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Review of AT Paradigm 8mm and Pentax XF 8.5 mm
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Impressions of the 8mm Astro-Tech Paradigm and 8.5mm Pentax XF, with comments on the 12mm eyepieces in both classes.
Astro-Tech 8mm Paradigm on left, Pentax 8.5 XF on right.
I’ve been observing for 12 years, with equal interest in deepsky and planetary targets. I now have a nice case of all-purpose widefields: Naglers 31, 20, and 16, and Pentax XW 10, 7, 5, and 3.5. (The collection will be complete with the acquisition of the soon-to-be-released Delos 14mm, at which point the 16 may hit the shelf.) Many observers augment such a collection with a case of “planetary” eyepieces, such as orthoscopics. My home-built dobs are undriven, however, and everytime I have bought a short-focal-length ortho or plossl, I have sold it, in part because the field is too narrow to allow me to concentrate on the target between dob shoves. More importantly, a relaxed eye sees more detail, and the minuscule eye relief of the typical short plossl or ortho makes such relaxation impossible, at least for me. Perhaps if I had a driven scope that would change. As it is, I find the XWs to be splendid planetary eyepieces, as well as unmatched for perusing the deep sky.
But the case full of premium widefields is heavy. I have often thought about a lightweight case of general-purpose eyepieces, sharp enough to compete with a good ortho, yet having sufficient eye relief to be comfortable, and having a somewhat more expansive field: let us say, 60 degrees. The Radians were intended to fill this niche, but they were heavy—twice as heavy as the eyepieces under review; and while sharp, they had a fussy exit pupil, notable lateral color, and the dreaded insta-just. And they were expensive. Hence, coupled to my longing for a lightweight case of 60-degree planetaries has been the lure of China.
What are the options? The TMB planetaries, while quite comfortable and decently wide, are, sadly, a little mushy and lacking in contrast. From Explore Scientific, only the 82-degree series offers the small sizes and shorter focal lengths I desire. But these appear to be knock-offs of the Type 6 Nagler, a design which I have all but abandoned. (I still have the 13mm.) More to the point, my only direct experience of ES was with the 30mm 82-degree eyepiece; it was nice, but not the equal of the 31 Nagler that replaced it. (And yes it was cheaper, but it wasn’t cheap.) So, up until now, my experience of Chinese glass, TMB planetaries included, has amounted to a shrug of the shoulders: not bad, but not good enough.
Hence I found the ad copy for the Paradigms intriguing. Is its design indeed “truly totally new...unique to the Astro-Tech Paradigm eyepieces...available for the first time in the United States?” Well, no. It appears to be a clone of the Pentax XF! (Thanks to Mike Marsden, aka MikePDX, for bringing this to my attention.) The use of ED glass might be new—the matter is unclear, judging from Amalie van Allmen’s XF review—but the Paradigm and XF lens designs of 6 elements in 4 groups appear to be nearly identical. Compare the XF schematic in Amalie’s review to the Paradigm schematic offered by the vendor, Astronomics. There is a two-element barlow at the bottom of the chrome barrel, and a 1-2-1 arrangement in the main eyepiece body. Furthermore, the Paradigm seems to be the same eyepiece as the Celestron X-Cel LX and the Meade HD-60. It is, however, the cheapest of the three at $60.
As it happened, before I stumbled on the Paradigm ads, I had already decided to try the Pentax XFs. “Made in Japan!” Yay! Pentax! Yay! Prompted by reports of the imminent demise of Pentax eyepieces, and encouraged by Amalie van Allmen’s glowing review of the 12XF (and ignoring her warnings of field curvature), I bought them both, hoping to fill a perceived gap between my 7 and 10 XWs, on the one hand, and the temporary gap between the 16T5 and 10XW, on the other. They seemed like a bargain—could they be 60-degree versions of the XW? Alas, no: the 12XF lasted one night. I cannot stand field curvature, and this one has lots in an f/5 reflector. Curiously, the clone 12mm Paradigm does not—it’s quite nicely flat! So apparently its makers have tweaked the design. Certainly there are other noticeable differences between the prototype and the imitation, particularly at the 8mm focal length.
