Don Pensack

Is the 5mm Nagler DioptRx-compatible? Yes, with the TeleVue adapter that allows it. Do I recommend the DioptRx on the 5mm? No, I don't. The eyepiece has only 12mm of eye relief before the DioptRx is added and it reduces the eye relief to about eyelash length or less. Either you pull your eye back every time you blink, or you get eyelash oils on the DioptRx every time. Your scope is f/7. A 5mm yields a 0.7mm exit pupil in that scope. For you to need glasses at that focal length of eyepiece means your astigmatism needs to be 3.5 diopters or worse. If you have that much astigmatism, you do need the correction. But if you don't, the eyepiece would be usable without glasses (you simply refocus the scope for refractive error, either myopia or hyperopia or presbyopia). The alternative, if you do have that much astigmatism, is to get an eyepiece that is usable with glasses on, like the 4.5mm Baader Morpheus, 5mm Baader Hyperion, 5mm Pentax XW, etc.

These filters have no red transmission whatsoever and render superb contrast: TeleVue BandMate II Nebustar Lumicon UHC Gen.3 These filters have red transmission and also excellent contrast: Astronomik UHC Visual DGM NPB The debate is about the efficacy of the filters with red transmission, given that the main lines of interest are in the blue (H-ß) and blue green (O-III). I think that depends a lot on the nebula. Because allowing red to pass does increase light pollution passage, but also passes all the red lines in the spectrum for most nebulae (H-α, S-II, N-II). Our eyes have nearly zero response in the red at night UNLESS the nebula is bright enough to star activating cones in the eye (e.g. M42), so it is a mystery how the red passage makes the nebula appear larger and more details visible. But there are many nebulae where the passage of red does help quite a bit (M42/M8/M17/M20/M16/Eta Carina), and many where it doesn't. Where red transmission doesn't really matter, the main thing that helps is a narrow bandwidth, and all the above are narrow.

I think either works very well at f/8 but that mirrors are preferred at shorter f/ratios. Prisms also help if the scope has insufficient in-travel at the focuser for certain eyepieces. It's been years since I compared diagonals, but I was looking for sharpness of star images and good collimation in my tests. And the very best and very worst were mirrors, with the prisms somewhere in the middle. I thought that even at f/12 (the scope in the test), prisms had a little extra chromatic aberration at the edges of the 82° eyepieces I used in the test. I had a few samples of a couple of the diagonals and the sample-to-sample variation was as great as the range in the test. I came to the conclusion that, mirror or prism, quality of the optical surfaces was what mattered. You could adjust collimation, but not optical quality. Currently, I use a mirror diagonal in my f/7 triplet apo, and the image is quite sharp. I don't use a refractor often enough to re-do the experiment with several diagonals.

We've been through this before. A very small number of their products are unique to Svbony. Most of them are available from other labels and in most cases were available under other labels first, in some cases long before. They are to be commended for doing a good job of promotion, and offering a wide variety of products. But, in terms of eyepieces, except for a couple of the zooms, and the plastic lens eyepieces, I see nothing that wasn't offered by other companies earlier. They are not a manufacturer for other big names. You are just being a shill for their brand name. They sell eyepieces made by KunMing United Optics, Barsta, Suzhou Synta, and Tianyuan--maybe others. Like Omegon, or Levenhuk, or Telescope Service, etc., they are selling rebranded products from other suppliers. That's fine, and the prices are OK. But let's not pretend they are some large manufacturer of optical equipment. Hong Kong is not a big source for optics made in China.

size/diam./apparent field/eye relief Svbony Zoom 3-8 1.25 56 10 Svbony Zoom 7-21 1.25 40-57 16.3-18 Svbony Zoom 8-24 1.25 38-56 18-19.5 Svbony Zoom 9-27 1.25 40-60 17.5 Svbony Zoom 10-30 1.25 33-51 17-18.5 Svbony Zoom 7.2-21.5 1.25 42-65 18-20

