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The Pentax 5mm XO: A Planetary Weapon of Choice

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The Pentax 5mm XO

A Planetary Weapon of Choice

Over the years I have heard nothing but accolades for the Pentax 5mm XO when discussing planetary performance. Unfortunately, due to its comparatively high price and the lack of many focal lengths, I¡¦ve never been able to convince myself to purchase one. After the 6mm Planetary Eyepiece review I conducted, where I compared some truly world-class glass, I thought my quest for the holy grail of planetary eyepieces would be over. As satisfied as I was after experiencing so many top-tier planetary eyepieces, past praises for the untried 5mm XO still haunted me. Finally, through the kind generosity of CN member Ed Kessler who loaned me this eyepiece, I¡¦ve been able to put my nagging questions firmly to rest.

Focal Length:


Lens Structure:

5 Elements / 3 Groups

Apparent Field of View:

44 degrees (measured at 45Ė)

Height & Width:

1.8 x 1.9" (measure confirmed)


4.2 oz

Barrel Diameter:

1.25" (measure confirmed)

Filter Diameter:

28.6mm (measure confirmed)

Field Stop Diameter:


Eye Relief:

3.6mm (measured at 2.5mm)

Eye Lens Diameter:

6.1mm (measure confirmed)

The Pentax 2.5mm and 5mm XO series of eyepieces ("O" for visual Observation) were released by Pentax for the 2003 Mars opposition event. Based on my research of public online archives and marketing releases, Pentax¡¦s optical design goals for the XO series were for the achievement of an orthoscopic planetary eyepiece with maximum transmission, maximum contrast, and maximum suppression of optical aberration for telescopes with focal ratios as short as f/4. Additional goals included ease of use and to be constructed of eco-friendly materials containing no environmentally harmful substances (such as arsenic which can be used to whiten or reduce the green coloration of glass). As a result of these goals the XO series incorporated high-refraction low-dispersion lanthanum glass elements, the Pentax proprietary SMC full-surface multi-layer lens coatings with the laminated optical elements additionally treated with partial coatings for additional improvements, and the use of computer simulations to determine the best placement for internal baffles and blackening to maximize contrast and fully suppress any stray light. As visualized in the illustration below, the 5mm XO is a 5 element / 3 group design (blue are lens elements), which incorporates five different main baffles (red arrows show locations), and incorporates thorough blackening and micro baffles throughout.

For the 5mm XO, Pentax states it achieves a maximum transmission of 98%, which would mean that a 99.6% to 99.7% efficiency would be required for each air-to-glass interface to achieve the overall 98% transmission at the green wavelengths, an amazing achievement for the circa 2003 technology. Based on available data, below are graphs illustrating the 5mm XO¡¦s transmission curve as well as the distortion level of the design. The blue line of the transmission graph shows the percentage of transmission (y-axis) by wavelength of light (x-axis). The green line of the distortion graph depicts rectilinear distortion, commonly referred to as barrel and pincushion distortions. For this graph the solid green line shows the distortion level in percent (x-axis) as a function of the increasing distance off-axis from the center of field (y-axis) -- quite impressive being less than 5% when compared to other designs as can be seen here - http://www.users.bigpond.com/PJIFL/eyepieces_distortion.html).

As good as charts, graphs, and design parameters may be, once an eyepiece enters the complexity of an optical chain, the best laid plans of optical designers can often go astray. So with all the lofty intentions of the Pentax designers I wondered exactly how well this eyepiece would fare once I put it to the ultimate challenge -- not in some sterile bench test, but in the real-world, under real skies that take no prisoners, and in real telescopes with all their flaws and challenges trying to keep up with changing temperatures as an evening¡¦s observation progresses.

For my first evening out I took my Takahashi TSA-102 f/8 APO, pointed it at Jupiter, and loaded my personal standard for performance the 5mm TMB Supermonocentric. After observing for several minutes and noting various surface details, levels of shadings, and colors (primarily of the orbiting moons), I was ready for the 5mm XO. With my first view through the XO all I could think was ¡§Wow! Impressive! Definitely living up to its reputation.¡¨ And as this evening progressed, and the many subsequent evenings using the 5mm XO in the TSA, my smaller 80mm f/6 APM APO, and my Orion XT10 f/4.7 Dob, my overall feeling was that compared to the other eyepieces in my inventory, even the venerable TMB Supermonocentric, this diminutive 5mm XO could definitely hold a place as a planetary weapon of choice! Focus snap, crispness of the details, depth of contrast, and aberration control over its entire field of view were nothing less than spectacular. My judgment of the XO¡¦s performance by target or usage characteristic over the many evenings of use was as follows.

Focus Snap

Finding best focus with the 5mm XO was never a chore, it decisively came to sharpest focus almost immediately with little need to ¡§play¡¨ with the coarse or fine focusing controls. When I placed other eyepieces or eyepiece and Barlow combinations of the same effective focal length for comparison, even against the superbly executed TMB Supermonocentrics, it was immediately apparent that the XO provided a superior ability to snap to focus quickly and effectively.