The Pentax XF is a nice eyepiece to look at as well as to look through—it has a handsome finish, with its brushed nickel barrel and trim lines. But the Paradigm is no slouch in appearance either, with the possible exception of a cheap looking baffle ring in the 8 mm length. (I was also slightly dismayed by a sizable nick in its chrome barrel.) The dimensions are similar, with the Paradigm slightly heavier, shorter, and fatter:
5.4 oz (153 gm)
6.3oz (177 gm)
The difference in width is a bulkier eyecup housing on the Paradigm; the difference in height is a much deeper recess of the field lens on the Pentax. When the rubber exterior is removed from the eyecup, the Pentax shows a rather thin plastic twist-up ring that does not inspire confidence for the long haul; it has four click-stop positions. The Paradigm has a sturdy helical screw arrangement with a wide thread, so one can adjust the eyecup quickly as desired.
The threads of the Pentax are evenly blackened, and the field lens housing nicely machined. In contrast, the 8mm Paradigm’s baffle ring is a rather crude flat plastic disk, and the thread blacking on mine is mostly rubbed away or just not there—I found this curious, since the 12 paradigm is well blackened, and has a nicely executed field lens housing. (All eyepieces were bought new.) In use, both the XF and the Paradigm have sharp, clear field stops, and the lack of blacking on the Paradigm’s threads is irrelevant, since any hypothetical reflections from that area are effectively blocked by the field lens baffle.
The somewhat crude baffle on the Paradigm (left), and the nicely finished XF (right).
Eye relief is very comfortable on both. Without glasses on, I needed to pull out the eyecup on the XF to its maximal height; during the day, this was almost, but not quite, sufficient—if one more stop had been available, I probably would have used it. Likewise, I used the Paradigm with its cup screwed out perhaps 2/3 of its travel at night, and nearly all the way during the day. With glasses on, I could just see the field stop in both eyepieces with their cups at minimal height—but this will vary from observer to observer.
Apparent field of view. Switching between the two eyepieces, the field in the Paradigm appeared constricted compared to that of the Pentax, out of proportion to the magnification difference. So I measured afov in the two eyepieces as well as in the 12mm Paradigm. Here are the results:
Eyepieces were placed on a level surface about 3 feet from a clear white wall; a focused flashlight was shown through their field lenses, projecting an image of the field stop onto the wall through the eyelens. Between the eyelens and the wall the location of the focused image of the flashlight was found by holding a card vertically and determining best focus. The diameter of the image of the illuminated fieldstop was measured on the wall. The afov is obtained by dividing the radius of the projected image by the distance from focus to center of the image, taking the arctangent, and multiplying by two (thanks to D. Knisely for articulating this method clearly in an old thread).
In this, I think my measurements were accurate to within 1/4 inch, and perhaps to 1/8 inch. Coumpounding errors of 1/4 inch in both measurements would result in an error approximately 0.6 degrees, while errors of 1/8 inch would cut that in half, since arctan is linear in small angles. Furthermore, repeated measurements yielded a spread of only 0.2 degrees, but there might have been some systematic error. At any rate, the 8mm Paradigm’s afov is not 60 degrees: it’s less than 57 degrees, which was something of a disappointment.
Coatings. Besides its field curvature, the XF departs most notably from the XW in its coatings. I found the coatings on the Paradigm to be superior to those of the XF, and this difference declared itself in observations of galaxies and of Jupiter. When placed on a table near a light source, the XF shows many bright internal reflections; the reference 7mm XW is extremely muted by comparison, and the Paradigm is not far behind the XW. Likewise, in ambient daylight, the Paradigm shows far less reflection than the XF.
Left, internal reflections under a light source: XW on top, Paradigm lower left, XF on lower right. Right, reflections in ambient light (“sidewalk test”): Paradigm above, XF below.
Observations were made over four nights in my 10-inch f/5 reflector (Zambuto primary), both with and without Paracorr; daytime observations were made in my f/7.5 Orion 80mm ED refractor. A few observations (as noted) were made in my 18-inch f/4.2 reflector, using Paracorr exclusively, and in my 8-inch f/6, which has an exquisite Zambuto primary. All observations were made at home in Petaluma, California, under variable seeing and with transparency varying from roughly mag 4.5 to 5+. So this is by no means a critical review, but a collection of impressions briefly gathered.
