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Don Pensack

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About Don Pensack

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    Audio, films, traveling, and astronomy of course.
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  1. The Paracorr offers a top with a bit less wobble and slightly better correction of eyepiece field stops as large as 46mm and better correction at f/3-f/3.5. Other than that, the HRCC would be fine IF you have sufficient in-travel of the focuser to accommodate it. You should note that the visibility and appearance of coma is related to apparent field. Use 50° eyepieces in your scope, and you likely won't notice coma. Use 100° eyepieces in your scope and you will. Though the linear size of coma changes at the edge of the field with true field (it gets smaller as the focal length of the eyepiece shrinks), the apparent size (what you see) of coma only changes with apparent field. Also, note that a short focal length scope will have inherent curvature in the focal plane that will mildly defocus eyepieces that have large field stop diameters. Another way of looking at it is that coma is made worse in appearance by field curvature, so keeping apparent fields narrow will reduce the ability to see field curvature. So, suggesting an apparent field wide enough to yield fairly large true fields, as well as avoiding defocus from field curvature, and having just about zero astigmatism, I think I would recommend TeleVue Delites. You might find you don't need a coma corrector with them, at 62°, but a coma corrector would yield perfect stars from center to edge in them. Also, they're all parfocal, so would use the exact same setting of the coma corrector for each +/- 0.5mm or so.
  2. Repeatable and identical registration in a focuser for collimation tools, eyepieces, or star diagonals demands one of two binding systems: --a centering collet which squeezes down uniformly all the way around and centers the inserted accessory --a two screw binding. While Rusted is probably correct that a 3-screw binding would work so long as the first two screws are very tight first, I would bet that, in the field, they might not always be tightened in exactly the same sequence, which is what is necessary to always guarantee identical registration each time. And since the linear contact on one side of the inserted accessory will add as much or more friction than a 3rd thumbscrew, I see no reason to use a 3rd screw. Though no one has yet made such a focuser, one with a long collet from opening to, say, 1.5" further in, would work with all eyepieces, undercut or not. You can find similar 1.25" adapters, but not, so far, focusers containing that idea. The Baader Click-Lock system is getting close, but it's still too short internally to work well with all undercuts (there is a wide range of widths and shapes to accommodate, there).
  3. Two thumbscrews are better than one because two screws prevent wiggle in the eyepiece and automatically register the eyepiece barrel against the other side of the focuser. Three screws are a bad idea, though, because the 3rd screw presses the eyepiece away from contact with the focuser and can cause registration issues thereby.
  4. The conically tapered undercut was designed to work with a thumbscrew. When the screw tightens on the eyepiece, it actually pulls the eyepiece down into flat contact with the focuser. If the screw is a bit loose, it still catches the eyepiece. An excellent idea with thumbscrews. However, the taper can permanently distort a brass split ring binding (mistakenly called compression ring), and result in damage to the collet in a twist-lock or click-lock binding. The split ring and twist-lock binders were designed for smooth barrels. So what we have now is focusers, barlows and coma correctors designed for smooth barrels, but we're using eyepieces designed for thumbscrews. A great state of affairs, and one seemingly without resolution. The eyepiece manufacturers aren't going to abandon the undercuts, and focusers and other accessories aren't going to abandon whatever binds the eyepiece without a mark (I blame the used market for that). Tele Vue, at least, has beveled the lip of the undercut so it won't catch on the lip as the eyepiece is removed. Personally, I went back to a smooth focuser with a large nylon 1/4" x 20 tpi thumbscrew--no mark and compatible with all eyepiece barrels and undercut shapes.
  5. Don't worry about central obstruction. It is unimportant for anything except lunar and planetary observing at high powers. As for what you typically see at the edge of the field in ultrawide eyepieces at f/5: Coma (from the mirror--needs a coma corrector to solve) Astigmatism (all ES eyepieces have this except the 92s, but it can be in the optics as well) Field curvature (causes slight defocus, making other aberrations look worse) This can be in the eyepiece or the scope if the focal length is short enough (say, under a meter). Lateral chromatic aberration (sometimes referred to as chromatic smear, looking a bit like a prism has spread the colors) Edge of field brightening (EOFB) wherein the center of the field looks like it has better contrast than the edge Edge of field vignetting, wherein the edge appears to be darker than the center of the field Of these, astigmatism is the worst, and if combined with coma, you get stars at the edge that look like seagulls. But all f/5 scopes have visible coma. Only the central 2.75mm of field (remember, 2" is 50.8mm) is essentially coma-free, so: --can you accept 2" eyepieces? Because all currently-available coma correctors are 2". --If you cannot afford the TeleVue Paracorr II, I also recommend the Explore Scientific HRCC, which is a lot less. --do you have quite a bit of in-travel of the focuser left in the scope? The Paracorr needs about 14mm of in-travel at the focuser compared to where the eyepieces focus without it, and the HRCC needs 32mm of in-travel at the focuser to get to a focus position. The good news is that a coma corrector will allow all your eyepieces to focus at the same point of focuser travel, so you won't need any additional in-travel except to accommodate your near-sighted friends, and that's only a few millimeters at most. You could be where I was many years ago. I saw horrible coma, and got a coma corrector to eliminate it, which it did, only to show me my low power eyepiece had strong field curvature (it got replaced). Once coma is removed, all the other aberrations become apparent. Alas, eyepieces without astigmatism at f/5 are few and far between. Only 2 from ES, none from Baader, 49 from TeleVue, none from Olivon/Barsta, a few from Nikon, none from Vixen, none from Pentax, none from APM, etc. But, zero astigmatism may not be a necessity if coma is gone and astigmatism is very small. But if you want refractor-like star images to the edge, prepare to pay.. As we say: Wide-field, well-corrected at the edge, low price--pick any two!
