| Reports | TAC | Join mailing list |
by Jay Reynolds Freeman
On the evening of Friday, May 7, 1999, I took my recently-purchased 1987 Astro-Physics 155 mm f/8 triplet refractor to Fremont Peak State Park (near San Juan Bautista, California), for its second time out in my possession. It was the best of nights; it was the worst of nights. After a disturbingly un-California-like winter, any dark, clear sky was much to be appreciated, but on this occasion, the early onset of fog and high humidity put an unhappy damper on observing.
Setting up the telescope is becoming much easier with experience. I have used my Losmandy G-11 mounting in the field several dozen times, and am getting to where I can deal with it quickly and efficiently. I have bought two sets of three of Losmandy's knurled knobs to speed up the attachment of the equatorial head, and of the pier extender that I use with the 155, and they are a noticeable improvement over having to fuss with Allen wrenches. The other telescope I use regularly with this mounting is my Celestron 14, though, so the big win in operational happiness is *not* having a 52-pound lift to attach the optical tube assembly. It also helps that I only have to install one 22-pound counterweight, not three.
The main shortcoming of the big refractor is the wide range of eyepiece positions. A Christen triplet is *heavy* -- there's a lot of glass in the front end of that tube. In consequence, the optical tube assembly balances nearer the front end than the back. The distance from the balance point to the eyepiece position is about three quarters of a meter. The range in eyepiece height is rather more, since the "high" position has the tube looking horizontally over the top of the mounting's right ascension axis: Swinging to look vertically lowers the eyepiece, not only because of that three quarters of a meter, but also because the intersection of the optical tube assembly and the declination axis is swinging in an arc about the right ascension axis. All in all, the difference is something like a meter, and that's quite a lot for someone who has been spoiled by the much smaller range of eyepiece positions of a C-14. Perhaps I shall ballast the lower end of the tube -- actually, the lower end of the Losmandy dovetail plate -- to reduce the range. The mount has more than enough excess capacity to handle a couple of lead bricks.
I took a brief look at Mars shortly after setup, but the seeing was poor and the planet was presenting one of its more featureless aspects, so I did not bother to try to make sense of such faint detail as may have been lurking at the limit of vision. I instead went on with my standard means of familiarizing myself with new equipment, a Messier survey. I had logged a few the first time out, mostly wintertime galactic clusters, which were all easily resolved at 78x, I trust to no one's surprise. Now Leo and Virgo were riding high, and it was time for some serious galaxy work. I went through all the Messier galaxies in those two constellarions, using a 12 mm Brandon eyepiece for 103x, and had just crossed the border into Coma Bernices, when an abrupt rise in humidity and the approach of tenuous wisps of fog prompted me to wimp out and cover my optics. I had been too lazy to pack my big battery, so I could not use my Kendrick anti-dew system, but it had been a tiring week, so I was just as glad to have an excuse to quit early.
The refractor was doing very well on the Messier galaxies. A magnification of 103x is higher than many people would use for deep-sky work with a six-inch telescope, but I have had good luck with a 1.5 mm exit pupil for galaxy observation in telescopes with apertures from 55 mm to 350 mm, and it seemed to be the right choice for the Astro-Physics as well. One of my first targets was M104, the Sombrero galaxy, and I was pleased to see it showing all the gee-whiz detail that people like to look at: concentrated nuclear brightness, fat bulging lens, and the long dark streak of the edge dust lane, clearly bordered on both sides by brighter material. Nearly all of the Messier galaxies had detail of some sort, such as bright nuclei, visible elongations of varying orientations and proportions, mottling, or dust lanes.