No. 25nm is a ridiculous bandwidth for an H-ß filter. You'd be better off getting a narrowband O-III + H-ß filter with about the same bandwidth that would also work on many other types of nebulae. The best H-ß visual filters will have bandwidths in the 9-12nm range, and H-ß photographic filters in the 3-8nm range. The narrower the filter, the greater the contrast enhancement. Here are some H-ß filters that meet the requirements: Arcturus 12nm bandwidth Astronomik 12nm bandwidth Baader 8.5nm bandwidth Custom Scientific 10nm bandwidth Intercon Spacetec (ICS) 12nm bandwidth Orion 12nm bandwidth Optolong 12nm bandwidth

The 6mm and 8mm Ethos, if used as 2" eyepieces, need 0.7" of out travel from the telescope's focal plane. If used as 1.25", however, they require 0.3" of in-travel from the focal plane, which is easier to accomplish in most scopes and/or in a Paracorr. I think of them as 1.25" eyepieces, not 2", despite the 2" skirts.

2" because with H-ß, you will use that 34mm a lot. Plus, a 2" filter will fit the bottom of the 2" to 1.25" adapter for use with 1.25" eyepieces. if the adapter is short and you fear the 1.25" barrel hitting the filter, you can always add a 48mm extender to the adapter to move the filter farther away. 48mm extenders are available in 3-50mm lengths and many in between. One good example: Baader 14mm Fine Tuning Ring.

Interesting. I prefer wider and wider fields of view the higher the magnification. I have non-tracking scopes. A 22mm Apollo, if scaled, would be the weight of an ES 92. The 23mm Pentax 85° shows how hard it is to push a design that far without optical consequences. The 25mm EX 100° also shows the same thing. One thing I would count on Tele Vue not to do is to make serious compromises in design. But will we see an entire line? First, Al is retired and Paul is gone. Second, their factory is completely maxed out and they would have to sacrifice something else for the factory to fit in another eyepiece, and they are already in the 4 to 6 month delay status on some of their eyepieces and other items. It would also represent a huge investment--maybe $125k per focal length.

Without the fancy box or medallion, and fast air freight and speeded-up production costs, it could have been in the Ethos price range. A line of them might be at the Ethos price points, and I, for one, would buy every focal length they make up to maybe 16-17mm. My eyepieces longer than that right now are just fine.

And if they removed a lot of the weight. 12mm ES 92--1017g 11mm TeleVue Apollo 11 85°--612g, or 405g lighter, or about 0.9 lbs! Yes, 92° is wider than 85°, but not by that much. Both are usable with glasses.

That depends. Do you like to seek out the large and very faint nebulae right at the limit? If you do, then an H-ß filter can help. Here are some objects where an H-ß filter will help a lot: From David Knisely: While the H-Beta is probably one of the less-used nebula filters, the commonly expressed idea that it works only on a handful of objects is not necessarily true. Here is a list of some of the more prominent objects that the H-Beta may be at least somewhat useful on. Some may require larger apertures (and some may be slightly better in other filters), but a few have been seen from a dark sky site by just holding the filter up to the unaided eye and looking at the sky. Some of these will also be helped by a narrow-band filter like the Lumicon UHC. 1. IC 434 (HORSEHEAD NEBULA) 2. NGC 1499 (CALIFORNIA NEBULA, naked eye and RFT) 3. M43 (part of the Great Orion Nebula) 4. IC 5146 (COCOON NEBULA in Cygnus) 5. M20 (TRIFID NEBULA, main section) 6. NGC 2327 (diffuse nebula in Monoceros) 7. IC 405 (the FLAMING STAR NEBULA in Auriga) 8. IC 417 (diffuse Nebula in Auriga) 9. IC 1283 (diffuse Nebula in Sagittarius) 10. IC 1318 GAMMA CYGNI NEBULA (diffuse nebula in Cygnus) 11. IC 2177: SEAGULL NEBULA (Diffuse Nebula, Monoceros) 12. IC 5076 (diffuse nebula, Cygnus) 13. PK64+5.1 "CAMPBELL'S HYDROGEN STAR" Cygnus (PNG 64.7+5.0) 14. Sh2-157a (small round nebula inside larger Sh2-157, Cassiopeia) 15. Sh2-235 (diffuse nebula in Auriga). 16. Sh2-276 "BARNARD'S LOOP" (diffuse nebula in Orion, naked eye) 17. IC 2162 (diffuse nebula in northern Orion) 18 Sh2-254 (diffuse nebula in northern Orion near IC 2162) 19. Sh2-256-7 (diffuse nebula in northern Orion near IC 2162) 20. vdB93 (Gum-1) (diffuse nebula in Monoceros near IC 2177) 21. Lambda Orionis nebular complex (very large, naked-eye) 22. Sh2-273 "Cone" Nebula portion south of cluster NGC 2264 In addition, a number of the brighter nebulae like NGC 7000 or M42 will respond to H-Beta use for revealing certain specific detail, although other filters may provide a somewhat better view overall.

