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?
There is a modest error in my baffle plans. It could be solved with mirrors.
Edited by starcanoe, 13 November 2018 - 12:44 PM.