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  1. Background The late Thomas M Back developed his planetary series of eyepieces working with Burgess Optical. Those who want to know more about Tom and his works might like to read this 2006 report: http://www.cloudynig...hp?item_id=1549 . Tom was enthusiastic about the eyepieces and wrote about the 4mm on 21 October 2005 as follows: "I first did a bench test with my Strehl .997 TMB 100mm f/8 SD apochromat, on the autocollimator. As you may know, I was very happy with the performance of the prototype of the 4mm. The production model is as good or better. The Airy disc was textbook, the off-axis aberrations were as low as any eyepiece I have tested in this field size and focal length, and the contrast and sharpness was superb."At the time these eyepieces were a radical departure, at $99 much cheaper than, but similar in specification to the Televue Radian and Pentax XL, which were market leaders. Tom set high standards for his eyepieces as in this claim: This is the first wide angle, long eye relief eyepiece that has the sharpness, contrast, and lack of scatter that the best orthos have...These are bold claims which I will attempt to address in this review. In the years since, there has been extensive discussion about their quality. Some changes have been made and some are now labelled Planetary II, but I am not a historian so will concentrate on the features and performance of these four currently available eyepieces. Eyepieces under test The four eyepieces that I have in front of me are all of 4mm, to the same optical design and I think they are representative of those currently on the market, as follows: TMB ® Optical Planetary II, hereinafter TMB ®, which is now only available from Astronomics in the US, though mine was bought in 2012 from High Point Scientific who were then also distributors. This is the only range authorised by the Estate of the late Thomas Back and on which royalties are returned. Astronomics will ship to the UK and these eyepieces were on sale at $40 at the time of writing, though only the 5mm, 6mm and 9mm were in stock. Clearly taxes and carriage would add significantly to that price. TMB Optical Planetary II SW, hereinafter TMB SW, is widely available including from a number of UK dealers, from one of whom mine was bought in 2011. I have four eyepiece in this series and for some time they formed the backbone of my eyepiece collection. At the time of writing this model is listed on the Burgess Optical website. The range includes 4.5mm and 7.5mm models, which were not part of the original set. They are quite widely available in the UK price for about £50 or slightly less. Teleskop-Service HR Planetary, hereinafter TS HR 60°, is available from Teleskop Service and a number of other dealers, some which are in the UK. Markus Ludes of TS has written that the design was supplied to him directly by Tom Back. The 9mm has a revised barlow arrangement and there are additional 15mm, 20mm and 25mm eyepieces to a similar design. My example was obtained second hand. The price in the UK from Modern Astronomy is £49. Sky-Watcher Planetary 58° UWA, hereinafter UWA, is widely available from Sky-Watcher dealers, but has no Sky-Watcher mark and the eyepiece appears to be identical to those sold under the BST and Olivon labels and formerly known as "TMB design". At the time of writing Sky-Watcher only offers the 2.5mm, 4mm and 5mm but other suppliers offer the same range as for the TS HR above. For some time I owned the 15mm and 25mm. The Sky-Watcher models are widely available and cost about £40. Overview These four eyepieces are very alike so I shall cover the similarities in design and performance first and differences afterwards. The optical design is quite novel but there is no patent that I can see. The the names TMB and TMB Optical are US trademarks. The design is certainly not a Plössl (four lenses arranged in two convex cemented achromatic pairs) despite the claims of a number of suppliers including, disappointingly, Sky-Watcher. The range originally had 2.5mm, 3.2mm, 4mm, 5mm, 6mm, 7mm, 8mm and 9mm models. I dismantled the UWA to show the construction. The bottom row of components consists of the housing, a singleton eye lens, a spacer and a thicker black coated cemented doublet lens, (so three lenses in total) and the retaining ring which incorporates the field stop. This main group is identical for all the focal lengths between 2.5mm and 9mm. They have a focal length of about 16.5mm and together are achromatic, but are not corrected for spherical aberration. Without the barlow they give a very blurred image in an F/6 telescope. The spacer is smooth anodised black in all models and while it is out of the proper optical path, its reflection may contribute to the sensitively of these eyepieces to ambient light. The negative Smyth (Barlow) lens in the barrel consists of a cemented pair which not only serves to shorten the focal length, but also corrects the spherical aberration in the main group. There are thus just five lenses in three groups in total, perhaps the minimum for an eyepiece range with constant eye relief and a reasonably wide field of view. The eyepieces under test are the same size and have a similar screw up eye guard. They weigh about 170g (6oz). The eye relief is about 15mm and is enough for me even when wearing spectacles. Despite some suppliers quoting an apparent field of 60° and others 58°, I found no significant difference in true field of view. The lenses are located directly in the aluminium housings. This construction means that lenses can come loose from time to time and the relevant retaining ring needs to be tightened to stop any rattling. I tested these eyepieces in my two refractors (70mm F/6 and 120mm F/7.5) and in my 200mm F/4.5 Newtonian with a coma corrector which extends to focal length by about 10%, to F/5. They all gave a good sharp colour-free image across the field with a clearly defined field stop. I have been unable to distinguish the image sharpness from one to the next, but testing has been limited due to the amount of recent cloud. I have rarely had truly dark skies, but all gave good views of