No question the 13mm was engaging. I used eyepieces all the way to 4.7mm in that scope. But I started out with a 32mm Plössl to get as wide a true field as possible in the scope with 1.25" eyepieces. In my light-polluted environment, though, the 24mm yielded the same true field with a darker background. A 13mm (at the time it was a 13mm T6 Nagler) was excellent, and used a lot, but it had an appreciably smaller true field. The 13mm Ethos hadn't come out back then. I used the 24mm and 13mm a lot with the Mak. Field stops of 27mm and 16.7mm respectively. The 13mm Ethos would be in between. It would still suffer from a small exit pupil, though. Even a 24mm had an exit pupil smaller than 2mm. The 127 Mak cries out for a low power eyepiece of 40mm, but in a 1.25", that would be a very narrow apparent field. What the scope lacked was large exit pupils and wide fields. I would have been happier with a short focal length refractor. Not long after, that is exactly what it was replaced with--a TeleVue NP101, which had a focal length close to 1/3 that of the Mak. The same eyepieces worked great, and by then I had a 13mm Ethos that got a lot of use--MORE than the 24mm. In that short scope, though, an 8mm eyepiece gave the same magnification as the 24mm in the Mak, and I used eyepieces down to 3mm with it.

The 24 was wider in true field, and brighter.

When I owned one of those 127 Maks, I, too, used about 2 eyepieces. That was because a 32mm and 24mm yielded the same true field, so why not use a 24mm and get a darker background sky? And because a 13mm yielded a 1mm exit pupil, which was high power in that scope. 24mm and 13mm would do fine for most observing. In my dob, however, I can get much larger exit pupils for low power and a 1mm exit pupil is a high power (320x). So I have 10 eyepieces in that range and use at least 8 of them on any random night. It could be fewer if I used a Barlow.

Louis, I can use the TeleVue Apollo 11 with glasses on (it's not too difficult), and that one has 18mm of eye relief from the glass and 14.3mm from the folded-down rubber eyecup. The 85° Pentaxes, though have only 12mm from the rubber. Some who have shallow eyes and a short eye-to-glasses distance might be able to use them. I could not do so comfortably, but only by mashing my glasses into my eye socket until my eyelashes brushed the glasses. The picture from "globular" of the 16.5mm shows why--there is a lot of aluminum above the lens to mount the eyecup onto. With a different eyecup design, that aluminum top could have been 3mm shorter. But, as it sits, even if the eyecup is removed it still has less than 14mm of eye relief, alas.

I think this same zoom is sold by Orion (US) and might be the same zoom sold as 7.2-21.5mm by Artesky and Tecnosky. Most of the 7.2-21.5mm zooms have a narrower field. but those have the same element count, etc., and look identical. The Svbony is less expensive.

I don't worry about where to put filters. I use a Paracorr and mount all filters on the Paracorr. In my refractor, I put all the filters on the 2" adapter. Hence, all 2" filters, even with 1.25" eyepieces. With the threaded-on adapters, all the weight of the eyepiece is hanging on a few mm of 28.5mm thread. I don't think it is more secure than using the eyepiece as a 1.25"--maybe less. Though, in practice, it works fine.

I found the difference just enough that the 85° were a NO-GO with glasses, while the 70° are very comfortable. I'm very sorry about that, because the optics in the 16.5mm are excellent.

I measured the depth and assumed the eye relief from the glass was 20mm. The depth of the lens below the rubber eyecup was exactly 8mm.

