I'm building a 200mm F6 newtonian bino and trying to decide as to how much baffling I should do. I have two CF tubes internally matt blackened (I'm experimenting with that) and they are 250 ID by 1160 long. I have been looking at several sources for calculating the diameter and spacing of baffles and there appears to be wide discrepancies in the number required.
Using Stellafane's Newt for the Web, 10 baffles are calculated, but if I use Ivan Krastev's calculator at http://www.myoptics....bafflecalc.html or Richard Combs' calculator at http://atmpage.com/baffle.html I get about double the number of baffles. Stellafane's calculator has few baffles between secondary and aperture (245mm) and most between secondary and primary, whereas the other calculators have many, about a third the total, between secondary and aperture.
Given that I want to achieve good contrast but not really want to do *that much* extra work if it is unnecessary, just how much baffling should I do? Baffling!
I plan to make the baffles using CF tape and black tinted epoxy resin pressed in a flat circular mold with a fixed OD. Then I would make a fixture for machining the various ID's in a lathe.
We currently have a 12"/F5 binocular telescope that was built by Mark Suchting as part of our 3RF OZsky Equipment. It works very well. We also have a 22"/F5 binoscope that was built in the USA and it doesn't work so well due to stability issues. It will eventually be re built by Peter Read at SDM Telescopes. In the images below you can see the white 12" binoscope towards the rear of the observing field and in the image of the 22" binoscope you can see how flimsy the truss poles are. The flimsy truss poles cause it to wobble unnecessarily which makes it extremely difficult to keep the images merged.
I would recommend building it and getting used to it first, then baffling it to suit the eyepieces you most frequently use. The thing with bino's is that you will see a lot more detail for pretty much any given magnification, so you will spend more of your time with longer FL eyepieces. In my experience, the increase in detail perception follows the rule of the square root of two. ie) If a 12mm eyepiece is your favorite eyepiece in a monocular telescope, then you will see the same amount of detail in the binoculars with 17mm eyepieces. (all else being equal)
I found that at F/5, ninety five percent of the time the 17mm eyepieces gave the best views, only swapping to 12mm for viewing the planets.
Once you have narrowed down the range of magnifications you are likely to employ, this will give you an idea of the size of baffle required under the base of the focuser. I would then experiment with different sized baffles at this position to optimize over all contrast. This is the single most important baffle in the entire OTA. The best position for this baffle will be approximately 23mm off the side wall off the tube (102mm away from the centre of the secondary mirror)
For the rest of them, there is only one which needs to have a sharp edge, that being the one that defines the aperture stop at the top of each tube. Make it a couple of mm larger than the clear aperture of the primaries, and the rest of them can be cut from 2mm ply (or kydex) using a router and radius bar, and make the diameter of hole a couple of mm larger than the aperture stop. This way, you can cut a whole stack of them at once.
Here's one of my tops made with 2mm ply, cut using a radius bar and router.
Incidentally... speaking purely from an optical perspective, the best material to make baffles from is black perspex.
You don't want baffles that are matte black which would scatter light in all directions. The idea is that the 5% (or whatever) of the light that isn't absorbed by the black surface is directed away from the focal plane.
fwiw) I have noticed that the two baffled tops attract mosquitoes during the day whilst they are sitting in the shed. If you disturb them, there is as many as a dozen fly out and buzz around, then (eventually) return. I suppose they recognize it as the darkest place in town and an ideal place to lay up for a snooze.
Hi John,
Without wanting to put too finer point on it, the issues with the 22" run far deeper than just the size of the truss poles.
best,
c
Hi Clive,
We are fully aware of that and that is the reason they will be completely rebuilt from scratch. I didn't need to go into all that here in this thread. The sad thing is we all (including Mark) tried to tell the builder from day 1 that they wouldn't work using his design parameters and he wouldn't listen. The guys at 3RF in the USA, where they already had a very similar pair that didn't work, also tried to tell him. About the only things salvageable from them are the primary and secondary mirrors, the focusers, the Argo Navis and the Sci-Tech drives. The Sci Tech drives are excellent BTW and highly recommended for those who can cover the price of admission.
Hi John,
Thanks for the comments. My bino design doesn't require truss poles as I am using fully enclosed tubes. That 22" bino appears to have concrete reinforcing steel for trusses and seems to be poorly engineered.
I haven't been in contact with Dave Moorhouse but I learnt a few years ago, when I was just getting into this stuff, that Dave had sold his big bino. Clive and Mark have given me some good advice in the past. My aim has been to design with good stiffness, portability, lightness.
My OTA's are almost complete and I am about to fix the secondary spiders in place. They comprise two flat pieces of cf 1mm thick and triangular in shape. They are attached to an aluminium boss to which the secondary mount is attached. I am a bit nervous about getting them glued in the right place.
Hi Clive thanks for the timely advice. I have removed the gloss off the inside of the tubes which is black epoxy, and the finish seems acceptable. I was thinking of spraying with Krylon ultra flat black after the baffles, etc., are glued in but I may not need to.
When you say the baffle is to be positioned "under the base of the focuser," I take that to be just below the light path from the secondary to tertiary.
As for the Aussie mozzies.... Well I'm in mooloo land (Waikato) and we do have annoying bugs but not like them WA mozzies
I am enjoying this build but it is taking a while. I have no work at present but I do have free use of machine tools in the mean time so hopefully I can get the framework and tripod done before more work comes in and I get busy again.
When you say the baffle is to be positioned "under the base of the focuser," I take that to be just below the light path from the secondary to tertiary.
Just wondering why a gloss black finish for that "red" area? Wouldn't it reflect stray light onto the primary? I had thought that I might consider black velvet. I am inclined to consider baffling last if necessary, as you recommended, especially as it would slow things down if I paid attention to it now. I must get this project finished this year!
I have read such a lot about binos and about the experiences of those who have used them, but in my isolation have not actually seen one personally. The nearest to it are my Celestron SkyMaster 20x80 Binoculars which function to spur my build on.