Crispness of Images

The term ¡§crispness¡¨ is something I have come to use more and more, as opposed to the term sharpness or resolution. The reason for this is that I feel it is a term where it is evident that there are many variables which contribute to an image being perceived as crisp (i.e., resolution, contrast, scatter, minimized distortions, etc.), whereas many times terms like sharpness or resolution can be interpreted more strictly and narrowly which is not always the intent when trying to convey the impression of clarity that an image an eyepiece provides. On its own, with no comparison, the 5mm XO¡¦s image was always pleasingly crisp, giving the impression that I was resolving all the details possible given what the atmospheric seeing would allow.

When comparing its crispness to other eyepieces, it was readily apparent of the superior performance of the 5mm XO, even to my personal standard of the TMB Supermonocentric. The personal surprise for me, was both the degree of difference and even how atmospheric seeing seemed not to level the differences as much as I am accustomed. When judging most premium-level eyepieces, using the very stringent demands of planetary observation where very minute differences can make a difference in the view, it is usual that one eyepiece differs from the other by what is generally termed as a ¡§hair¡¨ or as ¡§splitting of a hair.¡¨ In the case of the 5mm XO I felt the difference was immediate and obvious. At no time did I have to spend tens of minutes or more and many switches back and forth to determine which eyepiece provide the ultimate level of crispness and clarity of image. It was always obvious when switching to the 5mm XO that the image improved, and that the image degraded when moving to other eyepieces like my TV Plossls, Meade 3000 smooth-sided Plossls, Radians, or the TMB Supermoncentrics. And while it is not surprising for an eyepiece to provide such an immediately noticeable difference, what was personally surprising was that on evenings of less than excellent seeing or transparency, when the atmosphere I have found usually levels any differences between eyepieces, the XO still held its ground and provided an overall more crisp and clear image.

Apparent Contrast

Again, instead of simply using the term ¡§contrast¡¨ I prefer to instead use the term ¡§apparent contrast.¡¨ The reason for this is that contrast, and the judging of contrast, can often be quite a difficult thing to do effectively and divorce from other non-contrast related variables, such as transmission and scatter. I therefore use the term apparent contrast as a means of conveying that any comparative judgment may include drivers from sources other than simply levels of contrast, but that again things like transmission, scatter, and other factors may also contribute to the overall perception that darker areas seem to have a level of increased darkness or lighter areas seem to be lighter. For the 5mm XO, this was again an area when compared to other similar effective focal length eyepieces that it was immediately apparent that it was doing a better job. On Jupiter in particular, it was always obvious that the atmospheric bands and structures appeared richer through the XO, gradations of shading in the polar regions showed better as well, and definition in and around the Great Red Spot was either newly visible or more pronounced with the XO. When observing the Moon however, apparent contrast improvement was not so obvious as will be further detailed in the section on Lunar observing. Overall, on Jupiter, the XO performed obviously better in terms of apparent contrast of the image when compared to my other eyepieces.

Off-Axis Performance

Overall the off-axis performance of the 5mm XO was excellent, regardless of the scope I used. In the fast f/4.7 Orion XT10 Dob the 5mm XO showed a very slight amount of field curvature and lateral color in a region far off-axis, perhaps five percent or so from the field stop where it took approx 1/5 turn of fine focus know to correct the field curvature. I also noticed some very minute astigmatism in this region as well but it was only detectable if I racked the image out of focus, in focus the star image appeared non-astigmatic. No rectilinear distortions were evident as I panned star fields. Moving to a slightly slower 80mm f/6.25 APO, astigmatism was no longer present and only the slightest amount of field curvature was seen right at the field stop, taking only a slight touch of the fine focus to correct. In the 102mm f/8 APO stars were sharp to the edge -- as example, Polaris remained a nice airy disk with the companion fully visible with no change in appearance right the to field stop.

Eye Relief

I would characterize the eye relief as tight and at times felt it was difficult to get close enough to see the entire field of view. Also of note is that the measured eye relief was shorter than the manufacturer¡¦s claim (I re-measured several times to verify and each time came up with the same result). Even though the eye relief was definitely tight, as I used the eyepiece more and more, I became more adept as getting close enough to take in the entire field of view. I also found it quite acceptable to back off from the eyepiece and not take in the entire Field of View for more comfort, as it was easy to then simply use oblique viewing to observe objects as they drifted to the very edge of the field stop.