The 2" skirt fits OVER the 1.25" focuser and a thumbscrew in the 2" skirt tightens down against the 1.25" focuser. The TeleVue Hi-Hat adapter fits up inside the 2" skirt to grab a longer section of the 1.25" barrel and the slot in the 2" skirt allows the thumbscrew in the adapter to be accessed. TeleVue's eyepieces and adapters were made to work with each other. I do note, however, that the 2" skirt idea is now down to only 4 of TeleVue's eyepieces. If you want to use the eyepiece as a 2", the Ethos 4.7 and 3.7 and all the APM XWAs and the TeleVue Apollo have a threaded-on 2" adapter that works fine. It's a "better mousetrap" for a dual-size eyepiece. And I agree, Louis, the Barsta 70° eyepieces have a nice adapter.

You might like this other site, which is comprehensive, but a little less technical, by Bruce MacEvoy: https://www.handprint.com/ASTRO/ae1.html https://www.handprint.com/ASTRO/ae2.html https://www.handprint.com/ASTRO/ae3.html https://www.handprint.com/ASTRO/ae4.html https://www.handprint.com/ASTRO/ae5.html https://www.handprint.com/ASTRO/ae6.html Not all his data or illustrations are correct, but, by and large, the information is quite accessible by someone not educated in optics.

Judging by the lenses added in the lower barrel, it doesn't look like they could push the design much farther and keep it a 2" eyepiece. But APM is unlikely to pay Mark Ackerman to design a 35mm when the 30mm is already quite good. Here is an illustration of the UFF designs, with corrected field stops that correspond to the field diameters seen in the eyepieces. Note: the 10mm is actually 10.5mm:

I noticed a slight bit of EOFB in the 17mm T4, but not in the 12mm. It has been reported that the longer the f/ratio, the narrower the EOFB, but the brighter it is as well. That points to it being related to inadequate internal suppression of scattered light from spacers, or lack of baffling, lens edges, or possibly angular magnification distortion in combination with those. Many new 1.25" eyepieces come with a threaded-on 2" adapter, and that adapter has an additional baffle in the bottom of the adapter. A brief look in several of them with and without that additional baffle showed me that contrast in the outer field improved when the 2" adapters were added. Accordingly, if you have one of those eyepieces (Ethos 3.7mm and 4.7mm, Apollo 11, APM/Astrotech/Stellarvue XWA), I recommend using the eyepiece as a 2" for maximum contrast unless you experiment and find that, in your scope, there is no visible difference (some scopes have pretty aggressive baffling).

Yes, rectilinear distortion is a radial distortion. It stretches things at the edge along a radial, but not tangential, line. Angular magnification distortion compresses the field in both radial and tangential directions, which is what happens when the entire field has a lower magnification. Distortion is distortion, however. Neither form, in the percentages found in ultrawide eyepieces, allows the moon to stay round at the edge. In star patterns, you'd never know, however, unless the eyepiece is panned or the power is high enough the field moves across the eyepiece fairly quickly. And you might never know if the scope tracks. AMD usually is visible on straight lines as barrel distortion (negative RD), as the edge is compressed. That leads to the visibility of "rolling ball" or "globe" distortion, as if looking at a globe rather than a flat field. Zero AMD and high RD (positive, or pincushion) makes the field appear somewhat bowl shaped, but it is not different than how the eye sees the sky, as closer at the zenith and farther at the edges. Since star patterns do not follow straight lines, this is usually the preferable type of distortion. Pulling back a bit on the correction of AMD and allowing some to be in the field reduces the amount of RD seen and this is not uncommon in eyepieces (e.g. 14mm Morpheus). But the Moon will still distort as it nears the edge. Distortion is distortion. One thing I learned is that you don't view the moon at low power near the edge of any ultrawide eyepiece because of distortion and because those eyepieces are rarely free of lateral chromatic aberration. But having a fairly large amount of positive RD in the form of pincushion does not preclude having a flat field. The 30mm UFF is an example--obvious pincushion, yet a map-flat field. It's too bad the shorter focal lengths of UFF don't have the same internal prescription. That would have been an impressive 70° line, albeit more expensive than they are.