I could not help but notice many non-Messier galaxies as well. I casually logged NGC 3384 and 3389, near M105. NGC 3628, not far from M65 and M66, was almost a duplicate of the Sombrero: Its edge dust lane was delineated by bright stuff on both sides, but the nuclear concentration was not as great. I could see NGC 4388 and 4387, closely south of M84 and M86 -- 4388 was an obvious edge on. And it was simple to follow the brighter galaxies in Markarian's chain on the way from M84 and M86 out to M88 and M91 -- it was easy to see NGC 4435, 4438, 4458, 4461, 4473, 4477, and 4459, all without pushing the telescope, just casually wandering from stepping-stone to stepping-stone as I hunted Messier targets.
The six-inch instrument that I have most extensively used in recent years is an Intes Maksutov, generally with similar magnifications when chasing galaxies, typically 97x. The Astro-Physics refractor was obviously outperforming the Intes on deep-sky stuff, and a moment's thought showed why. There is a small increment in performance from the fact that the refractor is slightly bigger -- the actual apertures are 155 mm and 150 mm for the two telecopes. But the main difference stems from light loss in the Maksutov-Cassegrain because of the secondary obstruction -- exactly 50 mm diameter -- and because the reflective coatings on the primary and secondary are less than perfect.
Coatings are the tail that wags the dog in this matter, and coatings vary enough that generalization is difficult. Aluminum in poor condition might reflect 80 percent or less, so that light losses from two such reflections would reduce the incoming beam to 64 percent or less of its original intensity. On the other hand, whizzy high-tech coatings in sparkling new condition might reflect close enough to 100 percent for the losses to be negligible.
Let's assume my Intes has mirrors that reflect 90 percent of the light, and calculate from there. Both the Intes and the refractor have two air/glass transmissions (the refractor is a cemented triplet), and I use a star diagonal with both, but we can expect the effects of these things to be similar for the two telescopes, and so ignore them when making the comparison. Transmission through the glass certainly causes negligible losses. On that basis, the effective collecting area of the 155 mm Astro-Physics is (pi / 4) times the aperture squared, or (pi / 4) times 24025, or 18869 square millimeters.
The Intes has effective collecting area of (pi / 4) times 150 squared, times 8/9 (obstruction by the secondary), times .9 squared (the two extra reflections), which works out to 12753 square mm. The Astro-Physics therefore gathers more light than the Intes in the proportion 18869 : 12753, or 1.483 -- 48.3 percent more light. That works out to a limiting magnitude difference of 0.428 magnitude, which is certainly enough to be noticeable. I would have said that the refractor was doing vaguely half a magnitude better than the Intes, and so it was.
The same formulae can be used to compare this refractor with the more ubiquitous eight-inch Schmidt-Cassegrain telescopes. With truly ratty coatings, that reflect only 80 percent, a 203.2 mm SCT with a one-third-diameter secondary has an effective collecting area of 18849 square mm -- such an "eight-inch" telescope would provide the observer with slightly *less* light than my "six-inch" Astro-Physics. With perfect (100 percent) coatings, the SCT collecting area rises to 28826 square mm, which gives it an 0.46 magnitude advantage over my refractor. With 90 percent coatings, the SCT's advantage drops to 0.231 magnitude. I expect that few SCTs in operation have coatings worse than 80 percent, and that none have coatings that are perfect. With average coatings, an eight-inch SCT is not quite as far ahead of an Astro-Physics 155 as mindless comparison of their apertures would suggest.
Of course, an eight-inch SCT doesn't cost as much as a high-end six-inch refractor, nor is it as heavy and cumbersome. Neither does it require as big and expensive a mounting as a Losmandy G-11.
The refractor might have another mild advantage for deep-sky work compared to some compound telescopes, perhaps including some SCTs. High-end refractors are generally extremely well-baffled -- there is none of this nonsense about light squeaking past the secondary mirror and sneaking down the baffle tube. I did not mention that point in comparing the Astro-Physics to the Intes, because the Intes is quite well baffled in its own right. But I am not sure that is so for all of the commercial SCTs.
Anyhow, it was an interesting night. I will continue to report as I gather experience with my old/new six-inch Astro-Physics.