New Toy

1997 Oct 18

Prompted by discussion on sci.astro.amateur, and further seduced by the fact that no one in my local observing group had one, I recently bought a used optical tube assembly (OTA) for a Meade 127 mm (five-inch) f/9 "ED" refractor. After preliminary tests and evaluation, I report the following:

  1. The Meade 127 ED I bought has a quite good achromatic objective, though by no means a perfect one.
  2. Mechanical quality of the OTA is good to excellent.
  3. At present new and used prices -- I paid $1200 for a bare-bones OTA with simple tube rings and with finder bracket (but no finder) -- I consider the unit a good value.
  4. The Meade 127 ED is not as well color-corrected as Vixen and Takahashi fluorite doublet refractors, or Astro-Physics triplets.
  5. The Meade 127 ED is not an apochromat, in either the technical or the colloquial sense of the word. Meade calls it "apochromatic"; Meade is lying.

Truth is in the details, however, so here they are.

According to the seller, I am the third owner of this particular 127 ED, and the consensus of the two previous was that this unit is one of the better ones Meade has turned out. Reading those words made me a little nervous, however, not just because they might be false, but also because they might be bad news even if true. Meade's optical quality control has been a matter of much debate recently. Three of the four Meade objectives I have previously owned have had good to excellent optics, and the fourth was cheerfully replaced by the factory under warranty, but even so, the buyer of a used telescope, with no warranty protection, does not sleep easily.

The unit showed up in due course, well packed by the seller in a special home-built shipping unit -- he observed that I was acquiring not only a fine telescope, but also a handsome piece of student furniture. I opened it eagerly, hauled out the OTA, sat it on the counter like a small beached whale, then pulled the dust cap and found -- hoorah! -- that the optics had escaped intact from the clutches of UPS. I took it home, cleaned it off, and set about figuring how to set it up. I won't tell you what I used for a mount -- suffice it to say that the Society for Prevention of Cruelty to Great Polarises has a warrant out for my arrest. Seriously, I was surprised to find that the GP does in fact handle a telescope of this size, though I expect that even a little wind will render it unusable. But that was the biggest mount I had, and a recently-purchased half-pier brought the eyepiece up to more comfortable levels, and the OTA proved lighter than expected, so I was indeed in business.

Though light, the OTA seems solid. The cell has push-pull adjustments. The dew cap is held on with a setscrew. The huge rack and pinion focuser moves smoothly, with little play, and with adjustable tension. There are at least four internal baffles.

The finder bracket arrived late, in a separate package, but that didn't stop me, and the notorious curse of foul weather for months after acquiring a new telescope held off. I was able to perform a cursory test within a few days.

As a rule of thumb, a doublet using extra-low dispersion glass ("ED" glass) as one of its components, will produce only about thirty percent as much longitudinal chromatic aberration as one made with good glass of more conventional types. Five-inch refractors made with conventional-glass doublets are starting to show embarrassing amounts of color, at focal lengths as long as f/15, so I was eager to see whether the designer of this faster instrument had asked too much of the fancier optical material. The quick answer was that the tradeoff was well made -- for at 190x, the Meade showed only the barest hint of a violet fringe on a vertically-running portion of the edge of the Moon's disc -- less than I see in my 80 mm f/11.4 Vixen conventional doublet. (It is important to pick a vertically-running part of the edge of the Moon for this test, for portions inclined more toward the horizontal might show color from atmospheric refraction alone.) Bright stars, even Vega, showed only a faint violet haze, again much less than similar conventional doublets. The 127 ED certainly showed a lot less color than an old-style five-inch f/15.

