VERNONSCOPE BINO-VIEWER OUTFIT
To compliment my TEC140APO and enhance my visual H-alpha work I decided to invest in a high quality bino-viewer. I had previously used both Denkmier I & II and Baader bino-viewers at several star parties. The pseudo-3D views they provided were impressive, but their cost was an issue for me.
At AstroFest 2003 I purchased an economical bino-viewer from Astro-Engineering. It is the same model as the Seben bino-viewer. However its optical path distance (opd) was only 25mm less than the 170mm back focus on the TEC140APO. It did not leave adequate focus travel when using a zenith prism necessitating the use of a 1.25-inch Meade x2 Barlow.
For use with the Seben bino-viewer I also purchased a pair of TeleVue 8-24mm zoom eyepieces. Unfortunately when the zoom rings were rotated, the focussing sleeves on the bino-viewer also rotated, defocusing the image, which was irritating.
What I needed was a more compact bino-viewer with bigger prisms, and because my eyes have very nearly the same focal length, I did not require individual eyepiece focussing. I also decided to invest in a matched set of parfocal eyepieces.
My eyepiece collection already included Zeiss & U-O Abbe Ortho's & Tak Hi-LE Ortho's, and based on comparison trials made by Rodger W. Gordon, it seemed worthwhile considering Vernonscope Brandon Orthoscopics.
The Brandon Orthoscopic is a reversed assymetric Abbe duplet, designed by Chester Brandon, an American optical and instrument designer, in 1942. Brandon's design is the reverse of that of a duplet Orthoscopic eyepiece designed by Albert Konig, on behalf of Carl Zeiss, Jena, in late 1937, filed in Germany 28JAN1938 & US Patent 2,217,281 filed 18JAN1939. Brandon's design comprises a doublet field lens with an almost flat first surface negative meniscus flint in contact with a bi-convex crown and an eye lens with an almost equi-convex crown, nearly touching the second surface of the field lens, and a negative meniscus flint in contact , shallower convex side facing the eye.
Manufactured by Vernonscope, it remains the sole American made eyepiece. All air-glass surfaces are MgFl2 a/r coated, the scratch-dig ratio is to mil-spec, and the purity of the optical glass is higher than in most commercial eyepieces.
In May 2005 I took a look on Vernonscope's website and noticed they had just introduced their own US made high quality bino-viewer.
I approached the Vernonscope proprietor, Don Yeir and ordered a matched set of Brandon Ortho's, 32, 24, 16, 12, & 8mm, a x1.4 Dakin Barlow, a 48mm 2-inch Brandon Ortho, a 2-inch 45º Amici diagonal and a Vernonscope bino-viewer in a signed custom pine case. Don had not received such an order before and I negotiated an attractive and competitive price. The order was confirmed at the end of June 2005 and fulfilled mid-October '05.
Before describing my observations with this outfit I think it appropriate to explain the three crucial properties of an eyepiece that so often mar even quite expensive designs, and which influenced my choice of Brandon Orthoscopics. These are ghost images, internal reflections and scatter, and distortion.
There are two common causes of ghost images. Internal reflections that come to a focus at or near the eyes anterior focal plane, and a counter-reflection between the Cornea and the last surface facing the eye. If an air-glass surface facing the eye is concave, this situation may arise.
The number of possible ghosts is proportional to the number of air-glass surfaces, given by the expression where is the number of air-glass surfaces. (I refer to this in my monograph, Evolution of the Astronomical Eyepiece available as a free download off my websites publications page).
The Brandon Orthoscopic has four air-glass surfaces and hence six possible ghosts. There are in practice none because the design has only two air-glass surfaces concave to the eye. The first is the first surface of the field doublet and it is so shallow that its image surface lies far behind the eye. The second is the first surface of the eye doublet, again with an image surface behind the eye. The last surface is slightly convex and forms a virtual image well in front of the entrance pupil. So although there are six possible ghosts with a 2-2 design, the clever choice of surfaces elliminates three and avoids the visibility of the others.
Another significant advantage of the Brandon design is that a high throughput is achieved with a standard MgFl2 anti-reflection coating. Multiple layer a/r coatings widen the spectral bandwidth over which total, or almost total destructive inteference of surface reflections occurs within the film layers. The reason MgFl2 a/r coated lenses look deep purple is because the coating thickness is a in the yellow-green @ 550nm. Total destructive interference of surface reflections within the film occurs only at that wavelength, and partially at other wavelengths. The greater the difference in wavelength from the central wavelength, the greater the reflection loss because of incomplete destructive interference. The light reflected back is a combination of violet and deep red, hence the deep purple tint. Anti-reflection coatings have no effect on the colour of the transmitted light.
