I will make this report part a running log of AP-10 experiences,
with emphasis on things others might want to know.
December 20-21, 1999: I observed at a close-in hilltop site above
Palo Alto, all alone. Sound or motion in the brush became a sneaking
mountain lion, and stillness meant it was poised to pounce. Yet the
stars scarcely twinkled, so I hollared "Supper time!!" to any predators
present, and set up. Seeing was better than during my first-light
session. Near the zenith, I could see Airy discs of stars often, though
the rings were a blur, never even partially defined. It was easy to see
six stars in the Trapezium, even at only 155x. The image of Sirius was
better than on first-light evening, but I still couldn't split it.
Better seeing made accurate focusing easier. When the seeing was
momentarily at its best, there was a "snap" to focus; that is, there was
a well-defined tiny region of focuser travel in which the image was
sharpest. That is a very good sign about the optics' quality.
At 309x (12 mm Brandon), Saturn showed disc detail new to me. The
broad brownish belt in the south temperate zone was accompanied by a
narrow equatorial one. The south half or third of the southern
hemisphere was slightly darker than the rest, and more neutral in hue,
not brown. The colors and subtlety of disc shading reminded me of a
young Siamese cat, one whose "points" have not developed full intensity.
The Crepe Ring and Cassini Division were well defined, and the inner
part of ring B had a slightly warmer color than the outer part. The
broad minimum in the middle of the A ring was again present, but I did
not see the narrower one further out, nor any spokes.
I looked at Jupiter but have no special report. I spent a while
chasing faint galaxies, near the _Millennium_Star_Atlas_ chart limit,
east in azimuth from Polaris, in Cepheus and Camelopardalis. The sky
suffered from light pollution, yet galactic centers are generally
relatively high in surface brightness, so I could add magnification to
darken the background sky and increase perceived contrast. At 155x
(1.64 mm exit pupil), I found all the galaxies on my list, some twenty.
The AP-10 can indeed do deep-sky work that I call serious.
There was intermittent wind when I was looking at the planets, and
the AP-10 was not in the lee of my car. The G11 got jiggly enough at
309x to interfere with seeing details. Yet the wind disturbed the
seeing, so details went away, hence the mount was really not a problem.
December 22-23, 1999: I set up at a parking area near a reservoir on
the west side of California's Great Valley. Intermittent high haze and
cloud crossed the sky, but for part of the evening, seeing was good
enough near the zenith that stellar images showed a solid Airy disc and
a complete, but rippling, first diffraction ring, at all times. Those
conditions permitted a convincing star test, made at 464x using a Vixen
8-24 mm zoom eyepiece, with the small central baffle in place, so the
central obstruction diameter was only 2.3 inches. The test took place
late, after the instrument had been set up several hours. I had not
noticed thermal effects earlier, and the evening was not particularly
challenging thermally -- the telescope had been stored at 15 C, and
ambient temperatures were 9 or 10 C -- I just didn't get around to a
star test for a while.
First, the first diffraction ring was a great deal fainter, compared
to the Airy disc, than with telescopes with one-third diameter central
obstructions, like most f/10 or f/11 Schmidt-Cassegrains, and like my
Intes 6-inch Maksutov-Cassegrain. Furthermore, I could not even see a
second diffraction ring of this fifth or so magnitude star, though
perhaps I might have if the image had been completely steady. The
faintness of the rings not only reflects the AP-10's smaller central
obstruction, but also indicates that the optics are well-enough figured
to exploit the advantage of the small obstruction.
Second, I compared out-of-focus diffraction patterns far enough out
of focus that what was showing was a bright central spot surrounded by
three bright rings. The rings were in motion, just as they had been in
focus, but were complete, and as far as I could tell, considering the
motion, the patterns were identical inside and outside of focus. I am
sure I could do a better star test in more nearly perfect seeing, but
what I saw during this one leaves little doubt that the optics of this
particular AP-10 are excellent.
Jupiter and Saturn were well-placed, and I spent considerable time
looking at them, and also showing them to the five or six other
telescopists who were present. Jupiter showed rich belt detail, and
provided a fascinating opportunity to watch an Io transit. The moon and
its shadow crossed the face of the gas giant, giving views much like
some of the Hubble shots that have recently been released. Someone
remarked that he had been wondering if NASA had correctly balanced the
colors in the released images, and we could verify that they had. The
contrast in color between Io and Jupiter was striking; Io looked like a
pale gold Christmas ball suspended in front of the ruddy and creamy
colors of the gas giant. Even if I had not known about Io's
composition, I might have used the term "sulfur-yellow" to describe the
hue. The color contrast was particularly striking just as the moon was
about to begin its egress from the transit, when it was seen against the
limb-darkened edge of the Jovian disc.
Saturn also showed pleasant colors, though subtler ones. The
details were essentially as I saw them on December 20-21, though more
steadily seen, as the seeing was much better. Several people present
said that they had never had a better view of Saturn in any telescope.
The view certainly surpassed any that I myself had had, and its
competitors include my own 1987 6-inch Astro-Physics triplet refractor
in excellent seeing, and the 36-inch doublet at Lick Observatory in
seeing that was notably less than perfect for it.