By the time the instrument had settled down thermally, I was ready for a star test. I chose one of the stars at the corners of the Great Square. Racking the eyepiece inside focus until four or five diffraction rings were visible gave a pleasingly regular pattern -- just like the ones in Suiter's book for a good, unobstructed system -- embedded in only a trace of violet glow. Racking the same distance outside focus produced a puzzle: The pattern was not clean at all -- there was a broad greenish haze with a smaller set of purplish blue features superimposed, the most prominent of which was a single blue ring, of diameter about seventy percent of the diameter of the largest diffraction ring seen at the corresponding inside-focus position. I suspected a combination of spherochromatism and perhaps a zonal aberration -- I wish I knew more about the star test -- and decided to pursue the matter on another night. I did notice, however, that as I was beginning to rack the eyepiece outside of focus, for the first tiny movement the central disc of the diffraction pattern stayed quite sharp and turned purple: Thus red and blue light were both coming to focus longwards of green, so that the instrument was an achromat, not an apochromat, at least, not in the strict technical sense of the word. (See the appendix on "What's an `Apochromat'?")

I also looked at Saturn. The seeing was not perfect -- my star test had the in-focus Airy disc always visible, but the rings always in motion. Nevertheless, Saturn looked very fine in those occasional instants when the seeing was best -- at 190x, I could see the Cassini division as a feature of noticeable width, not just a line. I could see the crepe ring and the prominent dark band in the southern hemisphere of the planet. It was a promising view.

Later in the week, I went out earlier in the evening, for a longer test run. I had dug out an old set of Wratten eyepiece filters, to investigate the star test at various wavelengths. While waiting for telescope and atmosphere to settle down, I took a look at Jupiter, at 190x. Despite unsteady conditions, the Galilean satellites all showed distinctly non-stellar discs, of different sizes. Jupiter went behind a tree shortly, but later on I enjoyed views of Saturn and of the Moon similar to those of the previous evening.

For star testing, I chose Vega -- a demanding target. With no filters, 190x revealed the same patterns as before. I repeated the tests with red (Wratten #25A), green (#58) and violet (#47) filters in alternation. The outside-focus ring was still present with each of the filters, though it was diminished in intensity with the green or red filter, compared to the violet filter and to no filter at all. Furthermore, the distribution of intensity in the inside-focus pattern was different for the three filters. Compared to the edge, the relative brightness of the center was about the same with the green filter as for no filter at all, but the center was relatively brighter with the red filter, and relatively dimmer with the violet one. This change in intensity is spherochromatism, also known as chromatic variation of spherical aberration -- the system's figure is different in different wavelengths. Spherical aberration of opposite signs in red and violet is another hallmark of the plain achromat, and another indication that the design is not an apochromat in the technical sense of the word. The Wratten filters were very useful for these tests, though they are far from perfect. Wrattens have very broad bandpasses which are not sharply delineated: A good set of narrow-band filters would have been much more illuminating.

These star tests were performed with a 6 mm Vixen Lanthanum eyepiece. I repeated enough of them with another eyepiece -- an 8 mm Brandon -- and with another star -- Polaris -- to be confident that the results were not peculiar to just Vega or to just that eyepiece.

I attribute the less-than-perfect star test to a combination of longitudinal color, spherochromatism, and possibly a minor zonal error. All the text and pictures I have seen, that aid in interpretation of star tests, are for monochromatic images. I know very little about how to unravel test results in which different colors of light do different things.

Polaris was of course split wide open, so for a lark I tried a much more demanding multiple star, gamma Andromeda. The wide pair of this system was well separated, again "of course", but I was delighted to see at 456x just the barest hint of elongation of gamma-two Andromeda in what, on checking, turned out to be the right position angle. I only logged the elongation as "suspected", but even that is quite an achievement for five inches of aperture.

