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Unusual 4.8mm Orthoscopic


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This unusual ‘alleged’ Abbe fully multi-coated orthoscopic has a focal length of 4.8mm. I say ‘alleged’ as I don’t actually know if this is indeed a genuine Abbe design. From what I can gather it has two lens groups consisting of a triplet and a single lens. So, the odds are it is an Abbe eyepiece and very probably manufactured in China. Mine has no brand name but these eyepieces have also been sold as Kson, Ascension, Apogee and University Optics orthoscopics.

 

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The eyepiece has no dust caps but it is supplied in its own plastic bolt case. The housing is almost exactly 50mm tall and appears to include the tapered barrel as an integral part of its aluminium housing. The barrel itself has a 28.5mm filter thread and is competently baffled. I discovered that my Baader, GSO and Lumicon filters all threaded fairly smoothly into the barrel. However, Meade and Barsta 2x Barlow elements were not easily threaded in all of the way. A GSO 2x short Barlow element threaded perfectly. The eyepiece features a flip-up rubber eye guard which is a rarity on most orthoscopics. I couldn’t separate the barrel by unthreading it and I assume it is essentially a monocoque design, which is also unusual in my experience. The housing is very light as a whole and feels a fair bit lighter in the hand when compared to both a 5mm Kokusai Kohki orthoscopic and a 6mm Vixen NPL Plossl. 

 

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The build quality seems perfectly adequate even though it’s obviously a relatively inexpensive Abbe eyepiece. Especially compared to Takahashi and Ohi built orthoscopics which can cost up to three times as much as the forty quid (£39.60) I paid for this particular eyepiece. The AFOV is a claimed 48° which is a good 6° wider than most Abbe orthoscopics and I suspect the extra field is created by the deployment of a larger field stop than usual for its focal length. As it is purportedly a true Abbe the extra field is probably to aid target acquisition and would be expected to exhibit an amount of lateral edge astigmatism. There is a claimed eye relief of 4.2mm which is 0.15mm longer than a 5mm Ohi Abbe orthoscopic.  

 

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I got first light with the 4.8mm using my 72ED DS Pro. The seeing was a good Antoniadi II~I but the transparency was distinctly below average. The orthoscopic gave a convincing sharp and well contrasted view of Saturn at 87.5x. Saturn was fast approaching transit and I could see a fair bit of detail on the planetary surface and the rings, including the Cassini Division. In fact, I thought the contrast was better than the 6mm Vixen NPL that I directly compared with the 4.8mm ortho’. 

 

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I then turned to Jupiter. The detail was very well contrasted in the equatorial belts with excellent visual acuity. I occasionally witnessed some small amount of chromatic aberration around the planetary limb but it wasn’t particularly bad. I could sometimes detect an amount of scatter near the field stop and even some sporadic ghosting, although again, it wasn’t really a problem. Europa was right next to the limb and about to transit. I observed it once it was past the limb itself and could still see the moon above the surface of the gas giant for several minutes . 

 

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Meanwhile the already poor transparency was getting worse. I decided to split some doubles while I could still locate any before the steadily encroaching cloud enshrouded them. The extra 6° of field helped me find Iota Cassiopeiae more easily, but it took a 2x Barlow to properly reveal all three stars of the system at 175x. The stars were all well defined and their individual colours were easily perceived. The expected lateral astigmatism was basically undetectable when using the Barlow. But even without the Barlow it didn’t seem particularly problematic and was far closer to the field stop than I expected. I split a few more doubles at 87.5x including the Double Double, Struve 2470/74, Albireo and Almach. I could also quite distinctly see the Ring Nebula, albeit often using averted vision. Eventually I turned back towards Jupiter and could make out the GRS pretty easily at 87.5x. 

 

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Apparently orthroscopic eyepieces are expensive because it is costly to properly and accurately cement the triplet part of the element. The use of an inexpensive housing may be a way to keep the overall unit cost down.

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