Planets (Jupiter)

Jupiter provided wonderfully crisp and detailed views using the 5mm XO in all my scopes. Fine planetary details were more easily visible and all details were rendered as more obvious and with more apparent contrast when compared to my other eyepieces. I judged scatter as being either similar or very slightly more than in the TMB Supermonocentric which I judge as excellent. Jupiter¡¦s brightness using the XO seemed slightly better than other eyepieces and the tone I would say is fairly neutral, although just a little warmer than the TMB Supermonocentric which is one of the most color-neutral eyepieces I have used. The XO rendered the colors of Jupiter¡¦s moons not quite as richly as the TMB Supermonocentric did, and in particular Ganymede could be seen as a degree more orange in color with the TMB Supermonocentric. Overall the XO¡¦s performance on Jupiter was quite impressive, and I preferred its rendering of the jovian planet over that of the TMB Supermonocentric because more details visible within the NEB and SEB, shadings were more prominent within the GRS, fine structures around the GRS were better defined, and polar shadings were both richer and more extensively defined in the XO than all other eyepieces in my collection.

Lunar (The Moon)

Lunar performance seemed to be more level across the various eyepieces I used; all showing extremely crisp and detailed views of the lunar surface. Focus snap with the XO was easily best, and again the tone it produced seemed slightly warmer when compared to the TMB Supermonocentric. Areas of the lunar surface which were away from the terminator and illuminated more or less flatly and face-on were noted to have better apparent contrast using the XO, being for example more richly rendered in the area around the Apollo 17 landing site which I generally use as one of my standards for assessing contrast levels. Obliquely illuminated areas near the lunar terminator showed details slightly crisper that through my other eyepieces, including the TMB Supermonocentric, examples include the outcroppings in Crater Shiller and details around Promontorium Kelvin. While this improved performance could be detected with careful examination, it was not so obvious as the improved performance the XO showed for Jupiter relative to other eyepieces, instead being what I would call a hair-split better.

Where the XO did perform obviously better than all my other eyepieces, was in the rendering of white ejecta material in and around craters. As example, the white wisps of ejecta within the craters around Aristarcus in Schroters Valley on Luna were both crisper and more detailed using the XO. With my other eyepieces, including the TMB Supermonocentric, the white rays of ejecta within the prominent craters in the valley (white arrow in picture) were observed as a more non-descript lighter and milky-white brighter area. Through the XO however, a fine latticework of detail was clearly evident within the rays of ejecta extending into the crater, showing significant fine structure and many variations of shading.

Stars (Albiero)

Overall the XO rendered star colors as richly as other eyepieces, including the TMB Supermonocentric. However, transmission did seem very slightly less when compared to the TMB Supermonocentric when viewing Albiero. Scatter also seemed very slightly more with the XO than with the TMB Supermonocentric, but this was again a real hair split. Overall I did not prefer view of this colorful double any better through either the Pentax XO or the TMB Supermonocentric.

Nebula (M57)

My initial views of the Ring Nebula were on evenings when the atmospheric transparency was less than optimal and my moderately light polluted observing location was not at its best. Under these conditions, where the background sky was brighter than I liked, the Ring Nebula was rendered with a starker contrast between the Ring and the background sky with the TMB Supermonocentric than with the XO. However, as the weeks progressed and I had many more evenings of darker and more transparent skies, I was no longer able to detect any difference in how the XO performed on the Ring versus the TMB Supermonocentric. Therefore, under darker skies both eyepieces provided excellent rendering of the Ring Nebula and the faint companion star just outside the Ring.

Globular Clusters (M13)

In all three of my telescopes, from the 10¡¨ Dob to he 80mm APO, the 5mm XO provided a better view of the M13 Globular Cluster, increasingly so as the aperture of the instrument was reduced. Basically the level of distinct and separate stars that could be easily seen across the entire cluster was more pronounced using the XO. This advantage was most notable in the smaller 80mm instrument where other eyepieces showed a more or less non-descript cotton ball view of the globular cluster, using the XO individual stars were clearly and prominently evident.

Open Clusters (NGC 869)

My final observations with the 5mm XO were of the Perseus Double Cluster. Similar to the views of M13, the 5mm XO rendered these open clusters a degree better than my other eyepieces. With the XO brighter stars across the clusters popped with more prominence giving the clusters a more three-dimensional character than the other eyepieces could produce. Again, this ability of the XO over the other eyepieces was more pronounced as the aperture of the telescope was reduced, making the XO a valuable asset for my smaller 80mm telescopes where every ounce of performance capability from an eyepiece can be extremely valuable.


Overall I felt the Pentax 5mm XO was an outstanding world-class performer -- and interestingly the only eyepiece I have come across where I can say that its non- planetary purist more than four element design had absolutely no visible degradation to the planetary view to my eye. While not a large Apparent Field of View at 44 degrees, it did not feel restrictive like a typical 40-42 degree ABBE Orthoscopic or 30 degree Monocentric. As a result, I found it very pleasurable to use as a non-eyeglass wearer and I found myself reaching for it more and more to the exclusion of my other eyepieces of similar focal length. As reported, contrast, transmission, scatter, and sharpness were impressive to say the least. As a planetary eyepiece I felt the Pentax 5mm XO is squarely in the class of the venerable Zeiss ABBE Orthoscopics and could easily stand as the prized part of an eyepiece arsenal for the lunar and planetary enthusiast. Very much recommended.


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