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Show us your TDB (Turco Dark Bucket)!

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#1 jtsenghas

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Posted 09 November 2018 - 10:52 AM

When Ed Turco finished his six-inch "Definitive  Newtonian Reflector" in 2015, one design feature that I really liked was his "Dark Bucket" light trap opposite the focuser for improving contrast.   I had the privilege of setting up my Tardiscope next to his scope at Stellafane  that year and can vouch for the effectiveness of what I like to call the TDB.  This is a a darkened and flocked coffee can that really reduces a lot of the unwanted ambient light from being directly seen from the eyepiece.  For reflectors with small secondaries this is really a worthwhile modification, in my humble opinion.

 

Ed's Dark Bucket also has a removable plug on the end that makes sense for allowing fan ventilation for his scope during initial cool down because his scope is otherwise enclosed with an optical window at the aperture end, and the secondary is mounted to that window to reduce diffraction.  That plug also allows him access to his secondary. Such a plug wouldn't be needed for most scopes that adopt this idea of an inky black well that surrounds the secondary as seen from the focuser.

 

Ed's TDB.JPG

   photo courtesy of Ed Turco and used with permission

 

As of now I haven't seen a single example of this feature seen anywhere else.  I'm curious as whether anyone else has one to show or is willing to try adding such a feature after being introduced to this concept.  If so, SHOW US YOUR TDB!!!


Edited by jtsenghas, 09 November 2018 - 12:26 PM.

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#2 jtsenghas

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Posted 09 November 2018 - 11:03 AM

I'm currently building a 12.5" f/4.3 truss telescope that has to have a lightweight UTA, but one feature it might employ is a variation on the TDB.  It occurred to me that if the light trap expands in diameter away from the focuser the the side walls could be entirely invisible from the focuser.  It would be better for me if I make it lightweight and collapsible.  My current thought is to make a deployable inverted pyramid or circular "teepee" made of black velvet (turned inside out with the dark side inward).   I can think of a couple ways it could be attached with clips on the UTA structure, yet pack flat.   This means light can't hit the bottom of this device directly and only secondary reflections off the black velvet face of it could be seen at the eyepiece.  I recognize that it would have to be more than twice the secondary diameter at the smaller opening, but, as I said, it could be a lightweight tent-like structure. 

 

Thoughts of designs of TDB's on this thread are every bit as welcome to me as actual examples.  Show us your ideas!!



#3 ckh

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Posted 09 November 2018 - 07:51 PM

A gothic arch shape is supposed to be good at trapping light. If you make it a figure of revolution the point is the bottom of the dark pit. Maybe if you inflate middle a little it would be even better.

gallery_240847_5047_274.png



#4 Oberon

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Posted 09 November 2018 - 09:09 PM

The tube is a baffle, and TLB is a baffled baffle. I’d call it a “double baffle” myself as that describes its function, and thus opens up the options to alternative designs.

Anyone building an ultra light knows that the minimum size for a baffle is approximately the size of the entire UTA diameter. Therefore for a TLB to work properly its aperture needs to be roughly the size of the entire TA diameter, which in turn makes it harder (or more complex) to be baffled as a double baffle. Not saying it can’t be done, just saying you soon realise that to ensure that the eyepiece only sees a double baffle you soon lose the initial dimensions and simplicity of the TLB.

So...the real trick (for me at least) is to identify practical techniques that ensure that the eyepiece can *only* see surfaces that are double baffled.


Edited by Oberon, 09 November 2018 - 09:11 PM.

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#5 starcanoe

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Posted 09 November 2018 - 09:15 PM

So...the real trick (for me at least) is to identify practical techniques that ensure that the eyepiece can *only* see surfaces that are double baffled.

 

And that is where those ultrashort focusers bite you in the butt.



#6 jtsenghas

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Posted 10 November 2018 - 10:40 AM

Anyone building an ultra light knows that the minimum size for a baffle is approximately the size of the entire UTA diameter.  

I can see how an ultralight with just a single ring UTA or short UTA  benefits well from a huge baffle. If, however, the length of the UTA is closer to the diameter a considerable smaller TDB would have a bottom that is never struck by direct light. That's what I'm considering.  It's a tough call because the UTA needs to be very stiff to be able to suffer such a hole. I'm working on a few ideas to make it stiff and light. 


Edited by jtsenghas, 10 November 2018 - 10:47 AM.