  6. The Delite design has a very small spot size all the way to the field stop, so you should see somewhat sharper stars near the edge in the Delites. The true test, though, would be to see how the stars compare with the Nagler T6 when the star is in 10° from the edge, i.e. 30° off axis instead of 40° off axis. My recollection is that, compared this way, the two eyepieces are more similar except for the eye relief.
  7. Your Meade S4000 from the '80s were the 5-element Kowa-made versions. The one with no markings was made to be included with a Meade telescope, and has a 99.9% chance of being 4 elements, made in China.
  8. 4mm Plössl? How do you use it? The eye relief would be about 2.8mm, close to the average distance from the cornea to the pupil in the eye. I presume you don't see the entire field but move back and forth to see the edges? As an experiment some time, try a 2X Barlow on an 8mm Plössl. The 8mm Plössl has about a 5.6mm eye relief, but the Barlow extends that to close to 6mm, which would be a lot more comfortable. In addition, the Barlow would provide additional baffling. There is one good thing about a 4mm Plössl, though--if the lenses are removed it can be used as a collimation cap.
  9. Meade Series 4000 "Super Plössl" came out in the '80s. It was made in Japan by Kowa, had five elements in a 2:1:2 configuration, no rubber eyecup, and multiple 1.25" incarnations, as well as one 2" (56mm focal length) In 1994, maybe late 1993, this eyepiece line was updated to have rubber eyecups. Within the year, the configuration was changed to that of a standard 2:2 Plössl, though the name "Super Plössl" was left on the eyepiece, still made in Japan. Manufacturing moved to Taiwan briefly in less than a year and was there a short while before moving to China. The name stayed the same, though the first Chinese one was actually the 5th version of the eyepiece. Over the years, there have been at least 3 different Chinese manufacturers making the eyepieces, and maybe more since people have noted at least 3 different colors of logo on the eyepiece and different fonts as well. If you look at the eyepieces on the Meade site, you can see at least three different fonts just in their pictures. I had the complete set of the first 5-element version made by Kowa, and they were excellent. I would guess they'd be the same with rubber eyecups, though I am not certain how you would differentiate between the 4 and 5 element versions in the rubber eyecup versions made in Japan.
  10. https://www.explorescientific.it/out/media/50c37e2d0427ea14408f9fe6735dd29c.pdf Eyepiece spec chart for ES. Focal planes above the shoulder are labeled minus because you have to move the focuser in. Focal planes below the shoulder are labeled plus because you have to move the focuser out.
  11. If you view without glasses, long eye relief eyepieces can be hard to use. The cure is for the eyepiece to have an eyecup design that allows the eyeguard to be raised to a height preventing drifting too close to the eyepiece. Good examples: Delites, Delos, Pentax XW, Baader Morpheus with extra spacer added. All these are usable with or without glasses. Examples of eyepieces hard to use without glasses: ES 92s, APM 84°, Baader Hyperions, TeleVue 32mm Plössl.
  12. The 17mm ES 92 has its focal plane at the shoulder of the eyepiece. If the 12mm focuses 2mm farther out, its focal plane must be 2mm lower in the eyepiece barrel. It's not surprising a 20mm APM won't come to focus, as its focal plane is nearly 10mm above the shoulder and it requires a lot more in-travel of the focuser. Since a prism eats up a lot of back focus, you need to stick to eyepieces with lower focal planes.
  13. You bring up a good point. Since an eyepiece manufacturer cannot know what focal ratio of scope the eyepiece will be used in, why not design all eyepieces to work superbly at f/4. We know that such eyepieces will work just fine at f/10, and that eyepieces designed to work well at f/10 or longer do NOT work well at f/4. Why should the user of an f/3 to f/6 scope get stuck with eyepieces that yield a lot of f/ratio-induced astigmatism just because the owners of f/8 and f/10 scopes don't want to pay 1 penny more for their eyepieces? Either the manufacturers of eyepieces should specify "not to be used under f/6" or all eyepieces should be designed to work well at f/4 and then everyone will be happy with the results. I test a LOT of eyepieces, and I am constantly amazed at how poor some eyepieces are. And that is in a field-flattened, coma-corrected, f/5 newtonian with 1826mm of focal length. Ironically, some very inexpensive eyepieces do quite well, but they also have narrower apparent fields of view.
  14. It's usually about the true field rather than the apparent field. I use mostly Ethos eyepieces on my scope with a 1826mm focal length and mostly Delite eyepieces on my scope with a 714mm focal length. 100-110° yields adequate true fields on the longer scope and 62° yields adequately-wide true fields with the 714mm focal length scope. I have 8 eyepieces for the long scope (and that's enough) and 12 for the short scope (and that's too many). Recently, I've been experimenting with eyepieces, and i guess the #1 thing that immediately disqualifies an eyepiece for me is astigmatism in the outer field. #2 is if it has barrel distortion or noticeable angular magnification distortion. Pretty much every eyepiece is sharp enough on axis. It's keeping that sharpness to the edge that separates the wheat from the chaff.
  15. Just a follow up note: The 40mm Pentax XW has been reintroduced and will be at dealers at the end of December. US price $399.95. Depending on the scope, this is a very viable alternative to the 41mm Panoptic. My personal preference is the Panoptic, but a lot of people go the other way on that choice.
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