My third night out with the 127 ED was for the occultation of Aldebaran by the Moon, on October 18-19, 1997. I went to Fremont Peak, near Salinas, California (full report below). Seeing was better than on the first two nights, and the telescope had a long cool-down riding in the back of my car. Jupiter at 228x showed much rich detail, and the steadier view of Saturn confirmed all the features I had glimpsed fleetingly on the preceding evenings. I had not brought my Wratten filters, but I demonstrated the by now familiar star test results to several other observers, using Altair and 228x. We also repeated those tests with yet another eyepiece -- a 6 mm Zeiss Abbe orthoscopic, with no significant difference except that the Zeiss orthoscopic showed less intrinsic glare than the Vixen Lanthanum -- as had the 8 mm Brandon. The other observers present all seemed impressed with the telescope, and promised not to tell the SPCGP where I was.

I did a little Messier hunting -- no surprises there -- then moved on to the Moon. Plato was several days away from the terminator, so much that there were no shadows in or near it, but even so, I could see at 228x three or four white spots on the floor of the crater, three of which were at positions of some of the craterlets shown in Rukl's atlas. Nearer the terminator, in an area of contrasty shadows, I could see the well-developed rille system on the floor of Atlas. The 127 ED was showing all the detail depicted in Rukl.

Not too many telescopes were present, but I got to do one neat comparison test. Someone had an Intes 6-inch f/12 Maksutov. I was curious to compare it to the 127 ED, because I own an Intes 6-inch f/10, yet cannot do side-by-side comparisons with my own telescope since I have only one mount large enough for the 127 ED or for the Intes. The Meade seemed to be giving a slightly more contrasty view of Saturn than did the Intes; that's about what I would have expected, from comparing my own Intes with other five-inch refractors.

Other comparisons I can do from memory: The Meade 127 ED does not give quite as good images as late-model Astro-Physics 130 mm refractors, or as Takahashi 5-inch fluorites. Indeed, the Meade 127 ED shows more in-focus chromatic aberration than either Vixen and Takahashi fluorite doublets (which are almost color-free, but will show a trace of violet haze if you know where to look for it), or Roland Christen's late-model Astro-Physics triplets. These latter telescopes appear to show no chromatic aberration whatsoever, on any object, and are the only refractors which I have ever seen that warrant the adjective "apochromatic" in the colloquial sense (see appendix). (Note that I do not know for sure whether the Christen designs are in fact apochromats in the technical sense.)

(The 127 ED is a great telescope for comparison with Astro-Physics 130s. What the Meade does better, it does by its own superior virtue; what the 130s do better is because they have 3 more mm of aperture...)

Thus although the Meade 127 ED has a fine objective -- at least, mine does -- it is not quite the telescope for purists that the modern fluorite doublets or the Astro-Physics triplets are; anyone who buys a Meade ED will sooner or later see a Vixen or Takahashi fluorite, or a Christen triplet, and be disappointed, for it is these telescopes that create that rabid fanaticism which causes enthusiasts to swear, "You can have my refractor when you pry it from my cold, dead hands."

When disappointment arises, perhaps the unhappy owner will say to Meade, "What you sold is not what you advertised -- an apochromat; I want my money back." I will be curious what Meade does in such cases, because their advertising is manifestly absolutely false -- they wouldn't have a leg to stand on, to justify the label "apochromatic", if it came to a court case. You can tell the judge I said so. I am not certain the error is deliberate -- folks who prepare ad copy often don't know the first thing about what they are selling -- but as a corporation, Meade is lying: There is surely enough optical expertise within their walls to know the ED doublet they sell is not an apochromat in the technical sense, and is not sufficiently color-free to warrant the colloquial meaning of "apochromat".

Yet there is no reason to allow the reputation of a nice achromat to be besmirched by the shoddy business practices and dishonest advertising of its manufacturers. My Meade 127 ED optical tube assembly is a fine telescope, and at $1200 (used) was an excellent value. I expect to keep it for a long time to come.

Appendix -- What's an `Apochromat'?

"Apochromat" is one of those words that has a precise technical meaning, but has entered popular usage with a colloquial meaning that is somewhat different. Here is an overview of what the term means.