Jupiter, Andromeda, the Great Orion nebula inluding the trapezium and other objects. The eyepieces are nice to use. The eye has to be kept quite precisely in position, otherwise the view is cut off sharply, though newcomers have little problem using them. Any tendency to kidney beaning can be controlled by screwing up the eye cup. I have owned planetary eyepieces of this type for some time and when I have directly compared a planetary against an ortho, the latter has never given a worse image and on occasion has delivered very slightly more contrast on fine detail. However I do not have a 4mm Ortho and was not able see any difference in sharpness, compared with either a 6mm ortho and 1.5x barlow or a 9mm ortho and 2x barlow. There are however issues and two are common to all the models on test. Firstly the large eye lens is pone reflecting any stray light, but this is a minor problem if you can shade it or stick to a dark site. The second problem is the annoying ghost image, seen particularly when viewing planets. This dances in front of your eyes and to me is a serious distraction. When viewing an extended object the ghost is not visible but surely will still be there filling the view and presumably reducing the contrast. I have seen such ghost images in other eyepieces, but it is particularly prominent in these. Differences There are some differences in detail in construction and performance as follows. The TMB ® comes with the same end caps as the HR and UWA eyepieces. The exterior, including barrel is in shiny black. Internally, it has the most effective anti-reflection treatment with a matt screw thread on the inside of the barrel and threading on the barlow lens retention ring. Flare from a bright object just outside the field of view was well suppressed. The TMB SW is entirely finished in an attractive matt black. The eye guard screw is loose and has no grease on it unlike the other three eyepieces tested here and indeed some of my other eyepieces in this range. It comes with different end caps from the other models, which all have the same ones. It has a deep, 0.6mm, shoulder on the barrel unlike the others, which have a similarly, ludicrously deep undercut. I like undercuts but only when they are minimal, perhaps 0.2mm deep, as on my Vixen and Antares Ortho eyepieces.The internal barrel is threaded, but has a bright finish which is not effective in controlling flare. There is also no threading on the barlow retaining ring and indeed this assembly is different from all the other models here. The arrangement is not effective in controlling flare from bright objects just out of view.In the past on one occasion with another eyepieces in the series (9mm), this flare was so bad that it effectively prevented the viewing faint galaxies signposted by Mars. My Antares ortho did not have a problem, so it was an issue with the eyepiece and not the telescope.The eye lens has a coating which shines mainly cyan (red) in colour while the other three eyepieces in this test have a coating which shines mainly magenta (green to blue). see the second photo below. I have three other eyepieces TMB SW eyepieces and one has this cyan coating while two have magenta coatings. Despite the different coating, I was not able to detect any difference in overall view, tendency to ghosting or sensitivity to external light.So far as I can tell, the HR and UWA physically differ only in the eye guard finish. They both have a semi-gloss barrel with a chromed nose, with the deep undercut discussed above. The internal barrel is smooth, but there is threading on the barlow lens retention ring. There is significant flaring and I noted that the previous owner of the HR had taken to the trouble to flock the inside of the barrel. The other difference between the HR and UWA may be something or nothing dependent on your point of view. The HR is clearly branded by Telescope Services, an organisation that surely has the technical expertise to check for faults in the event of problem. In contrast the Sky-Watcher is totally unbranded and the eyepiece appears identical to other unbranded UWA eyepieces. That Sky-Watcher describes the UWA as a Plössl, when it is clearly nothing of the sort, does not fill me with confidence that it knows anything about the eyepiece.Summary These are nice eyepieces to use, but I think that ghosting in all models and the flare in three models mean that they do not meet Tom Back's aim, that they should match the best orthos. They come in a wider range of short focal lengths that any other eyepiece range so are can be useful for filling gaps. There are certainly worse eyepieces available. The best of them is the Astronomics TMB ® in which flare is well controlled unlike in the others. Those interested in such things are assured that some revenue is going to Tom Back's Estate. I shall be retaining this eyepiece. I have real concern over the variability I have seen in the TMB WA, but it may be that familiarity breeds contempt. I also have concerns about the use of the TMB trademark particularly on those focal lengths, 4.5mm and 7.5mm, which have emerged since the death of Tom Back so presumably were not designed by him. Even if this does not concern you, it would not be practicable for most users to fix any flare that may arise from the lack of threading on the barlow assembly. That the shoulder is nicer than the undercut on other models is just about the only plus for this eyepiece. The TS HR is backed by a significant name in astronomy and Markus Ludes worked with Tom Back on a number of projects, but not this eyepiece. If you were to buy one, flocking the barrel would most likely remove flare. The Sky-Watcher Planetary is largely similar to the HR but has no marking to indicate the maker or to differentiate from other apparently identical eyepieces advertised under different names. If you are unconcerned by this and were to buy one, then flocking the barrel would most likely remove flare. Endnote I would welcome comments. I intend to retain the eyepieces test for a couple of months so I can properly answer any queries. If anybody would like to do their own tests, they should send me a PM.
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