Re: [ATM] Baffles

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

John, Anthony,

> My article was in Sept. 1996 and was advocating Newtonian type baffles as 
> seen in Telescope Optics (not refractor).

Looked throygh my collection - I likely had it once but couldn't find it 
now. And I don't have Telescope Optics either, so I can only comment on what 
has been on this thread.

>> For a telescope, I would suggest taking a photo through the empty focuser
>> opening (or simply checking visually) during daytime to see if any
>> significant stray light is visible, and if so, identify the path.
>
> Identify the path? Difficult! CCD flats with and without baffles would 
> likely be better.

Depends on the purpose. To illustrate the principle, see the very 
exaggerated image at:

http://web.telia.com/~u41105032/bilder/PICT1924a.JPG

This was taken with the focuser removed and the camera somewhat inside focus 
to exaggerate (also somewhat off axis). None of the normal baffling is used. 
I have put a black sheet over the tube top to keep the mirror dark - you see 
a white "leak" below center (above, a little of my thumb and the camera 
strap). This way, you can see any part of the tube, or crack in the 
baffling, and identify any offending light path. You see a little of the 
secondary cage, this is not possible from the focuser.

> For an image of my "tube within a tube" see
> bmaa.freeyellow.com/Constellation/NOV2000.html

Fine. How do you let out air currents in the larger part without letting 
light in between the tubes?

As I said, I took some pics to show my experiment of yesterday. They are 
illuminated by the camera flash, but visually, a flashlight is fine. The 
"baffle" strip is held a little away from the "wall" for clarity. As said 
before, the test pieces are not very suitable for real tubes or baffles, but 
they serve to illustrate the point. You can easily do a similar experiment 
with whatever coating you consider for your own tube.

The first
http://web.telia.com/~u41105032/bilder/PICT1915a.JPG

shows illumination only along the line of sight - the "baffle" strip is very 
much brighter, and here, a baffle would defeat its own purpose (to some 
small extent).

http://web.telia.com/~u41105032/bilder/PICT1921a.JPG

shows the same setup in front of a mirror, creating grazing reflections. 
Here, the "tube" is brighter over a wide angle of scatter, and baffling 
helps.

Let me point out again that in refractors, the baffles are seen from their 
shadow sides, in the Newtonian from their lit sides. Thus, a baffle in a 
Newtonian serves no purpose unless it is darker than the part of the wall it 
hoides from view! Trying to shade all of the tube wall from sight is what I 
call "refractor baffling", and is a solution to a very different problem. 
This said, while refractor baffling a Newtonian is a waste of effort at 
worst, it is not likely to create any serious problems.

Nils Olof

ps Anthony, I tried to mail you but the mail wasn't delivered promptly - a 
problem?


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