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The Siebert 10mm Monocentric ID

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The Siebert 10mm Monocentric ID

by Bill Paolini – wapaolini@hotmail.com


Thanks to the generosity of a fellow CNer, I had the opportunity to try out one of the Siebert Monocentric ID eyepieces (http://www.Siebertoptics.com/Mono.html).  Siebert advertises his monocentrics as having a wider and flatter useable field of view compared to traditional monocentrics.  They also state that the eye relief is more generous and that they are 100% color corrected across their field of view.

The monocentric design, is a three element one group eyepiece.  Characteristic of this triplet design, unlike the Hastings or Steinheil designs, is that the radius of curvature of each element shares a common center.  So the arcs of each lens surface share the same center point.  For these single group design types, there are no additional air-to-glass interfaces other than the top of the eye lens and the bottom of the field lens as all three elements are cemented into a single group.  Monocentric designs are often considered as being as high transmission as feasible since there are only the two surfaces to lose light from the glass-to-air interfaces.  If a modern 99.75% efficient multi-coating were to be used in this design, the resulting theoretical transmission could be as high as 99.5% for the entire eyepiece!  With a standard single-coat of 98.25% efficiency, this design would yield a lower theoretical 96.5% transmission.  Siebert does not state the type of coatings on this eyepiece, so it is unknown if they are multi-coated or simply single-coated.  The common weakness of a monocentric, is the limited apparent field of view (AFOV), typically being around 30 degrees, and an off axis that exhibits a good amount of field curvature in shorter focal ratio scopes.  This design type tends to work best with focal ratios of f/7 or greater.

The Siebert 10mm Monocentric ID eyepiece is very light, typical of all of Siebert’s eyepieces.  Housing construction is Derlin, and the barrel is aluminum.  Overall the fit and finish is adequate, but rather utilitarian with more of a home-constructed look rather than a highly finished and polished look of typical production eyepieces.  Again, a typical trait of Siebert eyepieces. 

Pictured below are close ups of the eye lens and the interior of the barrel with field lens.  The interior of the barrel is painted generously with flat black paint.  The Derlin housing and gasket serving as a retaining ring appear to be sanded to a rough texture, possibly as an anti-reflective strategy.  Additionally, just above the filter threads, there is a thin strip of adhesive flocking, again probably as an additional stray-light control measure.

The field stop is not a knife edge but rather a broad flat surface blackened with paint to stop any reflectivity.  So a little crude in execution for this component which consequently provided a not so precise or uniform field stop edge while viewing.  Eye Relief was measured at approximately 7.5mm.  Eye relief also felt fairly comfortable, probably due to the placement of the eye lens which protruded was slightly above the housing of the eyepiece.  The AFOV measured in bench testing approximately 35 degrees.  This measure appears accurate as when visually compared to the AFOV of a TMB Supermono, the Siebert Monocentric ID is larger, yet smaller than the 40+ degree AFOV of an ABBE Ortho.

I performed casual testing over several nights on Mars and the Moon using both my XT10 f/4.7” Dob and by TSA-102 f/8 refractor.  I compared it to the performance of the 10mm Astro-Physics Super Planetary (AP-SPL), which is also a three element design but being two groups instead of one like the Siebert, and I also gave a few tests of the Siebert Monocentric ID against the venerable Pentax 10mm XW.

Observing Mars with both the TSA and the XT10, it was necessary to do the testing using two different Barlows as a 10mm eyepiece does not produce sufficient magnification in either scope.  A TeleVue 2x and a Siebert 2.5x Barlow were used. Comparing the performance of the Siebert Monocentric ID to a same focal length AP-SPL and Pentax XW, the AP-SPL showed the most detail, contrast, and least scatter of the three eyepieces.  Using a TeleVue 2x Barlow with the eyepieces to get to the higher planetary magnifications needed for Mars, while the Pentax XW gave a very good showing, it simply could not compete so closely with the Siebert Monocentric ID or the AP-SPL as it showed more scatter from its complex design, washing out some of the finer details visible in the other simpler design less glass eyepieces.  

The Siebert Monocentric ID and AP-SPL did place closely to each other, however the Siebert showed less distinct Martian details and showed slightly more scatter than the AP-SPL.  As example, in both eyepieces the polar cap was revealed very prominently, along with the dark region surrounding it.  However, the Mare Sirenum, Solis Lacus, and Mare Erythraeum surface features were less prominent and more difficult to see in the Siebert.  Solis Lacus in particular was only hinted at with the Siebert Monocentric ID while clearly evident in the AP-SPL, and not visible at all in the Pentax XW.