On the face of it a multiple layer a/r coating is superior to a single MgFl2 coating because reflection losses are lower across almost the entire visual spectrum. However there is a pay back. Each layer in the stack has a thickness that causes total destructive interference for a specific wavelength. Surface reflections not of that specific wavelength escape into adjacent layers. Some do get totally destroyed, but not all, because the wavelengths are infinitely variable across the visual spectrum, and the multiple layer coating can have only a certain number of layers, perhaps 9, maybe more, usually less. Light that is not destroyed by destructive interference is scattered back and forth within the stack before emerging and entering the eye. The combined effect across the visual spectrum is termed narrow angle scatter.
Narrow angle scatter is inconsequential at low power on either faint point sources or faint extended and diffuse objects. It is a nuisance on bright point sources and bright extended objects at contrast boundaries, where an otherwise sharp boundary is broadened slightly.
Eyepiece designs with more than six elements would have unacceptable reflection losses, even with MgFl2 a/r coatings. These designs are only feasible because of multi-coating technology, they depend on it for the design to be practical. The Brandon Orthoscopic, with its four air-glass surfaces and MgFl2 coatings on those four air-glass surfaces, has in fact a higher throughput than a multi-coated seven or eight element ultra-wide angle design. There is also no narrow angle scatter.
All that is needed in addition to elliminate all internal reflections and scattered light within the field of view (fov) is blackened lens edges, a chased and effectively blackened internal barrel wall, a sharp and blackened field stop with no bright filter thread before it, and a correctly profiled, chased and recessed eye-cup into which the eye socket can nestle, so elliminating stray light shining obliquely across the field of vision.
None of these requirements are difficult or costly to achieve, and yet many commercial astronomical eyepiece designs fall down in one or more of these areas.
The one remaining source of scattered light is imperfections in surface polish and purity and homogeneity of the optical glasses used. If you were to examine the surface and the interior of a lens in a Brandon Orthoscopic, using a low power microscope and a bright white light at the condenser stage, you would be struck by the almost complete absence of pits, sleeks, straie and inclusions.
Finally to be truly orthoscopic an eyepiece must not only be aplanatic (free of spherical aberration and coma) but also field distortion. It is not possible to correct both rectilinear and angular magnification distortion at the same field radius because the former is proportional to the field radius in radians and the latter to the tangent of the angular field radius, also expressed in radians. It is also not possible to simultaneously correct coma and distortion at the same field radius. When the apparent field of view (afov) exceeds a radian (roughly 57º) distortion becomes a problem, and it grows alarmingly as the afov approaches 90º. That is why the Abbe Orthoscopic is restricted to an afov 40º or so, and the Brandon to an afov 45º.
My first use of the outfit was on the Sun with my Solar Spectrum SO1.5 Å0.3 H-alpha filter and Baader x2 Telecentric amplifier with the ERF stopped down to 2-inches, effective focal ratio f/28. I used the 32mm Brandon Ortho's @ x60.
The field stop of these eyepieces is 28mm, so the limiting aperture in the system was the SO1.5 filter which is 25mm. The real field of view was 2200 arcsecs, sufficient to accommodate the Sun's disc and space all around the limb wide enough for Prominences.
The view of the Sun in H-alpha with a Å0.3 passband filter using a single eyepiece, already showed the Sun as a globe. However in the bino-viewer the pseudo-3D image was even more apparent, and it was easier to descry subtle low contrast Chromospheric detail.
One of the bug-a-boo's of H-alpha observing is ghosting, internal reflections and stray light. When I first started observing the Sun in H-alpha using a T-O Å0.9 tilt tune filter, most of my medium and low power 1.25-inch push fit eyepieces exhibited either off axis or axial ghosting.
I tried a Zeiss 2-1 aspheric 25mm Ortho, a Zeiss 25mm Konig I, a U-O 24mm Konig II and a Meade 2-1-2 28mm Super Plossl (Erfle II). None were completely satisfactory, they all ghosted to some extent or other, although the Meade 28mm Super Plossl gave the highest image contrast and least bothersome ghosts.
The worst ghosting was exhibited by the Konig's, due to a counter-reflection off the Cornea and the eye lens, being focused near the eye's anterior focal plane. It produces a maddening "gad fly" that dances around the fov.
