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Greetings fellow Herb,
Interestingly, I am making a very similar
telescope, an f/4 Newtonian. I also worried about the vignetting of the 75% ray
at the focuser when I first plugged my parameters into Newt. Eventually I
realized that I really don't care about whether or not the 75% ray is vignetted
at the focuser. What I care about is the diameter of the completely unvignetted
field and how steeply the illumination drops off from there. When I read that
you'd tried to fix the problem by making your diagonal smaller, it occurred to
me that you might be having the same interpretational difficulty I did at first.
What it boils down to is, the bigger you make the diagonal, the broader the 75%
light cone will be, but also the less you will care about capturing it at the
focuser. The idea is to get as much of the FOV into the 100% cone as you can
without other considerations, like central obstruction, becoming an issue.
If you make your diagonal smaller, you lose vignetting at the focuser
because you are substituting vignetting at the diagonal. In fact, if you're
doing photography, you WANT your 75% ray to be vignetted by the focuser.
That way you know you have at least 75% illumination at the edge of your maximum
field.
I ran your scope on my copy of newt, with 16" f/4.5
mirror, and a 2"X2" focuser. I get that you have a 100% illuminated area of 0.5
degrees and a 75% illuminated area of 1.5 degrees, which is Juuuust barely
clipped by the edge of the focuser. I think that would be just fine
for visual use. Your minimum magnification to avoid vignetting by the
hole in the front of your eye would be about 58X, which would give you
about a 1.4 degree field with the super-expensive 31 mm type 5 Nagler
eyepiece, which AIUI has pretty much the widest actual FOV on the market. You
probably wouldn't even notice the ~20% drop-off in illumination at the edge of
this vast, 82 degree apparent field.
I must caution you that I don't know
squat about astrophotography, but intuitively, if you're projecting the prime
focus image directly onto the film, it seems to me you couldn't use a field
wider than the 35mm (about 1.4") diameter of your film. If you have a lens in
the camera, the widest field you could use would be the 48mm diameter of the
lens tube with no aperture stop, again, less than the 2" diameter of your
focuser. In other words, vignetting by the edge of your film or the hole in the
front of your camera moots the question of focuser tube vignetting. Of course,
if you're using some kind of large-format film setup, the foregoing argument is
all wrong. If you look at the geometry of the
representative 75% light cone at the focuser, it's very nearly paralell to the
focuser tube. That means that what you see where the film would be is almost
exactly what you would get at the focuser entrance. So the bottom line is I
don't think vignetting at your focuser will cause you to lose any light you
could have used anyway, and as Mel pointed out, it will help to exclude stray
light. One suggestion I might make, though, if you're really concerned about
getting as much light as possible, would be to move up to a 3.5" diagonal. That
way you'd have a 100% illuminated field of just over 1" diameter, and the edge
of your film (by my verry rough calculation) would be about 90% illuminated.
Good luck, whatever yo decide to do, fellow Herb.
Later,
Herb (Kasler).
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