleton element will be compromising exterior light exclusion unless very well shrouded, what about inside reflections? Why are shrouds not made to go inside the truss poles, and why are shiny poles accepted?

Good question, which is why my poles are covered with flat black Aerocel pipe insulation.

It's not perfectly black at extremely low angles, but it's darker than a flat black tube.

My poles originally had a grey polyethylene pole cover, but it was white at a low angle.


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#12 Astro-Master

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Posted 11 October 2019 - 02:19 PM

I was amazed that Meade painted the edges of the OTA and lower tube ring white on their truss Dob's, and silver on the truss poles.  What were they thinking?


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#13 Starman1

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Posted 11 October 2019 - 02:22 PM

They're black now.  Better late than never.


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#14 stargazer193857

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Posted 11 October 2019 - 02:43 PM

As noted earlier, some scopes do have shrouds inside the truss poles. I've seen a few and would do the same. It would have more droop issues, catch wind more, and need outside loops or something to support it, but it can be done. A big spring is probably best, with lots of loops. Just need to fabricate the spring, and then hook the spring to the poles.

As for silver ones, I don't like those. But there are people who think you only need a baffle opposite the focuser and leave everything else open.

Edited by stargazer193857, 11 October 2019 - 03:00 PM.

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#15 Redbetter

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Posted 11 October 2019 - 04:20 PM

My Obsession has dark gray pipe insulation on the poles, not quite black, but dark enough with the shroud wrapped around it.  The foam also helps provide a bit more stand off for the shroud. 

 

I would NOT put a shroud within the truss.  This is based on the experience of even the external shroud getting wet with dew and sagging some near the bottom if not pulled taut with the straps in the grommets at the bottom stretched over the blocks. 

 

The only time I observe without the shroud is when there is strong wind, or on rare instances in which the session will be short with a few easy targets and no local light interruptions (traffic of any sort.)  The last time I tried to observe without the shroud and there was considerable traffic in and out of the lot, it was a miserable experience.  I spent a lot of time throughout that session waiting for someone to leave or turn off their lights.  Often light was shining up from below into the focuser--light that would have been blocked by the shroud. 

 

Folks forget that one of the biggest glare problems with a Dob/esp. a truss type is not light shining down the tube at shallow angles, but instead bright sources coming into the focuser almost perpendicular to the tube, but parallel to the back of the focuser.  I notice it this time of year as the brighter stars of Orion as well as Sirius are rising while I am still observing galaxies near the meridian.  I have made a black back stop extension for the UTA on the opposite side of the focuser but have not worked out how to effectively attach and employ it in the field. 

 

Another issue that arises is when some sort of light is shining on the back of the mirror.  This happens mostly during outreach when either a ranger sets some sort of "shielded" lantern down somewhere behind the scope or if someone is using their phone or a flashlight to look at the base of the scope.  Headlights will do it of course...


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#16 hakann

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Posted 11 October 2019 - 06:25 PM

One must understand at ATM it’s often a must to has high optics as the ’ease’ of it either to get it or do it.

-But what about optic sag, boundery layer, ground heat, body heat, secondary sag, incoming light in a open truss or thin shrouds, plywoods at factor 0.15 and so on.

Edited by hakann, 11 October 2019 - 06:43 PM.

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#17 Redbetter

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Posted 11 October 2019 - 06:32 PM

Huh?


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#18 Starman1

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Posted 11 October 2019 - 09:02 PM

One must understand at ATM it’s often a must to has high optics as the ’ease’ of it either to get it or do it.

-But what about optic sag, boundery layer, ground heat, body heat, secondary sag, incoming light in a open truss or thin shrouds, plywoods at factor 0.15 and so on.

Yet, despite the entire litany of problems, somehow it works.

Dick Suiter referred to the "wobbly stack" of potential problems, yet eppur su muove, 

and yet, it moves........



#19 CrazyPanda

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Posted 11 October 2019 - 09:19 PM

While solid tube OTA users seem scrutinous over the flocking or blacking of their OTA's interior, truss & collapsible scopes are accepted with shiny poles in carbon fibre, gloss black or silvery aluminium. Its a puzzlement to me & there seem to be different mindsets.

 

Given that any skeleton element will be compromising exterior light exclusion unless very well shrouded, what about inside reflections? Why are shrouds not made to go inside the truss poles, and why are shiny poles accepted?

This was a concern of mine when designing my 15", and I solved it by using this industrial heat shrink tubing:

 

https://www.digikey....Q1160-ND/754944

 

Gives the poles a nice soft, semi-rubber feel, insulates them, and does a great job of darkening them and reducing incident reflection. The only issue is it added considerable weight to the poles (made them about 65% heavier), but I took that into account when designing the balance point of the scope.