Neither eyepiece is particularly forgiving with regard to head and eye position. Nevertheless, both eyepieces are quite comfortable at night, as their eyecups do a good job of guiding the eye to the correct position. I had no blackouts at night, even on the Moon, but daytime was another story. The XF was designed for spotting scopes, and it is much easier and more enjoyable to look through during the day. The pupil can be a little hard to hold, but there is no kidney-beaning. The eyecup of the XF does not come out as far as I would like for daytime viewing, but still provides a sufficient guide to my eyebrow in order to hold the head in correct position. The XF showed sunspots with nice concentration and contrast. Colors are bright and saturated. In fact, both eyepieces showed lovely colors in birds and intricate feather detail.
I found the 8mm Paradigm difficult to use by day. Besides having a difficult pupil to hold, any tilting of the head or wandering of the eye produces kidney beans (again, this is not a problem at night). On the Sun in white light, and depending on head and eye position, it showed a bluish tone in the 10-degree-wide annulus at the edge, tending to purple at the very edge. This aberration seemed connected to the exit pupil—that is, if I looked through the eyepiece “just right,” I could make this color go away, retaining sharp images of sunspots near the edge, but accomplishing this was difficult. Hence I cannot recommend it for solar viewing. I found, however, that if I screwed out the eyecup nearly all the way and then held my eyesocket firmly against it, I could hold the correct eye position and avoid blackouts—but that’s a tough sell. The 12mm Paradigm seemed friendlier during the day than the 8mm, and provided very nice views of the Sun in white light.
Pincushion is evident in both eyepieces during the day; straight lines bend significantly near the edge of the eyepiece; the effect is marginally worse in the Paradigm. However, this distortion does not bother me at night—in general, pincushion only bothers me in long focal length eyepieces when I pan rapidly.
Controlling scatter. The galaxy NGC 404 sits 7 arcminutes NNW of Beta-Andromedae (Mirach). Scatter from the bright star was well controlled in both eyepieces, but the galaxy had more pop, or luster, in the 8mm Paradigm. Also, two faint stars within 3 arcminutes of Mirach were more easily seen in the Paradigm. As for scatter from outside the field stop, both eyepieces did fairly well; in the 18-inch, Vega showed its diffraction spikes (in both eyepieces) when beyond the field stop by about 10 degrees of apparent field, much as it does in the XWs, but this was not overly obnoxious: light from the out-of-field star did not flood the field, as it sometimes does in inferior eyepieces.
The waning crescent Moon (phase 32%) was observed in the early morning of 10 Sept. The views in both eyepieces were beautiful, with dark shadows, dark sky background, and pinpoint details. No color was seen; the unlit portion of the Moon stood out well in both eyepieces, and the lit portion, when outside the eyepiece, did not protrude into the view. The tone of the two eyepieces was similar: neutral to slightly cool. I saw no obvious lateral color. This is not an aberration that tends to bother me a great deal, but I do not look at the Moon in my 16mm Type 5, for example, due to its ugly purple ring at the edge.
Jupiter was viewed around 4:30-5:30 PDT on 10 Sept, with the GRS near the meridian. The planet was well up, the seeing quite good, and the 10-inch scope thoroughly cooled and collimated. Scatter from the planet was well controlled in both eyepieces: the sky was nicely black around the planet—particularly in the Paradigm—and the Galilean satellites were all fine little disks against this black background. I had my best views of the night in the 8mm Paradigm, which, with the Paracorr, hit the sweet spot at 180X; the 7XW, at 210X, was a bit much, washing out color concentration in the belts. Very intricate detail was seen in the belts, the GRS was nicely concentrated, and the turmoil in its wake finely rendered; the belt colors had excellent concentration. Jupiter is most surprising this year—the SEB is concentrated, knotted, and thin, much like the NEB of previous years, while the NEB is bifurcated, paler, and exceptionally turbulent. There is also a significant equatorial band. This was nicely on display in the Paradigm, and I was quite pleased, it being my first good view of the 2012 apparition.