I invested in a set of Coronado CeMax Plossls and a x2 Barlow, designed for use with an H-alpha filter. The 25mm CeMax Plossl worked best of all. No ghosting and only minimal internal reflections off the chased internal wall of the barrel.
Yet, the 32mm Brandon Ortho's were noticeably better than the 25mm CeMax Plossl. There were no internal reflections, no scattered light and no axial ghosts. The only off axis ghosting occurred when the Sun was placed just behind the field stop, outside the fov, so was never an issue. Image contrast was also best with the Brandon. The outfield surrounding the Sun's disc was black, with no visible scattered light, except when Cirrus cloud drifted across.
The next test was on the evening of November 9th. I observed Mars in Seeing IV (an essentially featureless disc with a vague hint of shadings) and Alpheratz (alpha-And) and its surrounding star field, in Seeing III. The sky was clear in the SE but the first quarter Moon hung low in the South behind a horizon hugging bank of Cirrus, making the entire sky milky.
I began with the bino-viewer used straight thru' with the 32mm Ortho's (x30). No ghosting, no distortion, no field curvature, no image deterioration at the out-field whatsoever; (afov 45º), no internal reflections and no scattered light. The only internal reflection was seen when Alpheratz was placed just behind the field stop. It reached the field centre and there was no counter reflection. The star field surrounding Alpheratz was tack sharp right across the fov. As the field was allowed to drift the relative star positions remained fixed, evidence of true orthoscopy.
I then added the APM zenith prism & TeleVue x2 Big Barlow (x60). Eyepiece performance was identical to the straight thru' view.
Next was straight thru' with my TeleVue x4 PowerMate (x120). Still no internal reflections or ghosts, including the "gad fly".
I also used the 48mm Brandon straight thru' (x20) then with the APM zenith prism & then the Vernonscope 45º Amici prism. This eyepiece is amazing. There was no astigmatism or lateral colour in the outfield. All the stars in the fov (approx. 2º.25) were tack sharp. No ghosts, scattered light or internal reflections even when Alpheratz was placed behind the field stop. A slight diffraction spike could be seen thru' Alpheratz with the Amici. The roof ridge is exceedingly fine and delicate, very awkward to see in daylight.
Next I fitted my TEC 5-port turret & the five focal lengths in the set, 32, 24, 16, 12 & 8mm. With & without the x2 Big Barlow. Powers would be approximately x30, x40, x60, x80, x120 & x60, x80, x120, x160 & x240. I could see a clean Airy disc @ x240.
The opd of the APM zenith prism is 55mm and of the Vernonscope bino-viewer 110mm (compared to 145mm for the Seben) which leaves only 5mm focus travel. This is insufficient for any of the Brandon Ortho's. The TeleVue x2 Big Barlow increases back focus an additional 65mm.
The Dakin x1.4 Barlow increases back focus only an additional 8mm used straight thru'. With the bino-viewer the Dakin Barlow amplification increases to roughly x2.
The Vernonscope bino-viewer does not have separate eyepiece focussing. If your eyes have the same focal length (not necessarily focus) this is not a problem. My left eye has a focal length (anterior) 0.25mm shorter than my right eye. Because Brandon Ortho's are parfocalised what you need to do is focus with your longer focal length eye and adjust the other eyepiece by backing it out of its collet until the image is in focus. A spacer ring needs to be machined to suit the gap between the barrel's shoulder & the collet. This spacer ring will suit all the focal lengths in the set.
If you have astigmatism exceeding 2 Dioptre you may find that whereas this arrangement works satisfactorily with the 8mm & 12mm focal lengths it doesn't with the 24mm & 32mm focal lengths, unless your eyes are equally astigmatic with similar sagittal and tangential focii.
Based on my preliminary trials I would rate Brandon Orthoscopics equal to my U-O & Zeiss Abbe Ortho's in every respect. The eye recess & eye-cup design is spot on for me. I always remove my spectacles, and I prefer the eye cup to nestle in my eye socket, eliminating any stray light from within the dome entering my field of vision. However if you need to observe with your spectacles on, the rubber eye-cups can be turned down.
The TEC140APO is a very high Strehl ratio visual refractor. If you want to minimize loss of image contrast on the Sun, Moon & planets, and are considering a high quality bino-viewer, I cannot recommend Vernonscope highly enough. Rodger W. Gordon also informed me he had prevailed upon Don Yeir to reintroduce the 6mm focal length early 2006. They havent been made since 1973-74.
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