#7 jtsenghas

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Posted 11 November 2018 - 06:57 PM

Here are a few more photos Ed Turco shared with me and offered to let me post here.  You can see that the bottom of the "dark bucket" can't be illuminated via the aperture.  It is entirely "double baffled", to use Jonathan's terminology:

DNR1.jpg

 

DNR2.jpg

 

DNR3.jpg


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#8 kfiscus

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Posted 11 November 2018 - 10:04 PM

That puts the "t" in telescope...
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#9 figurate

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Posted 12 November 2018 - 05:53 PM

And this is going to put the "oh" in telescope-  for reasons to be explained shortly, my light trap design goes by a different name than the thread title here.

 

I originally had the idea back in the mid-1990s while planning a 10" dob, but because that project never got going, I pondered an installation on my 8" reflector for some years before I could talk myself into cutting a hole in the tube so close to the tensioned spider vanes. At any rate, consulting my memory and examining some photos on my old computer indicates I fabricated my light trap around the time of December 2013, as I later documented with a photo in a thread on the Classics forum ('Classic or Modern' topic by Barrysimon615 from April 21 2014), in comment #35 posted on April 22 2014:

https://www.cloudyni...r-modern/page-2   

 

My design, shown below, for that particular telescope was built as a detachable part to avoid awkward situations in storage and problems going through doorways; I intended that the trap, which fits snugly with a gasket against the tube and is held on by a spring-loaded latch, would pop off in the event of a catastrophic impact with a door jamb rather than crack the tube as it could were it epoxied in place. While my tube hole is roughly elliptical, I elected to use a very lightweight rectangular container in formed aluminum that I had sitting in the garage for a housing (generally speaking, a larger trap or box than the hole diameter is a help) and I put black paper louvers inside. The tube is sealed with a tight-fitting curved plug when not in use, and I later took advantage of the location by putting on a thick slab of aluminum that serves as an eyepiece counterweight.

 

While I am pleased that the idea is gaining traction (of course it works well), the point here is that while Ed is justifiably well-known here for his optical fabrication and I am a relative nobody on the ATM forum, this online platform and our culture itself are best run on transparency and truthfulness. Obviously, it is quite possible that Ed came up with the idea independently; it always seemed extremely obvious to me, but taking the chronology given here as accurate, and unless Ed can show otherwise with an earlier documentation than my post above, I have to take issue with the terminology "Turco Light Bucket". The person who gets the credit for an innovation should rightfully be the innovator and not the early adopter.

 

While my 8" was originally a commercial scope, it is actually more of an ATM project than many scopes here in the ATM forum, with my own mirror, focuser, finder, finder mount, etc. I think the only unmodified piece is the RA shaft. To heighten the surreality of the situation, I have had a tube removable front extension for this scope for some time (posted photos on three or four occasions) and while the brand was originally "Optical Craftsmen" I call it my "optimal craftsman". I am remiss for not pointing this out earlier, because I have seen the threads on the topic without commenting- but again, if Ed came by the idea independently that is well and good, but based on the evidence, I believe I have first dibs here.

 

Fred     

 

   

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#10 jtsenghas

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Posted 12 November 2018 - 07:40 PM

.....Ed can show otherwise with an earlier documentation than my post above, I have to take issue with the terminology "Turco Light Bucket". The person who gets the credit for an innovation should rightfully be the innovator and not the early adopter....I believe I have first dibs here.

 

Fred     

That light trap is cleverly executed, especially with the use of louvers. I hope this thread doesn't degrade into a heated discussion regarding who was first in improving the darkness opposite the focuser. You may well have been ahead of Ed in execution.  Hopefully he and others won't now be digging through old documention in an attempt to say "I was first!" For that matter Ed found an imported model of telescope with a similar light trap and when he contacted the maker he was told the idea was developed by them.

 

I simply like Ed's turn of phrase in calling such an item a "dark bucket" since it is a good play on words with the term "light bucket" for large newts.

 

I hope feelings aren't hurt by not having the thread title changed to "Show us your Light Trap behind the Secondary!"

 

I hope we see more examples, however they were derived. Thank you very much for showing us yours  Fred. Well done,  sir! Well done! 


Edited by jtsenghas, 12 November 2018 - 08:36 PM.


#11 ed_turco

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Posted 12 November 2018 - 08:12 PM

My project took over two years in the writing and the building of my DNR.  Dratted RA made the project go for so long.  

 

I'd call it a tie.  For the sake of harmony, I have nothing further to contribute on this topic..

 

 

ed


Edited by ed_turco, 13 November 2018 - 11:07 AM.

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#12 figurate

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Posted 12 November 2018 - 09:00 PM

J.T. and Ed, I'm satisfied with your responses. You can understand my position I hope, and that's all I needed to communicate. It is certainly not unusual for people to be thinking in parallel lines, and I do like the "dark bucket" term just fine. Next time I do something like this I will post it in the ATM forum instead of Classics, where the idea didn't even evoke one peep from anybody at the time in 2014. And thanks for the reply.


