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# Refractor Baffle Calculator

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### #1 rnabholz

rnabholz

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Posted 28 April 2007 - 11:01 PM

I recently picked up a 60mm refractor ota 800mm FL (f13.3).

I had a 60mm f11.66 and had them both out tonite comparing views.

I was immediately struck by the better contrast of the f13.3 tube.

Now I know that contrast improvement can come from many sources, and I also know that the extra magnification provided by the extra focal length may have contributed to my impression.

I opened up the 800mm tube tonight to take a look at the baffling and now am wondering if just maybe the baffles are stopping the scope down - making the view the effective focal length even longer.

There is one tube baffle at about the midpoint of the tube - I can't really get to it to measure it very effectively, but the focuser has a baffle with a 1/2" opening (.965 focuser).

What I was wondering was is there any software or other tools out there that can help me understand the characteristics of the light cone of this scope so I can determine if the baffles are doing their job correctly or are maybe clipping things a bit.

Thanks

### #2 Mike I. Jones

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Posted 28 April 2007 - 11:52 PM

Rod,
I'll repeat a post and diagram I put up a few months ago, Hope it helps.
Mike

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The attached figure shows the layout and formula for baffling a refractor. Measure the back aperture of the lens in its cell and call that "DL". Measure the back focal length "BFL", and decide on the diameter of the fully illuminated field at focus "d".

Then decide on where you want to put baffles along the tube. Let the distance from the back of the lens to each baffle plate be "X", and use the formula on the drawing to calculate the corresponding aperture "DB" of the baffle plate at that position.

This formula neglects the sag of the lens rear surface. The last radius is usually long if not actually flat, though, so the error is small.

If the tube interior were perfectly absorbing right down to extreme grazing angles, baffling would not be needed. But that's not the case for real materials, and a small amount of light still reflects from the tube I.D. and fogs the focus with stray light. The point of internal baffling is to block these stray light reflections from reaching the focal plane.

I've built two refractors, using a dusting of Krylon Ultra-Flat Black paint on the inside of the tube. Looking at the exit pupil, I found that one baffle halfway between lens and focus was sufficient. Some folks like two baffles, equally spaced in thirds, or one baffle at the midpoint and another between that baffle and the lens. It didn't look like it needed but just one to me.

You do know about the neat little trick of stacking another eyepiece on top of the telescope eyepiece to inspect the exit pupil for stray light, right? The second eyepiece has to have a real focal plane to do this, such as a Plossl or orthoscopic eyepiece and not a Nagler, Panoptic, Radian, etc.

Mike

### #3 Nils Olof Carlin

Nils Olof Carlin

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Posted 29 April 2007 - 07:56 AM

How to calculate "X"!
The basic idea of baffling is to hide the tube wall from sight from the FOV - thus, the position of the baffles are determined by that.

Call the inner tube diameter Dt; and DL, d, X and BFL as in the figure. Set Y=BFL for the first baffle, T=DL, then set z=(Dt-T)/2; X/z=(Y-X)/d => X=z*Y/(d+z); this gives the distance to the *first* baffle!
For the next baffle, L=BFL-X where X is the position of the first baffle, and the new T=the diameter of the first baffle opening. The new X is the distance from the first to the second baffle - and so on.

Suppose the aperture is 80 mm, the tube dia. is 100 mm, the BFL is 500 mm and the field dia. d=44 mm:

Y=500, d=44,T=80, z=(100-80)/2=10, X=10*500/(44+10)=92 mm.
This gives, with Mike's formula, Db=73.4 mm.
Next: Y=408, T=73.4, z=(100-73.4)/2=13.3; X=95 (i.e. the distance from baffle #1 to #2).
The dia. of the second opening again with Mike's formula (using the sum of Xes as the new X!)=66.5 mm

And so on - until you have baffled as far as is visible through the focuser. How far that is, is left as an exercise to the reader - it might be simpler to make a scale drawing...

Nils Olof

### #4 rnabholz

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Posted 30 April 2007 - 10:26 AM

- it might be simpler to make a scale drawing...

Nils Olof

I believe that you are right, I can draw it out faster than I can do those calculations....at least at this focal length.

Mike and Nils, thanks for the response, I am printing it for reference and will work through the excercise.

Thanks again.

### #5 John Carruthers

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Posted 01 May 2007 - 08:19 AM