Fremont Peak

1997 May 31

The evening of May 31, 1997, provided a taste of summer weather at Fremont Peak State Park, near San Juan Bautista, California. I arrived in midafternoon, and set up in the southwest parking lot, overlooking the coastal plain. Declining temperatures from the very hot day had started a substantial sea breeze, that chilled us all and caused worries about off-shore fog coming inland and up the hill, but the wind died by the end of twilight, wind-chill temperatures climbed to comfortable, and no fog appeared. A long lane of high cirrus, extending NE/SW, was not far off shore at sunset. During the first half of the night, it drifted sideways over us, but after a gossip and night-lunch break at midnight, it passed on to the east, leaving clear sky and good seeing.

I had several telescopes, for a multi-part observing session. As I recently reported, I had had excellent luck pushing faint-galaxy observations with my Intes Maksutov well beyond what most people consider the limits for six-inch aperture, and in relatively bright, moonlit sky, too. I was anxious to repeat and continue that work in better conditions, and I did. At 121x, I observed NGC 4565A, 4565B, 4565C, 4559B, 4559C, 4286, and 4525. While I was looking at the region of NGC 4565, another observer asked for a peek. His comment reminded me how important it is to learn and practice, practice, practice, the skills of deep-sky work. He remarked that with averted vision, he could detect 4565: Yet, the companion galaxies I was chasing down, are three or four magnitudes fainter.

I switched to my 98 mm f/6.7 Brandon refractor for a Messier hunt. My Messier survey with this instrument is now almost complete -- I could have finished it that night, except my setup location did not have a good view of southern Sagittarius and Scorpius. Four inches and low magnification are a winning combination for Messier hunting -- all the objects are apparent at a glance, and at only 53x with an Erfle eyepiece, they were easy to find as well.

I did finish a Messier survey with a Meade model 165, a 50 mm f/12 refractor that is only a small notch up from the dread department-store variety. It has been clear for quite a while that even this tiny an instrument would have no problem chasing down even the most difficult of Messier objects -- I happened to view M74 and M76 early on -- and on this night I logged the last six. The survey was most hampered by the rather rinky-dink 5x12 finder on the little Meade, but it did show slightly fainter stars than I could see with the naked eye, and thus was usable. Between the finder and the rather flimsy mounting that I reported previously, the 165 is not quite a satisfactory beginner telescope, but it is close, and serves as a reminder that small refractors can do yeoman duty for newbie astronomers.

There were some other interesting telescopes and views near my setup area. One person had an Orion ShortTube 80 mm f/5 refractor. I had played with one in the store, but not much in the field. For daytime use, it showed a lot of chromatic aberration, which made bird-watching more fun if not more realistic, but at night that problem was less obvious. The owner had the roof-type 45-degree erecting prism in use for astronomy; thus, diffraction from the crease of the roof left a nice, reflector-type ray across images of bright objects. Even so, this little telescope seemed to be a fairly satisfactory rich-field unit.

Someone else had a shiny new Astro-Physics 6-inch f/7. All were well pleased with it, and several of us had fun pretending to mistake it for the Orion ShortTube -- the two refractors were similarly proportioned and colored.

Several large Dobson-mounted Newtonians were present as well; I had a nice view through an 18-inch f/4.5 Obsession, of galaxies NGC 3222 and 3227, side by side in Leo. The magnification was 50x, low enough that my pupils were not collecting all the light, but notwithstanding, the owner was well-pleased with her new 40 mm Pentax eyepiece for finding, centering, and public viewing with less frequent control inputs.

I left at 3 AM, about half an hour before Moon rise. It was a fine night.

At Fremont Peak State Park, near San Juan Bautista, California, on the evening of 31 May, 1997, several of us had planned an informal series of eyepiece comparison tests. Informal plans often come to naught, and when half the night had passed with no eyepiece comparisons, I was beginning to think that these arrangements would go that way, too, when up walked Joe Sunseri of Earth and Sky Adventure Products -- that's where I had gotten my 6-inch f/10 Intes Maksutov. I had sent Joe EMail about our proposed comparisons, and he had brought an eyepiece I was most curious about.

The unit was a 12 mm monocentric, made by the Russian company, Intes. Monocentrics have the reputation of small fields, superb correction, and extremely low scattered light -- qualities that might make them valuable specialized eyepieces for the dedicated observer of planets or double stars. I had never seen one, and was anxious to see whether the claims were true.

Strictly, "monocentric" is a technical term, not an eyepiece name -- it means that all the lens surfaces involved in the design have their centers of curvature at the same physical location, or very nearly so. One specific design, the Steinheil monocentric, uses several extremely thick pieces of glass cemented together; the resulting assembly is much thicker than its diameter. Others merely resemble fairly thick multi-element magnifying glasses, such as Hastings triplets. The unit Joe had was not a Steinheil (yes, I took it apart). It appeared to be a cemented triplet, with nicely blackened edges and very odd-colored coatings, almost the blue-green of a nice, fat, house fly. The designers had worked hard at eliminating stray light. The triplet was mounted between front and back retaining surfaces, with some 5 or 10 mm of clearance between its edges and the metal parts of the housing. Those parts were also well blackened, and threaded, too. All this, plus only two air/glass interfaces, gave the prospect of good performance.

