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Don Pensack

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About Don Pensack

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    Star Forming

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    Audio, films, traveling, and astronomy of course.
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    Los Angeles
  1. Because I have so many eyepieces, I don't use a Barlow any more, but when I did, the Baader VIP was my favorite--really nice. But my best views were always with the TeleVue Powermate, that just seemed to add magnification and had no other noticeable issues at all.
  2. The 17.5mm may be different internally--it has a different placement of the field stop inside the eyepiece than the others, and it is the lightest of all of them.
  3. Axial sharpness varies on paper, but not actually that much in the field. When the seeing is really superb, ALL eyepieces do well on axis. When the seeing is bad, all eyepieces do poorly. The eyepiece differences on axis are the residual 0.01% after seeing conditions are concerned. I found the APM excellent on axis and for nearly the entire field in a coma-corrected f/5 12.5" dob. Since I used it with glasses, my complaint wasn't the eyepiece, but the fact the frame of my glasses was visible in my peripheral vision inside the field of view. Think about it--do you have to lift your eye 42° to see over the frame of your glasses? For me, maybe 30°, which is why I don't notice the frames of my glasses with a 62° eyepiece. The 11mm Apollo had a sharper outer field but the difference was slight. My next outing, I'm going to specifically look for the edge of field brightening some have reported. I didn't see it the last time I was in the field, but I was doing an evaluation of several other eyepieces, so this one didn't spend that long in the focuser. I haven't owned or used a Plössl since the early '90s. My last long experience with the Delos was when they came out (not sure the year), and they struck me as being long eye relief, slightly narrower field, Ethos. I wasn't wearing glasses for eyepieces back then. Even today, I don't really need glasses at all below 12mm. Whether glasses are needed for the best images has an easy test: defocus the star on one side of focus and do the same on the other. Do you see any ovality to the ON-AXIS star image? They you could benefit from glasses. I can see very slight astigmatism that way down to a 4.7mm eyepiece, but the stars are sharp enough in focus at 11-12mm. And when I wear glasses? Stars are completely round out of focus in both directions all the way up to a 30mm focal length. The absence of astigmatism is a marvelous thing--if the apparent field is not larger than your glasses. My next observing pair need to be those over-sized '80s glasses that hit my cheek on the bottom and extend well above the eyebrow on the other.
  4. They are made in China. They might have been designed in Germany, however. I don't know the factory name, but I bet there are many in the business that do--especially those who import directly.
  5. Killing the mould with UV light wouldn't eliminate the haziness on the lenses, though. At this point, it's take it apart or throw it away. Better to take the chance to clean it at this point.
  6. A telecentric Barlow will usually workbest, avoiding the eye relief extension and vignetting of the conventional Barlow. TeleVue calls theirs the PowerMate, while ES calls theirs the Focal Extender.
  7. With different number of lenses, different field stop diameters, different eye relief, different eyepiece diameters, different eye and field lens diameters, and different % of geometric field distortion, I would say no, the APM is not a clone of the Docter/Noblex. Just similar in that it is an ultrawide with long eye relief. As for visibility of the angular magnification distortion, it depends on whether the scope tracks and how wide the true field is in the scope. I found the Docter intolerable in my dob, but it worked fine in a tracking SCT.
  8. And it is just that globe distortion (AMD) the Docter eyepiece has in abundance. Between that and the pincushion form of rectilinear distortion, I'll take RD any day. That article explains distortion very well, by the way.
  9. You are only referring to rectilinear distortion (e.g. pincushion), which the Docter has almost none of. On the other hand, it has a LOT of angular magnification distortion. If you looked at a billboard letter and moved it from the center of the field to the edge, it would not bend, but it would shrink substantially. That is not the distortion you are looking at and it is hard to see in a star field. But you would see it easily while panning across the Moon.
  10. I'm the opposite. Merging images is not the issue, it is my non-observing eye, which is dominant and has visibly lower resolution than my observing, non-dominant, eye. ε Lyrae is an easy naked eye double star to my right eye, but a single star to my left, even with glasses that correct perfectly for astigmatism. When I binoview, I see an improvement in contrast, but a significant loss of sharpness, even in high-end binoviwers. I enjoy the loss of floater issues when observing the Moon, but I see both a brighter and more detailed view with a single eyepiece using my observing eye. Too bad, too, since I have a 70mm IPD and can use a lot of eyepieces in binoviewers that many people cannot. BTW, I have the same issue with binoscopes.
