Comments on Gary Seronik's TMB Monocentric Eyepiece test report Sky & Telescope Aug. 2004 pp98-102
by Chris Lord
Chris Lord's comments on Gary Seronik's TMB Monocentric Eyepiece test report Sky & Telescope Aug. 2004 pp98-102
Gary Seronik's report dams TMB SuperMonos with faint praise. The report is biased against eyepieces with narrow afov in favour of those with anything that is wider, however marginal.
On the first page he poses the 'Aunt Sally' question, "Are these oculars really the last word in resolution and contrast?" As if over-stated claims on Internet use groups &c are held to be synonymous with those of the manufacturer. And why does Seronik presume, "To some extent, these eyepieces are the product of an unstated assumption: If an ocular were made in which every aspect was optimized to deliver the very sharpest, highest-contrast image possible, the view would be noticeably better than with a general purpose eyepiece." ?
I want to take this statement apart, because it is both tendentious and at the same time, in some ways, meaningless. Any planetary eyepiece has to have lenses made of the best, most homogeneous optical glass, polished to a high surface finish (minimum scratch / dig ratio), with almost no internal reflections, ghosting or scatter, or narrow angle scatter. The eye cup must be correctly recessed and chased, so as to accommodate the eye socket, with the pupil at the Ramsden disc, so the field stop is in sharp focus, and no stray light can shine across the line of vision. If Seronik presumes these properties are unstated, I recommend he read my monograph, "The Evolution of the Astronomical Eyepiece", where they are clearly stated.
How is it possible to optimize, "...every aspect of eyepiece design." This statement is proposterous. For example you cannot design an ultra-wide angle eyepiece without introducing distortion, and astigmatism. If you require true orthoscopy, you cannot at the same time have a afov greater than a radian or thereabouts. If you require a fixed eye relief of 20mm across the focal length range of the set, you are obliged to employ a Smyth converter (built in Barlow), at least in the shorter focal lengths.
And what is, ".... a general purpose eyepiece" ? Is it a Ramsden, a Huyghenian, a Plossl, an Orthoscopic, what?
Seronik doesn't even know what the difference is between a Steinheil Monocentric and a Hastings triplet Monocentric (see my critique of his report). In the caption on p99 he states that the 1883 illustration in Konkoly-Thege's 'Practical Guidance for the Employment of Astronomical Observations' to be the Monocentric's original configuration. It isn't, it is a modification made in 1880. The original design was symmetrical. He then erroneously goes on to state that the TMB symmetrical design is based on Steinheil's design. It is not, it is based on Hastings' design, and Hastings' design was based on Paul Rudolph's who took out a patent for Zeiss in 1911. It is not a modification of Steinheil's.
He then proceeds to test them initially with a 8-inch f/4.5 Newtonian fitted with a TeleVue Paracorr coma corrector! Why would anyone wanting to compare image contrast, resolution, and internal reflections and scatter, use any Newtonian, let alone one with such a fast f/ratio. The diffraction limited (1/4 wave) fov is only 196 arcsecs, without the coma corrector. But the coma corrector is itself a source of scatter and internal reflections, astigmatism and field curvature. The mirror coatings also scatter light. And I have yet to come across a fast Newtonian with a low(ish) profile focuser that doesn't suffer to some extent or other from veiling glare. He could have used a 3-inch off axis stop to reduce the effective focal ratio to f/12. That would have been something. But if you are testing for scatter you should not use a telescope with aluminized mirrors. They are in themselves a source of scatter and you cannot simply presume all the eyepiece types in the comparison will be equally affected.
His criticism of their poor off-axis performance is based largely on this stupid choice of telescope. If he had cared to test the Monocentric on a refractor with a focal ratio greater than f/15 he would have discovered that star images remain tack sharp across the fov. The f/ratio limit if f/6, but even on my TEC140APO (f/7) you can see some off axis astigmatism. The answer is to use a long focal length Barlow or a Telecentric. And may I add that any experienced planetary observer familiar with Monocentric eyepieces would have known that.
He also confuses light scatter within an eyepiece with "veiling glare". Veiling glare is caused by light shining directly into the drawtube, not internal reflection or scatter. Veiling glare is a problem with fast Newtonians, especially those with skeleton tubes, and ultra-low profile focusers.