But I am getting ahead of myself. I dropped the eyepiece into the telescope I had set up, which happened to be the Intes, and pointed it at Vega. The field of view was indeed narrow: No porthole on the universe this, the vista was more like looking up from the sewer through an open manhole. The apparent field was only about 30 degrees. Use of such eyepieces would require a good finder or finding eyepiece, and either a sidereal drive or extremely fidgety fingers with a Dobson. Furthermore, the design had a prominent out-of-focus ghost image. The ghost was at most an annoyance -- after a few minutes, we all forgot it was there. It would not interfere with observations of planets or double stars -- the drill is simply to set the object in view a hair away from the center of the field. That separates the ghost from the object in view. Yet I don't think this eyepiece would be suitable for lunar work -- all those ghosts superimposed would likely add significant stray light to the image. Still, the field around bright Vega sure looked dark. The usual soft glow that surrounds the image of any bright star or planet, was obviously much diminished from what I am used to.

It was time for some comparison tests. I swung the telescope to epsilon Lyrae, the double-double, which was still bright enough to provide a glow, and which also gave the opportunity to judge contrast in the dark space between each pair of stars. (Seeing was excellent, incidentally, and the clouds that plagued us during other parts of that evening were not then about.) I dug out several eyepieces from my own box, and hollared to other observers to bring other interesting units for test. Besides the 12 mm monocentric, we gathered up a Tele Vue 13 mm Plossl, a 12.4 mm Meade Research-Grade Erfle, a 10 mm Vixen Lanthanum, an 8 mm Brandon (of fairly recent vintage), and a 7 mm Meade Research-Grade orthoscopic.

The monocentric out-darked all the others, showing much better contrast in the narrow black space between each close pair of the double double, and less soft glow surrounding each of the pairs itself. Note that we were *not* attempting to judge the brightness of the background sky -- other things being equal, the higher magnifications should have won easily, just by spreading the light out more. Rather, we evaluated the brightness of the ball of light that more closely surrounds each bright image. We also switched eyepieces back and forth many times, to be sure we were not merely seeing small variations in transparency instead of real variation in eyepiece characteristics.

As I was saying, the monocentric won, and it was no contest. A distinct second was the Meade 7 mm orthoscopic, followed by the 8 mm Brandon, then a tie between the 10 mm Vixen Lanthanum and the 13 mm Tele Vue Plossl, with the Meade 12.4 mm Erfle last. Everybody was quite impressed with the monocentric, and beginning to be a little disappointed with the performance of several standard favorites.

It's not as good a test to compare eyepieces of different focal lengths as ones of the same focal length -- if any of the glow is due to scattering in the atmosphere or off the optics of the rest of the telescope, then short focal-length eyepieces might obtain an edge, by spreading out that scattered light over a wider area. But we had to test what we could find, and note that the monocentric outperformed all the shorter focal-length eyepieces against which we compared it.

(I should say in passing that I have hearsay reports that at different times, Meade sourced its "Research-Grade" series of eyepieces from several different vendors, so that different year's versions of what is nominally the "same model" eyepiece have substantially different performance. The Meade eyepieces used for this test were my own, and were purchased in 1979 or 1980; your mileage may vary.)

Later in the evening, after I had put the Intes away and set up my 98 mm f/6.7 Brandon refractor (yes, 98 mm -- no typo), another observer came by with some more eyepieces for test. In that instrument, we compared the 12 mm monocentric with a 7.5 mm Takahashi, a 12 mm Brandon, and the Meade 12.4 mm Research-Grade Erfle. We used epsilon Lyra once more, and once again the monocentric won handily. The Takahashi was second -- I did not think to pull out the Meade 7 mm orthoscopic again, but I believe that the Takahashi was approximately as good as the Meade 7 mm with respect to darkness of field. The 12 mm Brandon was third, and the 12.4 mm Meade Erfle last.

I had planned to bring my set of Ramsden eyepieces for our eyepiece comparison night, but forgot. My old Ramsdens are coated, and seem to give quite contrasty views, probably because they are assembled from military surplus lenses, and the military had no-nonsense specifications about quality of polish. I will try to remember to check the monocentric against a 12 mm Ramsden in the future. I have checked a Ramsden against a Takahashi, and found them comparable in contrast, so the monocentric will probably beat the Ramsden, but likely not by much.

Many of us also wondered how the monocentric would stack up against Pentax's high-end eyepieces, or against units by Zeiss or Clave'. Alas, none were handy for testing. Perhaps some other time.

I think that the bottom line on the monocentric is, that serious observers of double-stars or planets (but not just the Moon) should try one. Yet monocentrics are NOT general-purpose eyepieces: The narrow apparent field will be an impediment to many uses, and the strong ghost image will bother some (and will scatter light across lunar views). Notwithstanding, the very low scattered light from sources other than the ghost, will result in noticeably improved ability to detect faint companions of double stars, and to see low-contrast planetary detail.

Intes exports monocentrics in 6, 9 and 12 mm focal lengths. Earth and Sky Adventure Products has them at about $100 each; perhaps other vendors carry them as well.

Jay Reynolds Freeman; last updated: 1997 June 2