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#13 NHRob

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Posted 12 November 2018 - 09:30 PM

Ed,

  The light trap is a great idea and too often lost in the scramble for ever increasing aperture and shorter focal lengths.

It's not always max-photons.  Sometimes it's contrast!


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#14 jtsenghas

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Posted 12 November 2018 - 09:50 PM

It's not always max-photons.  Sometimes it's contrast!

YES! Exactly. Especially when there is ambient light to contend with.


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#15 starcanoe

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Posted 13 November 2018 - 08:41 AM

Back in 1999 give or take I had intended to do something similiar to a TDB.

 

I had a 12 inch mirror. The solid box tube was to be built in 2 sections. The first one would be 48 inch long (or perhaps cut down a bit in length). The second upper solid box would bolt to the first and would would be long enough to contain the focuser,spider, and a decent length past that. And most likely a very light "dew cap" would extend the tube even further. This would give me the advantage of not having a long unwieldy tube, parts that would be a bit more manage weight wise and none of the fuss, muss, expense, and more complicated engineering and construction that comes with a truss tube design. Also less problem with body heat currents or stray light.

 

The main thing though was the tube. It was significantly oversized. 16 inches on a side. The main reason for that was  that the upper section was going to have a fair number of baffles. I needed the extra space so the baffles could have a bit of height to them. The baffles would be slightly tilted so that from any point in the focuser tube one would only see the side that faced down towards the bottom of the tube (it would only be lit up from a secondary reflection off of a black surface...so two reflections off of black surfaces is going to be pretty dark).

 

Secondly, the height of the baffles and the spacing of them along the length of the tube was going to be such that any light that came directly from the sky would only be able to hit the front surfaces of the baffles (which the focuser could not "see"). So the walls of the tube would also be in shadow in regards to light coming directly from the sky.

 

So, any surface the focuser could "see" would only be illuminated by light that had been reflected off some other black surface first. Again, if you have 2 reflections off of black surfaces before it hits the focuser....those light levels have gone pretty far down. Use a really good paint or black surface and you've done good. Also, such a set up avoids any grazing angles for the light..which is usually where black paint starts to become not so black.

 

I had also considered making the area opposite the focuser a bit deeper so as to reduce the number of baffles. That and or or offsetting the optical axis towards the focuser side...again allowing for deeper and fewer baffles.

 

A few other considerations.

 

The baffle edges need to be considered. Those will reflect light directly so it helps to have as few of those as reasonably possible and probably more importantly as thin as possible (now the whole baffle does not need to be thin...just the edge).

 

The amount of light that enters the optical system in the first place is function of the length of the dewshield. One could consider that part of the tube in front of the spider in a reflector or in front of the lens in a refractor. And it is approximately a squared function. Twice as long......1/4 the light directly from the sky bouncing around down there....three times as long 1/9 as much light bouncing around down there.

 

Also, since much of the secondary light (light that has undergone one bounce first) that is getting into the system is coming from from the "dewshield" area it really pays to have that lined with your best paint or flocking material. (That and the surfaces the focuser can see directly).

 

Another factor. The focuser can also see the diagonal holder and spider vanes. You can build the diagonal holder so that the parts the focuser sees are in shadow and are only illuminated by secondary reflections. And it might make sense to have the spider vanes be highly reflective so that focuser is not seeing the spider vanes but optically speaking the tube walls.

 

 

The wood was cut out back then...been in storage until recently and is being repurposed for a 10 inch f8 scope (no tube current issues here!). Do wood and mirrors age like fine wine or cheese?....after 20 years maybe my Orion mirror  is a Zambuto?

 

EDIT.

 

There is a modest error in my baffle plans. It could be solved with mirrors.


Edited by starcanoe, 13 November 2018 - 12:44 PM.

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#16 figurate

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Posted 13 November 2018 - 10:15 AM

I've been interested in contrast and stray light mitigation since my early days of fooling around with an f/15 60mm refractor, and have investigated some effective schemes for refractor 'dewcaps' that don't involve dozens or even hundreds of closely spaced baffle rings. These ideas though, like the newtonian focuser trap, have to involve real-world tradeoffs to avoid becoming too unwieldy to use, and there is a practical limit to dew cap length. Like Starcanoe just mentioned, longer extensions reduce stray light. The way I look at it is, as the extension increases, you progressively limit the angle of light admitted into the aperture that can eventually wind up in the focal plane, while effective geometries or baffles inside that extension prevent any reflections from within that added component from propagating. The image I use is a sort of statistical 'fog' that can be reduced but never completely eliminated, and you always need to allow for some amount of angular field of view.