 

Word of warning, this stuff needs a proper heat gun to shrink. Hair dryer won't do.


Edited by CrazyPanda, 11 October 2019 - 09:26 PM.

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#20 stargazer193857

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Posted 11 October 2019 - 09:50 PM

To shroud or not to shroud...
Get the best of both with a tent observatory?

#21 Starman1

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Posted 11 October 2019 - 11:58 PM

the shroud also keeps dew off the primary and body heat out of the optical path.
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#22 Jon Isaacs

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Posted 12 October 2019 - 12:25 AM

 

Folks forget that one of the biggest glare problems with a Dob/esp. a truss type is not light shining down the tube at shallow angles, but instead bright sources coming into the focuser almost perpendicular to the tube, but parallel to the back of the focuser.  I notice it this time of year as the brighter stars of Orion as well as Sirius are rising while I am still observing galaxies near the meridian.  I have made a black back stop extension for the UTA on the opposite side of the focuser but have not worked out how to effectively attach and employ it in the field.

 

Another issue that arises is when some sort of light is shining on the back of the mirror.  This happens mostly during outreach when either a ranger sets some sort of "shielded" lantern down somewhere behind the scope or if someone is using their phone or a flashlight to look at the base of the scope.  Headlights will do it of course...

 

 

waytogo.gif

 

Mike mentioned grazing angles from the truss tubes.  Intuitively, very little of the light that actually reflects off a truss pole actually hits the mirror and even then, it is off-axis so it is really only the scatter from this off-axis light that enters the eyepiece.  

 

The bigger issue light essentially parallel to the focuser tube that sneaks by the secondary.  Baffling is something I have been working on with my 16 inch Dobstuff for about 12 years.  It's a single ring upper cage design and all is reasonably well if the skies are truly dark and there are no sources of artificial light.  But this is not always the case and so this has been a project.  

 

To baffle the upper tube, I eventually just built a full circle baffle that attached to the upper ring so it mimicks a double ring upper cage. Previously light would bounce off the poles and down the drawtube. 

 

The focal plane sits back from the secondary about 14.5 inches, this lowers the eyepiece height and the focal plane is well illuminated because of the 4 inch secondary.  This allows me to also baffle the focuser tube from stray light with a 42mm hole in the upper cage baffle. 

 

The original mirror box also has a full circle baffle and the back is a plate so there is no path for light to enter from the rear.  

 

Normally it's windy so a shroud is mostly a sail but I recently added a shroud and I will be seeing if it helps with scattered light. My thinking is that if the focuser and upper cage are properly baffled and the mirror box is properly baffled, the shroud does very little.

 

12 years ago:

 

6466630-Dobstuff at Starpad zoomed.jpg

 

Mirror box and upper cage baffles

 

Dobstuff chair and cart 2.jpg

 

With shroud:

 

Dobstuff with shroud 1.jpg

 

Jon


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#23 Starman1

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Posted 12 October 2019 - 08:48 AM

Don't forget light entering the bottom of the focuser from over the top of the tube opposite the focuser.

Sky light is a serious problem if it can enter the bottom of the eyepiece.

Try looking through the empty focuser and adding a light shield above the UTA that is tall enough you cannot see the sky at all at any angle.


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#24 Jon Isaacs

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Posted 12 October 2019 - 09:02 AM

Don't forget light entering the bottom of the focuser from over the top of the tube opposite the focuser.

Sky light is a serious problem if it can enter the bottom of the eyepiece.

Try looking through the empty focuser and adding a light shield above the UTA that is tall enough you cannot see the sky at all at any angle.

 

Don:

 

In that scope, the focuser baffle in the upper cage baffle addresses this issue. it probably reduces the field illumination of the 31mm Nagler but it did help with stray light. And too, one advantage of the large 4 inch secondary with the Astrosystems holder is that it also serves to block light. 

 

Jon



#25 Starman1

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Posted 12 October 2019 - 12:21 PM

Jon,

My focuser is 9" deep in my UTA, and the spider vanes block some light from over the top of the tube on the other side,

and STILL I notice the difference in the darkness of the background in a 30mm eyepiece with a light shield in place.

When you are looking at the sky with your low power eyepiece, try holding up a sheet of cardboard opposite the focuser above the end of the UTA.

If you can't see any difference, don't bother.

But for me, I could see the difference immediately with the shield in place, removed, and then put in place again.




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