[AZ-Observing] big scopes for planets
- From: "Mike Spooner" <spooner@xxxxxxxxxxx>
- To: <AZ-Observing@xxxxxxxxxxxxx>
- Date: Tue, 8 Jan 2002 22:17:47 -0700
I've been busy testing out an 18" personal mirror for my big Dob the past
few weeks and thought I'd pass some observations along. The mirror is
uncoated so while coating scatter is not a problem, the relative dimness at
higher magnifications has been limiting. The mirror exhibits minor
astigmatism on the bench but has very good spherical correction and a very
narrow turned edge. It is a zero expansion substrate so it's been somewhat
easier to get a repeated and reliable idea of the scope's performance. I
made this scope with planetary viewing in mind.
The seeing has been very good the past couple of weeks even though I've had
to dodge clouds on several nights. When I first star tested, I could see
some astigmatism as focus was approached but the planetary images were
fairly contrasty and detailed. I suspected my secondary mirror had slight
curvature from previous experience and so I decided to rotate teh primary
and see if I could determine if the astigmatism in the primary was at fault
entirely or if the diagonal was also affecting the image (diagonal curvature
shows up as astigmatism at teh eyepiece.) To my pleasant surprise, I found
that there was a rotational position where the two mirrors canceled the
errors. As mirror cells can easily induce astigmatic effects, I rotated away
and back again a few times to make certain I was seeing optical effects
only. Once this was ascertained, I got down to some serious planetary
viewing.
Saturn takes power well and I found that over several nights I could use 450
to 500x with no image breakdown (actually I went to 1560x on the moon before
I could even detect softness). I also rotated the mirror in and out of the
optimum position to see how it affected the planets. What I noticed was the
detail was not too greatly affected but the steadiness of the image was
highly compromised so that details like the Cassini division would fade in
and out much like poorer seeing. When optimized the images are very solid.
Upon switching to Jupiter, I found that it too would take surprising amounts
of magnification without losing contrast. With my 3mm Radian it was still
holding detail and contrast well but I was now running out of light for the
finer details. At 450x with a Pentax 5.2mm, it is giving the finest views
I've ever had of the big planet. I can hardly wait to get it coated but
don't want to lose it for a month or two that the gas giants are showing
off. Interesting delimma. It has certainly reassured my mind about the
viability of large aperture scopes for planetary work.
I now wonder if there are other large scopes that might benefit from this
form of tuning. It's not unlikely to find large mirrors with a trace of
astigmatism and diagonals with curvature are certainly common enough. If you
decide to perform this trial and error test, I'd suggest doing it at home so
as not to spend valuable dark sky observing time. It does take a bit of
effort as it may require a collimation touchup with each new rotation and
this is somewhat time consuming. But the results may be worth the
investment. I might add that if you can't see astigmatism using the star
test at 300 to 400x, it's probably not worth investigating.
I apologize if this is a bit off topic observing wise, but I consider this
as a special case of collimation (optical elements in refractors and SCTs
are rotated for best results as standard procedure). Perhaps someone else
will find benefits of crisper, more pleasurable images.
--Mike Spooner
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