I looked at several double stars as well. Sirius was low enough
that the seeing for it was considerably worse, though I did see the
central Airy disc at times. I thought I caught a few glimpses of the
Pup, as well, using an 8 mm Brandon for 464x, but the position angle did
not check with an ephemeris for Sirius B, so I guess I did not.
I should have looked at gamma Andromedae earlier, but did not think
of it till it was well down from the zenith and affected by declining
seeing. The split between gamma-one and gamma-two was of course easy --
this bright yellow and blue pair is resolvable in many binoculars. I
ran the magnification up to 742x (5 mm Pentax SMC-ED orthoscopic) to get
a careful look at gamma-two, but although its Airy disc was clearly
elongated, seeing did not let me check for any necking down or
separation of this close, unequal double. Gamma-two Andromedae has been
closing lately, it might not be within range of ten inches aperture now.
The good baffling and low scattering of the AP-10 appeared to make
the colors of double stars particularly pleasant. Stars like gamma
Andromedae, h3945, eta Cassiopeiae, and beta Cygni provided good
examples of contrasting hues. I also used the instrument for numerous
deep-sky targets, but have nothing to add to the initial impression I
reported, that it is no slouch for this kind of work, though not quite
the equal of my Celestron 14.
How The Grinch Went Cosmic: I took the AP-10 out on Christmas eve,
on the evening of Christmas day, and on the evening of the day after
Christmas. Seeing was soft on all three occasions, so I did not get to
try any more high-resolution tests. Rather, I used the telescope for
deep-sky work of the kind that makes up most of my observing, and
gathered more confirmation of my initial impression that it is not as
good as my C-14 for such purposes, but closer than you would expect from
mere consideration of clear aperture. The dark, well-baffled field,
with crisp images from edge to edge, helped a lot with chasing and
confirming faint fuzzies. There is no doubt that a deep-sky enthusiast
could spend a long time with this instrument before running out of
interesting things to look at.
I could see all five galaxies in Stephen's Quintet at 155x, and
could also see the four close companions of NGC 7331; namely, NGC 7335,
7336, 7337, and 7340, though the views were not quite as good as I
remember from the C-14, even though the sky was dark and transparent
when I turned the AP-10 on them. (I am not likely to be able to run a
side-by-side comparison of the C-14 and the AP-10, since I only have one
mount that can hold a telescope this large, so memory will have to do;
however, the objects just mentioned are among my favorites, and I have
looked at them many times with the C-14, usually at very nearly the same
exit pupil.)
I also continue to be impressed with how much easier to use the
AP-10 is than the C-14. The mounted weight of the former is only about
25 percent less than that of the latter, but the G-11 behaves much
better with the smaller mass, it is lots easier to slew and set by hand.
Set up and take down are both also faster, and with the AP-10 in my van
instead of the C-14, I have the front passenger seat available for a
guest, if need be. (That's where the C-14 OTA normally rides, but the
AP-10 fits behind the front seats, crosswise, in its case.)
December 27-28, 1999: Slumming. I did not take the AP-10 out, but
spent a while with my other 10-inch telescope, an f/5 Dobson with Nova
optics that I built for airline transport to places like Hawaii. I
tried a handful of deep-sky targets that I had recently looked at with
the AP-10, at similar magnifications (106x in the Dobson, 93x in the
AP-10), and insofar as memory permits, the images in the AP-10 were a
hair brighter. That is to be expected considering the Mak-Cass's
high-tech coatings (the Dobson has plain aluminum on the primary but an
enhanced coating on the secondary) and small central obstruction, and
again confirms my impression that the AP-10 does somewhat better for
its aperture than many 10-inch instruments that amateurs use.
This is probably the end of the initial series of my reports on
the AP-10, though I will certainly write up any more spectacular sights
that come with exceptional seeing. The conclusions I drew at the
close of part III still stand, except with a little more confidence
based on the additional experience. I shall restate them here:
1) The telescope is well-designed, and easy to set up and use.
2) I have seen no indication whatsoever of thermal problems, albeit
none of the nights I have had the instrument out have been very
challenging thermally. Nevertheless, the lack of such problems
without using the muffin fans or leaving the back dewcap off,
suggests that I will not have much to worry about from thermal
difficulties in the future.
3) The optics are excellent, well baffled, and easy to focus.
4) The telescope delivers a whole lot of low-contrast fine planetary
detail, even when seeing is so-so, and the more so when seeing is
good. It regularly does better than many other telescopes of
both greater and smaller sizes.
5) Between excellent baffling and high-technology coatings, the
telescope offers better deep-sky performance than a typical
10-inch Newtonian or 10-inch Schmidt-Cassegrain. It is certainly
capable of deep-sky work that will satisfy a beyond-the-fringe
raving lunatic of a deep-sky weasel, such as I.
6) More detailed optical testing and performance evaluation will
require a night of near-perfect seeing. I will report when that
happens, but don't hold your breath waiting.
For persons with ten thousand dollars to spend on an OTA, an
Astro-Physics 10-inch Maksutov-Cassegrain is certainly one to consider.