Moving to the Moon, both the AP-SPL and Siebert appeared to be much closer in performance than they were on Mars.  However, after lengthy detailed observing of highland formations, crater walls, ejecta patterns, and finally faintly lit structures just outside the terminator, it became clear that the AP-SPL was showing a slight edge over the Siebert Monocentric ID.  Compared to the Pentax XW, the Siebert Monocentric ID showed the smallest details just ever so slightly better defined.  The difference however was extremely small and required a lot of effort to tease out this difference.  What was overwhelming when observing the Moon, was the expansive AFOV of the Pentax XW where the view was sharp to the edge, providing a jaw-dropping view compared to the other more narrow field eyepieces.  This in itself, for the Moon, made the Pentax seem more impressive overall, regardless of any detail differences in the view.

Considering the off-axis performance, the Siebert Monocentric ID was performing less well than a TMB Supermonocentric.  In the XT10, regardless if the Paracorr was in place or not, only about the central 1/3 of the FOV or even slightly less was un-aberrated and sharp.  A TMB Monocentric will typically show about half of its FOV sharp in the fast F/4.7 Orion XT10.  The AP-SPL, by comparison, showed about 60% or slightly more of its FOV sharp.  In the TSA’s f/8 the Siebert Monocentric ID improved some with approximately 75% of its FOV being sharp and the AP-SPL showing probably more than 95% sharp.  Overall, I found the usable AFOV in the AP-SPL sufficiently large for productive observations in the undriven XT10. The Siebert Monocentric ID’s usable FOV in the fast XT10 was however just too small and was quite a burden for enjoyable or productive observing at the high magnifications in play (240x when used with the TV 2x Barlow).  So like the TMB Monocentrics, the Siebert Monocentric ID really needs either a long focal ratio scope of f/7 or f/8 and longer, or a scope with a driven mount for observing to be productive and easy.  Without a driven mount and observing with this small AFOV at magnifications of 240x and more, drift time in the small central part of the AFOV that remained sharp was simply too short in duration.   

When moving from the TV 2x to the Siebert 2.5x Barlow on the Moon, things changed radically.  Being called to produce the extra magnification using the 2.5x Barlow, making the eyepieces effectively 4mm eyepieces, the Siebert Monocentric ID was not able to produce near as crisply defined of an image as was the AP-SPL.  I have noted this behavior before in eyepiece tests, where eyepieces that work well with less powerful Barlows, may not work well with higher power Barlows.  In this instance, the Siebert Monocentric ID could not produce as sharp of an image as the AP-SPL under Barlow, even though both were using the same Barlow, same scope, and were producing the same effective focal length.  Moving the scopes back to Mars, this behavior was replicated with most all of the Martian detail being washed away, except for the polar cap, with the Siebert Monocentric ID with a 2.5x Barlow, while the AP-SPL still produced a nicely sharp and detailed image of all the Martian features.

As a side, neither the Siebert or the AP-SPL with Barlow showed their full FOV.  In both, the field stop became less distinct, fuzzy, and showed reduced illumination at the field stop. Most likely the field lens of each was not large enough to catch the entire light cone from the Barlow, so their FOVs did vignette. The Pentax of course had no such problem and showed the full FOV distinctly to the edge with either Barlow.

As a specialty planetary eyepiece, the Siebert Monocentric ID appears to be a *fair* alternative to the much more expensive and harder to find TMB Monocentric or AP-SPL eyepieces. However, its limited central sharp zone, due to off-axis aberrations in the eyepiece generated by fast focal ratio scopes, makes it not recommendable unless one has a driven mount where the planet can be kept directly on-axis. In addition, while the build is adequately sturdy the field stop was somewhat crudely executed and did not provide the knife-edge performance typical of mass production eyepieces. While this may be nit-picking, given the $150 cost, some aspects of the build might not be considered worth the investment to some observers, especially when a quality ABBE can be found for less expense.  From a purely performance perspective though, at its native focal length and using quality Barlows up to 2x, the Siebert Monocentric ID was able to keep fairly close in on-axis performance with the top-shelf AP-SPL, and was able to provide observations showing more Martian details when compared to even the highly respected Pentax XW.  In any case, the Siebert Monocentric ID is definitely an eyepiece needing a driven scope to realize its full potential for planetary viewing.

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