Seronik is surprised to discover he can cope with only 6.8mm eye relief on an 8mm Mono, and that the 32º afov is an advantage in positioning the eye at the Ramsden disc, where the field stop can be seen. Yet he also states that he did not like the limited narrow afov. Why? Why would a planetary observer need a wider afov, given that the planet is always centred first before observing?
Having posed the question as to whether the Monocentric design makes for a better planetary eyepiece than a, "... general purpose eyepiece", he then procedes to compare them with four other superb planetary eyepiece designs, presumably with the expectation of being able to detect some perceptible differences, which unsurprisingly he does not. The Monos may have a slight edge on starlike points, but apart from that there is very little in it. Personally having used all these eyepieces, I would not have expected to find any noticeable difference. However one test I would have done, and indeed have done, is to place a bright star, or planet, just behind the field stop, and see whether light is scattered across the fov. If you are hunting faint satellites of Saturn and place Saturn outside the field, absence of scatter with the planet just behind the field stop is important. This is one of the strengths of the Monocentric.
The criticism that their usefulness is hampered by their narrow afov is absurd. Getting rid of extraneous clutter is vitally important when observing the planets. Having used Tolles, Monocentric, Abbe Orthoscopic, and Clave Plossl and Konig eyepieces, my opinion for what it is worth, is that the narrow afov of the Tolles & Monocentric is no drawback whatsoever. If you have a driven telescope, with good slow motions, so you can accurately centre the planet and keep it centred, the afov is not an issue.
As for off axis image deterioration. There is no problem in optics that cannot be cured by increasing the focal ratio. No experienced planetary observer would use a Monocentric on a fast telescope without a long focal length Barlow, or a Telecentric amplifier, to increase it to at least f/10, and preferably f/15. The mere fact that Seronik proceded to judge the performance of a Monocentric on an f/4.5 Newtonian speaks volumes as far as I'm concerned.
Gary Seronik's TMB Monocentric Eyepieces equipment report (S&T Aug 2004 p98-102) in my opinion could have gone into a bit more detail about the original design of the Monocentric eyepiece. The illustration taken from Konkoly's 'Practical Guidance ...' 1883) depicts the original Monocentric eyepiece.
Steinheil's development of his aplanatic rectilinear camera lens led by 1880 to the Group Aplanat, which had surfaces whose curvature shared a common centre. This novel design of photographic objective underlay the Monocentric eyepiece. The lens consisted of a thick cemented triplet, with an equi-convex barium crown flanked by borate flint negative meniscus elements. Another assymetric version had an even thicker crown and forward flint with a thinner double extra-dense flint eye lens. This later version is that depicted by Konkoly. Eye relief was 0.85Fe, and the apparent field of view 28º. Steinheil Monocentrics may be used down to f/6.
A German patent was taken out on behalf of Zeiss by Paul Rudolph in 1911, registering their triplet design. Zeiss marketted this Monocentric variant in several forms until the mid 1950's.
There are two other designs of cemented triplet, the HASTINGS LOUPE and the assymetric LOUPE TRIPLET, both developed in the 1910's. They are still the most common form of achromatic, wide-angle, hand magnifier.
The TMB Super Monocentric, is not a development of the Steinheil Monocentric as Seronik implies. It is a variant of the Hastings triplet.
Amongst the comparison eyepieces, Seronik chose a Clave Plossl. Although Clave Plossls enjoy a high reputation amongst planetary observers, what few of them appear to realise is Ets Clave appropriated the design from Zeiss after WWII. The true Plossl eyepiece, as manufactured by Carl Zeiss, and after WWII by Ets Clave (now Kinoptique), has the crown elements almost in contact, and the eye doublet has a shorter focal length than the field doublet. This widens the afov to 45º at f/6 at the expense of eye clearance (0.7Fe). In its best form, this design is distortion free, and has no detectable lateral colour, even at f/4. Fields are dark and ghost free, and contrast is excellent. However, unlike the Abbe Orthoskop, and its derivatives, where longitudinal spherical correction is zero on axis, the assymetric form of Plossl leads to a zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of the field of view. At low to medium powers this is of no consequence, but it is noticeable at high powers (exit pupils less than 1.5mm).
Finally, on a personal note. No experienced Lunar & Planetary observer would entertain using an f/4.5 Newtonian; apart from the central obstruction ratio reducing image contrast, and difficulties preventing veiling glare, an 8-inch f/4.5 Newtonian has a very restricted diffraction limited field of view (barely 195arcsecs).