If you plot focal length of a given telescope vertically, versus wavelength of light horizontally, what you are hoping to see is a straight, horizontal line -- all wavelengths of light come to focus at the same point. Any telescope which uses only reflecting optics to form its image will meet that goal -- Newtonians do it regularly. Yet, put lenses in the system, and the nice straight line disappears. One task of the optical designer is to make the graph nearly enough straight, over the wavelengths of visible light, so that no error of color is apparent. Just how straight it has to be depends on the aperture and focal ratio of the telescope.

In a common-sized refractor whose objective is a simple lens, the curve of focal length versus wavelength is an inclined line -- blue focuses shorter than red -- of such steepness that only a small portion of the visible spectrum is in focus at once. Simple-lens refractors indeed show enormous color fringes about their images.

Add a second piece of the right glass, and the graph is a broad, flattish curve, concave up. The designs used generally have the low point of the curve at green wavelengths -- green focuses shorter than any other wavelength -- and red and blue approximately equally out of focus. This is the conventional achromat, in the technical sense. With common glass types, it is hard to make the curve flat enough to result in an image that appears color-free -- it takes small lenses and long focal ratios to do so. ED glass makes the task easier -- the curve is squashed by about a factor of four compared to regular types of glass. Fluorite flattens the curve by another factor of two or more. Thus ED achromats are better than conventional ones, and fluorite achromats better still, as far as color correction goes.

A third piece of glass, of a type well chosen, can make the actual graph fit the straight line even better. The curve now resembles a very stretched-out horizontal "S". It comes up from below the straight line at one end of the spectrum, crosses the line, flattens out, dips back below the line, and rises, crossing one more time, and continuing to rise, at the other end of the spectrum. This "three-crossings" property is one of the features which define an apochromat in the technical sense. Such a lens is sometimes said to "bring three colors to the same focus". I will get to the other property later. With good choices of glass, an apochromat can offer better color correction than a conventional achromat. Not all apochromats do that -- some early ones used the extra design freedom to make a lens that would be in focus simultaneously for visual light and for blue and near-ultraviolet light, so that one could focus visually and use early, blue/ultraviolet sensitive photographic emulsions to take a well-focused picture.

We are almost done. The second property that has to do with the technical definition of an apochromat is how well it corrects spherical aberration at various wavelengths. A conventional achromat has zero spherical aberration at only one wavelength -- typically, green. In telescope-maker's terms, the figure is good at only one wavelength. In a classic three-lens apochromat, there are two wavelengths at which spherical aberration is zero, and if the design is well done, the instrument will be better corrected for spherical aberration at other wavelengths, than an achromat.

Thus an apochromat, in the technical sense:

  1. Has an "s-curve" graph of focal length versus wavelength, so it "brings three colors to the same focus", and
  2. Has zero spherical aberration at two different wavelengths.

So much for the technical definition. If you've made it this far the rest is easy -- the colloquial definition of an apochromat is simply an instrument which shows no false color in its images. As I said before, there are vast numbers of instruments out there which are superb apochromats in the colloquial sense -- any Newtonian will do (except those few that use prism diagonals instead of mirror diagonals). (Newtonians of course have other aberrations, such as coma and the distortion of the diffraction pattern created by diagonal and spider, but they are indeed color-free.)

On the evening of Saturday, 18 October, 1997, I went to Fremont Peak State Park, near San Juan Bautista, California, with my new Meade 127 ED refractor. I have reviewed the 127 ED at great length in another posting (above), but I did get to do some observing of more general interest while I was there.

Two observers had new Zeiss bino-viewers, one mounted on a Meade LX200 12-inch Schmidt-Cassegrain, one on an Intes 6-inch f/12 Maksutov. These pricey items have stimulated much discussion among my observing friends of late, and I was anxious to try them out. Between the two telescopes, I looked at the Moon, Jupiter, and Saturn.

