Well, a truck hauled 18" scope should be a dream for photography. No
doubt it's well driven and controlled. However, since it's a Newtonian
it's quite possible that it won't work well for photography unless he
built it with that intention in the first place. Since you say you've
often talked of taking photos I assume he has already taken care of that.
The problem with photography using a Newtonian is that, for visual
usage, the focal plane isn't typically brought far outside the tube and
normally can't reach as far as the focal plane of the camera. In
addition, the size of the secondary mirror for visual use is undersized
for covering the full frame of a 35mm camera. We often complain about
poor contast on mirror lenses and the reason is the use of very large
secondary mirrors to avoid obvious vignetting in photographs. The large
secondary causes lots of diffraction and thus lowers the contrast. So,
if the scope wasn't built with photography in mind the secondary would
only be large enough to light the area covered by the eyepiece.
Your E-thing with its small sensor should fare much better in that
respect. Even so you may see some serious vignetting unless the
secondary is larger than is normal for visual use. Of course, the
vignetting is only important on deep sky stuff with an extended area
across the full size of the sensor. Planetary images will be so small
even with the 18" that vignetting will never be a problem.
Chuck Norcutt
Tim Randles wrote:
> I have a friend that builds newtonian telescopes, his "big bertha"
> requires a pickup truck and trailer to set up. He used a 18 inch
> concrete "sonotube" form for the tube and bought the mirrors from a
> place in Russia, spent several , hell dozens of thousands of dollars
> for this rig, a bit obsessed I think.. he's seen aliens and UFO's
> too... he lives on a huge mountain peak in the kootenay mountain
> area.. no stray city lights, no hazy atmosphere, he regularly looks
> at Saturn and all kinds of other stuff.
>
> We have often talked about taking photos, I was just curious about
> it.. I sure appreciate your typing speed and accuracy :)
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