by Jay Freeman
I will make this report part a running log of AP-10 experiences, with emphasis on things others might want to know.
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.
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.)
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:
For persons with ten thousand dollars to spend on an OTA, an Astro-Physics 10-inch Maksutov-Cassegrain is certainly one to consider.
| Part I |
| Part II |
| Part III |
| Part IV |