  11. Sorry, completely impossible. Orthoscopy implies a lack of distortion. The curves for rectilinear distortion and angular magnification distortion deviate above about a 30-40° field. Distortion goes up with apparent field, so there is no such thing as an ultrawide orthoscopic eyepiece. You can reduce rectilinear distortion to a minimum, leaving in a large amount of angular magnification distortion, as is done in the Noblex/Docter eyepiece, or you can reduce angular magnification distortion to a minimum, leaving in a lot of rectilinear distortion, as done in, say, the TeleVue Ethos. But you cannot simultaneously reduce both to a minimum in optics. This shows the curves for each form of distortion with apparent field on one axis:
  12. They exist, but for a variety of reasons I won't go into, are not preferred over the older ones. I do find it strange they did a redesign to lengthen the eye relief, yet didn't make them glasses compatible.
  13. The Baader has the click-lock feature, though the collet in the Anteres grabs a longer section of eyepiece.
  14. Let's unwrap that remark. 1) no commercial 4-element eyepiece with 2 doublets you have used in your life is of the 1860 original configuration of Georg Simon Plössl, so no one would know exactly how the eyepiece performed. Ray traces show it is pretty good in the center 20° of field and very sharp on axis, but we simply cannot know whether it is "better" than any of the modern varieties. 2) the revised version improved by Albert König in 1938 was the influence behind the reversed version of Chester Brandon (originally called "Brandon orthoscopics", and used in Questars) and the Clavé "Plössl" of the '70s, though the designers did improve on König's design. Both Clavé and Brandon paid little attention to correcting astigmatism in the outer field because most, if not all, scopes of the era were long focal ratios. An f/8 was considered normal for a 16" reflector! Fast scopes for f/5-f/6 soon revealed the poor edge correction in those designs. Those versions did all have non-symmetrical configurations, those glass types varied. 3) By the early '60s, the term Plössl in optics manuals and design handbooks had become synonymous with a 4 element symmetrical design, generally with outward-facing convex surfaces. Performance was good, but they suffered from lateral astigmatism in faster scopes. This eyepiece was sometimes called Symmetrical, but the descriptor "Plössl" had become a relatively generic term. Not in the public, however, where terms such as "Kellner Type III" or "Achromatic Kellner" were often applied. The term "Plössl" was really made popular by Clavé and TeleVue. Today, the term is totally generic, and several different internal configurations are sold as Plössls, which has come to mean: 4 element eyepiece made from 2 doublets. We can complain about it, but that is the way of the world, and we have to accept that that is the case. I also suspect, as an aside, that many of them work better than Plössl's 1860 design in the modern scopes where f/8 is considered long. 4) Nagler designed his 4-element Plössl to have concave outer surfaces for greater control of astigmatism, though this was not the first version he sold. He did get a patent on the concave design and it went into production. The goal was to raise money so he could introduce a 90° eyepiece he had designed, though optical correction characteristics eventually reduced that to 82° (though the production actually reached 84°). He called the 4 element eyepiece "Plössl" because that had become, more or less, what 4 element eyepieces constructed with doublet pairs had become described as. It also enabled a little wider corrected field at the sacrifice of a bit of edge illumination. So, a different level of performance than the eyepieces just called "symmetrical". 5) The original TeleVue Plössls had simpler coatings than the current generation, so along the way, they were improved. 6) König was granted 38 different patents for eyepiece designs so describing an eyepiece as a "König" is about as descriptive as saying "car". There are a lot of different eyepieces that could be called Königs. Many are really good on axis, but few of the designs work even modestly well in today's f/4-f/6 scopes. So, if conforming to Plössl's 1860 design is what is required to be called "Plössl", then there isn't one in existence today, and you probably have never ever seen one. But it's like calling cotton swabs "Q-Tips", or facial tissue "Kleenex". A particular product became a generic name. This statement, "TV Plössl is only a Symmetrical, and consequently incapable of a noticeably superior performance" is just a statement without any factual basis, and exhibits a bias not justified by the design.
  15. Looks like a comfortably long eye relief as well.
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