I have not had much luck with bino-viewers before, and I am afraid that my experience that evening was similar. The result is personal: I have two problems: First, I often have considerable difficulty fusing the images, even with great care adjusting the interpupillary distance. That's a bit of a surprise, since I use regular binoculars frequently, and have never encountered that difficulty there. Second, I tend to observe without my glasses, and there is a considerable difference in the correction for near-sighteness between my left and right eye, more than in the eyes of any of the bino-viewer owners that I know. In consequence, when I step up to a bino-viewer, I typically find that I can focus the telescope for one eye at most, and the only fix is the long, jiggly, fussy operation of loosening one of the eyepieces in its tube and sliding it back and forth manually, by trial and error, till I hit the right position.

That last problem really reflects a glaring deficiency in contemporary bino-viewer design. Although only Zeiss units were present on October 18, I think that all the current commercial units have the same fault: None of them allow any fine adjustment for different degrees of near-sightedness in the user's eyes. That lack not only makes set-up time-consuming and jiggly, but also means that it is very difficult to share views with friends whose glasses prescriptions are different from yours. I am left with the sense that none of these products -- binoviewers -- are really ready for the market. After all, when was the last time you saw a Zeiss binocular -- or a Tasco binocular, for that matter -- that did not allow some means of varying the focus from one eye to the other. For what these gadgets cost, surely it could not be too much trouble to put a coarse thread on one of the eyepiece holders.

One of the interesting things about being around a state park on a lonely night is the number of thumps and crackles that come out of the underbrush. When that happens, I generally lift my red flashlight to head level and sweep it horizontally, looking for reflections of eyes in the night. Usually there aren't any, but some times there are, and the deep red LED reflections look rather spooky. I was wondering what might be hiding behind a nearby fence post, as I shined my light at it, when the fencepost suddenly opened its low-set eyes and stared at me. Then it lifted its head to rather more than the fencepost's height, and stared some more. "Oh, look at the deer," I said. "Or maybe they're Velociraptors."

With an occultation coming up, and a nearly full Moon high in the sky, we spent a lot of time looking at Lunar features. My new Meade 127 ED shows signs of being a good telescope for that. At 228x, I saw three or four white spots on the floor of Plato, at least three of which were in the positions of craterlets shown in Rukl's atlas, -- no mean feat considering that Plato was so far from the terminator that no shadows were showing there. The pair of large craters, Atlas and Hercules, lay close to the terminator, and the shadows within Atlas clearly revealed its internal rille system, showing in essence all the detail in Rukl.

Users of larger telescopes were experimenting with high magnification. The seeing was not perfectly steady, but it was good enough to warrant going to very high magnification on all telescopes present. The Meade 12-inch LX200 was running at about 800x for a while, and though there was a good deal of chromatic aberration -- perhaps due to the Barlow -- the view showed more detail than at lower magnifications.

The star of the night, however, was the 18-inch Obsession at nearly 1200x -- with a 4.8 mm Nagler and a 3x Barlow. There were problems using this configuration, as you may imagine. The residual coma of the f/4.5 paraboloid was readily apparent at the edges of the field, something -- again, perhaps the Barlow -- was causing colored fringes that looked like chromatic difference of magnification, and since we could make the Dobson track smoothly at so high a magnification, we were reduced to slewing the telescope slightly, then sitting and watching the Moon glide by, then slewing again. Notwithstanding, the view was magnificent, with enormous amounts of detail; this was far and away the best view of the Moon I have ever had in any telescope. Aperture wins. Wow, does it ever.

When the occultation came, I was watching through the 127 ED at 228x. In an occultation last Spring, Aldebaran had seemed to take a substantial fraction of a second to disappear, but this time the vanish was as near to instantaneous as I could tell. The Moon slowly glided up over the star, and then alpha Tauri winked out.

I did not stay around for the reappearance, I packed and left right away. It had been a short night, but a rather enjoyable one.

Jay Reynolds Freeman 1997 Oct 21