 

It has been my experience that very long focal length refractors, f/20 for example, are sort of self-baffling regardless of whether baffles are actually in place- perhaps the long tube itself has a filtering effect on the specific direction of that stray light reaching the eyepiece end, but at any rate the exercise becomes something like a Zeno's paradox at some point (how long is most effective for a light shield, ten feet?). For myself, I would be more inclined to go with an unusually wide dew cap or light shield than an unusually long one.    


Edited by figurate, 13 November 2018 - 10:48 AM.

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#17 starcanoe

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Posted 13 November 2018 - 10:24 AM

Awhile back I was toying with the idea of a reflector that has an undersized secondary. Enough undersized such that the field of view is uniformly illuminated (ie no light drop off). However, the system aperture stop is now the secondary and that defines what the effective aperture is. I had this idea decades ago...and toyed with the idea again a year or three ago. The purpose was to make a poor mans small aperture wide field reflector rather than refractor. The two advantages were that you had one third the field curvature of a refractor (important for smaller refractors). And you could make it a relatively long f ratio and use simple eyepieces and still get good off axis performance. 

 

Anyway IIRC....I seem to remember that the way the light rays would converge on the focal plane from the undersized diagonal allowed for better baffling than your typical reflector. I think this was occuring when the secondary was significantly smaller than the focal plane image diameter...



#18 perfessor

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Posted 13 November 2018 - 10:38 AM

I like the removable lid, which allows you to periodically empty out the accumulated dark, before it spills out onto the primary.


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#19 Earthbound1

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Posted 20 March 2019 - 04:33 PM

A spring loaded baffle like the clown body in a "Jack in the box" or "Snakes in a can" is a potential design consideration for compactness...

Edited by Earthbound1, 20 March 2019 - 04:34 PM.


#20 figurate

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Posted 21 March 2019 - 12:36 AM

My 10" newtonian will have one, a retractable cavity that swings closed when not in use. I have also been playing around with an idea for a geometry to create a "black-body" newtonian using the 114mm Vixen f8 optics that came with a Polaris mount I bought on CL a while back. Just need one more key item (and of course lots and lots of time).


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#21 jimegger

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Posted 21 March 2019 - 01:21 AM

The light trap makes a lot of sense especially for getting that extra contrast when imaging in environments like where I live in a snow covered landscape. I have found a tube extension on my big Newtonian did help out a lot from all the reflected light off the surrounding snow. Next will come baffles and maybe a light trap .


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#22 jtsenghas

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Posted 21 March 2019 - 02:42 AM

The key to these most effective baffles and light traps appears to me to be to require light to reflect off of at least TWO dark surfaces before entering the eyepiece.

 

If a dark surface absorbs 90% of incident light then it is 10% reflective. A double bounce off of such surfaces makes it effectively 1% reflective. (0.1^2 = 0.01, or 1%)

 

No doubt, the incredibly dark surfaces of Joann Fabric's Royalty 3 Black Velvet has to do with the fern-like structure of the surface that makes most light reflect multiple times off the fibers before reaching the eyepiece or the next surface. Designs that force light to reflect off of two such velvet surfaces are already getting down to tiny amounts of light that may compete with single surfaces of such extraordinary materials as Vanta Black. 


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#23 calypsob

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Posted 21 March 2019 - 09:41 AM

Has anyone done any measurements to gauge the effectiveness of this? It would be pretty easy to slap on a dslr and take a fixed manual exposure with and without the baffle in place, you could then evaluate the histogram to see how the contrast improves or remains neutral.



#24 figurate

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Posted 21 March 2019 - 10:34 AM

My rule of thumb was always, three reflections or more and it's Don Meredith time ("turn out the lights"). From that standpoint, the most efficient approach in terms of money and time is a combination of both surface treatments and physical configuration; and my particular location is ideally suited for these kinds of experiments. Testing for stray light at the focuser would be helpful, but you would have to neutralize or remove the primary first because the two aspects of this signal-to-noise problem work in opposition, very high reflectivity vs (ideally) low reflectivity, making it difficult to measure otherwise.   



#25 Tenacious

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Posted 21 March 2019 - 11:49 AM

Has anyone done any measurements to gauge the effectiveness of this? It would be pretty easy to slap on a dslr and take a fixed manual exposure with and without the baffle in place, you could then evaluate the histogram to see how the contrast improves or remains neutral.

If you haven't seen it yet, have a look at this thread.  There is a photographic comparison in post #16.  No histogram...




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