At f/5, without the Paracorr, the image of Jupiter in the Paradigm started to fall apart about 50% out from center—perhaps the XF did slightly better than the Paradigm when the Paracorr was not in place. But, with the Paracorr, the views in the Paradigm were stunning to the edge: I held the GRS beautifully defined, along with much finer detail, right up to the edge of the field—really, I couldn’t ask for a much nicer view. There was only one minor gripe—at times, and I’m not sure why, the planet appeared color-fringed, a little bit like Venus does due to atmospheric refraction, bluish on one side and reddish on the other. Perhaps this was seeing-dependent, or perhaps it had to do with the exit pupil, as with the ring of color I saw on the Sun. It only occured once or twice, and briefly, during the hour I looked at the planet. The rest of the time there was no sign of spurious color, save for a tiny fringe at the very edge of field, much as the XW puts up.
Views of Jupiter in the XF were almost as good, but spoiled somewhat by the appearance of a fairly strong ghost image opposite the planet with respect to the center of the field—this was a true ghost, not an eyeball reflection. No ghost was seen in the Paradigm 8mm. However, a faint ghost was visible in the Paradigm 12mm—but not nearly as bright as the ghost in the XF. Ghosts are due to internal reflections, and I assume the ghost in the XF was related to the eyepiece’s internal coatings.
I viewed NGC 891 on Sept 9 under less than ideal transparency—a damp sky, sub-mag 5. I found the galaxy without much difficulty using the 12 mm Paradigm—a very subtle haze, not at all like the beautiful edge-on-plus-dust-lane seen in larger apertures at dark sites. Overall, it seemed in these conditions hardly to be worth observing. But moving to the 8mm, the galaxy acquired some real definition, the outline of the bulge making itself felt, as well as a concentrated glow in the core. By contrast, the galaxy remained rather washed out in the 8.5 XF. Similarly, NGC 7331 seemed much more alive in the Paradigm. The difference in view was out of proportion to the higher magnification of the 8mm: that would be 150X versus 159X in my 10-inch, a rather small difference, corresponding to pupils of 1.7 and 1.6 mm, respectively. I think the difference in coatings was in play again here.
Likewise, in the 10-inch, M15 seemed more interesting in the Paradigm. I can’t say that more stars were seen, but there was a better separation of the globular from the somewhat gray, light-polluted sky background—the core glowed with more luster in the Paradigm, and this made the cluster more present, so to speak. Nevertheless, the reader should take this with a large grain of salt—the underdog factor might have been kicking in at this point, and more importantly, the seeing was sub-par on this evening. A few nights later I viewed M15 again in my 8-inch f/6. The seeing was much better and it was darker out, and this time, both eyepieces showed the globular beautifully, as a sprinkling of many tiny pinpricks within a soft glow brightening to an intense small core.
At f/6, the edge correction is very good in both eyepieces—I saw no sign of astigmatism—and the field is quite flat. I was able to hold resolution of M15 right up to the fieldstop. Panning the Milky Way, I kept marvelling how beautiful the star fields were in the 8-inch f/6, which I had not used in a while—the proverbial “pinpoint diamonds on black velvet” thing. The colors of stars were striking, particularly in the 12mm Paradigm. I found the color rendition in both of the Paradigms and in the Pentax to be excellent, and star colors very saturated. I viewed a variety of colorful doubles in the 8-inch f/6: Eta Cass., Gamma And., Albireo, etc. Beautiful golden-orange and aquamarine tints, such as one only sees in such doubles, showed to equal advantage in both eyepieces. The carbon star Mu-Cephei was an intense reddish orange in the 12mm Paradigm.
At f/5, the Paradigm is flatter than the XF. As I mentioned, I can’t stand field curvature—it causes me to refocus an eyepiece endlessly. But while I found this intolerable in the 12mm XF, curvature in the 8mm XF appeared much more mild. It is there, and this caused me to refocus many times until I got used to the eyepiece, but it’s not a disaster. Also, it cleaned up nicely when the Paracorr brought the focal ratio up to f/5.75. (Perhaps the reduction of coma also helps, but I don’t know why.) Meanwhile, the Paradigms (both 8 and 12 mm) are really quite flat; as far as I can tell, the softness the 8mm showed at the edges at f/5 (for example, on Jupiter) was primarily due to the coma in the scope, and it also cleaned up nicely in the Paracorr.
I looked at a variety of open clusters in my 10-inch, including the Double Cluster (NGC 869,884), the ET cluster (NGC 457), and little NGC 6910, near Gamma-Cygni (Sadr). I was looking for overall quality of the view, and hoping to judge the eyepieces with respect to separation of doubles and ability to reveal faint stars. Seeing was not at its best, which made the latter two issues problematic. At times, it would appear as if the 8.5 XF showed more pinpoint-like stars, separating the close pairs more clearly, but then the view would sharpen up equally well in the Paradigm. I spent a lot of time going back and forth to avoid snap judgments, looking pretty hard at the fainter stars. At no time (on these targets) did I see a star in one eyepiece and not in the other. Overall, the Pentax seemed to give a more elegant, pinpointy view of the clusters, while the Paradigm seemed to have more pop/contrast (perhaps due to a darker sky background), if at times a less refined quality. It is hard to say, given the variability of seeing that night. I had a similar experience looking at NGC 869 on a later night in the 8-inch f/6; slightly tighter spot sizes, but curiously, less intensity in the XF compared to the Paradigm. But this was subtle and seeing-dependent; the cluster was flat-out beautiful in both eyepieces, with tiny faint stars popping in and out amidst their brighter neighbors. Bottom line, neither eyepiece had a clear advantage.
This ambivalence regarding stellar presentation/sharpness continued in the 18-inch f/4.2. I was using the Paracorr, bringing up the focal ratio to f/4.8. At first, I thought neither XF nor Paradigm could handle the light cone. I could not get M15 to stay sharp in either eyepiece when 50% out from the center. Similarly, Vega looked absolutely terrible 2/3 to the edge, a long blue-tipped ellipse in the XF and a horrible rainbow fan in the Paradigm. But both of these effects were due to the seeing and/or thermal conditions. When the seeing steadied, Vega stayed tight and round nearly to the edge in the Paradigm, at which point it bloated somewhat and showed a little purple on the side facing center, much as it does in the 7mm XW. In the XF, Vega was also quite tight but a bit elongated radially near the edge of field, lengthening to a definite purple-tinged spike at the very edge, similar to that shown by my 20T5 Nagler. Similarly, under steady seeing, M15 showed excellent resolution very close to the edge of field in the Paradigm—far closer to the edge than one would normally let it drift without recentering.
I viewed NGC 7331 also in the 18-inch. Both eyepieces showed the three brighter companions nicely, although the XF was beginning to fog and I had to be patient, allowing it to clear repeatedly. A very faint field star on the southern tip of the galaxy winked on and off in the Paradigm, while remaining pretty much invisible in the XF. However, this may have had to do with power or seeing variations; the 7XW also showed the star well, if intermittently.
In use in damp conditions, I found the XF to fog more readily when its eyecup was extended; I had to lower the eyecup and keep my face away from it to allow airflow. The Paradigms were much less likely to fog up. I’m at a loss to explain this, since the opening of the eyecup is the same size in both, about 1 inch; however, the twist-up cup in the XF is lighter and thinner that of the Paradigm.
The 8.5mm XF is a great spotting scope eyepiece. It is comfortable to look through and enjoys good edge correction down to f/5, although it is not completely flat. It produces very nice views, but it is not a 60-degree version of the XW. Its coatings are not optimized, as far as I can tell, for astronomical use, as evidenced by internal reflections and the ghost images of Jupiter. The 8mm Paradigm has a more restricted field of view and performs poorly in the daytime, but it holds up well against the XF at night. The Paradigm was a clear winner in my one early morning with Jupiter. I also generally preferred its presentation of DSOs. It has a flat, reasonably generous 57-degree field, very nice coatings, comfortable eye relief, and an ergonomically friendly design. It puts up beautiful views of clusters, galaxies, the Moon and Jupiter. It controls scatter well and produces brilliant stellar colors. As for the 12mm Paradigm, I found absolutely nothing to fault in it, save a very faint ghost of Jupiter. If anything, it’s superior to the 8mm, and I could happily spend an evening with nothing but it and my 8-inch reflector. The finish on the 8mm Paradigm is rougher than that of the Pentax XF, but so what? Anything that can provide me with a top-notch view of Jupiter, that very subtle target, is worth owning. I sold my Pentax XF and bought two more Paradigms: the 18 and 5 mm. Perhaps next I will compare the 5mm to my Tak 5mm LE.