Don Pensack

Correct, it is a Norin binoviewer and has been offered by many many companies, from Celestron to William Optics. The binoviewer itself is not too bad, though not friendly to larger field stop eyepieces, but the GPC that comes with is simply horrible and adds spherical aberration and chromatic aberration. I reviewed these a few years ago and found that just about any other brand of GPC was better corrected.

Dave, Here is why: If TeleVue switched to smooth barrels, it would say to the market that the undercuts are inferior. The eyepieces in dealer's showrooms would automatically be worth less and the average dealer would stand to lose thousands. If TeleVue just added the smooth barrels as an option, it would mean every dealer would have to have double inventory, because, inscrutably, some people actually want and prefer the undercut, and the dealer couldn't know which customer would approach first. TeleVue would have to double their inventory as well, something they cannot afford. In this Covid era, production is maxed out because demand is 3-4x the size of only 2 years ago and the factories simply cannot make more (one of the many reasons there are shortages), so the factory simply couldn't double the production to make both smooth and undercut barrels. Obviously, the smooth barrel version wouldn't cost more, but if the production quantity of both smooth and undercut barrels dropped because of a split inventory, it's possible BOTH versions would cost more. Going to a smooth barrel option in the aftermarket wouldn't be feasible, either. The number of different barrels they'd need would be large. And the individual costs would also be large because they would no longer be a part of an eyepiece "package". Ever price out a £300 bicycle in parts? 10x as much for the same thing. I recall hearing US$50-$100 for each after market barrel. Who would pay that on top of the already expensive eyepiece prices? And, there is no one at TeleVue, who is completely maxed out on time for the employees, to dismantle one with an undercut barrel and install the lenses into a barrel with smooth sides. And if they did so, there would be a large labor charge. So no, it isn't economically viable for TeleVue to offer smooth barrels. Instead, as a running change, they beveled the undercut to aid in removal. Then, let's look at current, in production, eyepieces with smooth barrels: A few APMs and Altairs (most other sellers of the same eyepieces have undercuts) Baader eyepieces Vixen SLVs (unclear whether they are still in production) And...?

Stupid. No reason to get the entire eyepiece wet. Water isn't a good cleaner on its own. You still need to clean the lenses. In 1/20 the time, you could clean a full set of eyepieces with Q-Tips and lens cleaning fluid and not have to let them dry. You have to realize that video is just bragging about being waterproof. Whoopee.

I haven't used the 5.2mm, but I did use the other Pentax XLs for several years. In my f/5-f/6 scopes, they suffered from lateral astigmatism. The XWs improved that, and even with a wider field. I view the XWs as a step up from the XLs. The 30mm APM has no astigmatism in the outer field at all at f/5. Comparing the 30mm APM to the 30mm XW, the APM was better-corrected at f/5.75, so if A > B, and B > C, then A > C.

Hmm. I measured those two eyepieces and got 49.8° on the Sirius and 49.5° on the GSO. My GSO also had a field stop <27mm, so it's possible they're changed the eyepiece over the years. That was ~2010.

Regular 90° prisms, yes. Amici prisms, no.

A good low power eyepiece for that scope could be 24mm +/- 2mm, i.e. 22-26mm. For a very low power eyepiece, usable primarily only in truly dark skies due to the brightness of the background sky in the eyepiece, would be a 36mm +/- 2mm. I don't think you would use that low a power very often because most objects would be very small in the eyepiece, but having an eyepiece in that range for low power, dark sky, observing isn't a bad thing. I still think you would use the 22-26mm more. One very good example of a superb lower cost eyepiece would be the APM 30mm 70° Ultra Flat Field. That sort of bridges the gap between those eyepiece focal lengths, and then you could easily skip straight to something like the 17.5mm 76° Baader Morpheus or a 17mm Astromania SWA 70° (a true bargain)[In the EU, the Omegon Redline] for the next magnification up. The APM is a lot better eyepiece than any of the Pentax XLs, and I think you'd use the 42mm only once in a blue moon. Assuming a 1200mm focal length, 30mm is 40x, and 17.5mm is 69x, not a very big step up in magnification. You could even have a 15mm be the next eyepiece up (80x).

Louis, 1) No 32mm Plössl has a 52° field--not one. The average is somewhare in the 45-49.5° range, the latter with a bit more RD. I've measured several different ones using the flashlight test. I have seen 52° in shorter focal lengths, though, but a typical range of Plössls will vary from 47-52° across the series. 2) 27.0mm is the average field stop of a full-field 32mm Plössl, which, on a 750mm focal length, yields 2.06°.

With my eyepieces, when used with glasses, they can go a few months between cleanings. With the eyepieces I don't use glasses with, a monthly cleaning after a full night at a star party is usually called for--especially if other people also look through the scope. Use Q-Tips and your choice of lens cleaning fluid--put a couple drops on the Q-Tip, not the lens, and clean the entire lens with the wet mop, then flip the Q-Tip over to absorb the junk on the lens, then grab a second Q-Tip and wipe until clean and streak free. The coatings are hard and not easily scratched if NEVER wiped when dry. I have had eyepieces cleaned over 100x that still looked brand new under a 10X loupe. There have been many recent threads and posts on eyepiece cleaning. A Search for "Cleaning eyepieces" using the search function at the top right returned many many posts on the subject.

I would have expected a bigger difference. But one measurement means more than any speculation.

Maximum field of view with 1.25" eyepieces is 27/750 x 57.2958 = 2.06°, not the 2.2° people have mentioned. That is still quite wide and good for the Beehive cluster, the Pleiades, etc. Your binoculars will have a larger maximum field, but at a much lower power. They won't have the resolution or light grasp of the telescope, but they will have a wider true field.

So the GPC 1.6x became 2.0x in the forward position.

Sure, there are two names for it: Rubber eyecup Rubber eyeguard

If you mean the rubber eyecup at the top, then yes. Often the mfr or importer will have spares and there are also after-market sources. You just have to shop for them--maybe make calls. In the US, AgenaAstro.com and Scopestuff.com. I'm sure there are UK sources as well. eBay is probably a decent source, as has been suggested. You would buy by diameter--there is no standard size. If you mean the rubber gripper band that goes around the outside of the upper barrel, then no. It is very rare for an importer to have spares. Maybe TeleVue. One possible solution is some thick o-rings, stacked in a set of 3 or 4. You could get the size that would be snug, and the groove in the eyepiece would hold them in place, and they'd have this cool '50s modern, space age sort of look. That would look nicer than a rubber band of the right size and width.

As long as the same eyepiece is used with or without the Barlow or GPC, then yes, magnification is inversely related to true field. So if you cannot focus without the GPC in front of the diagonal, it says the scope is not really compatible with binoviewers. You needn't modify the scope but you can lower the effective magnification by using a GPC with a lower magnification factor, like 1.0 or 1.2. Or use a binoviewer that doesn't require such a huge amount of in travel at the focuser. Or use a scope that has the amount of in travel necessary, like an SCT. Different diagonals are built in such a way that the length of the light path through the diagonal is not the same. Prism diagonals, for an example, have a shorter light path so automatically gain you some in focus. This has nothing to do with the focus in the field. Yes, two GPCs can have a negative effect on spherical aberration in the scope (which affects high powers most), vignetting, coma, etc. You are better off achieving high powers with shorter focal length eyepieces. Of course, shorter focal lengths in a binoviewer means longer focal length eyepieces than many will use for planetary observation because of the Barlow effect of the GPC. It isn't 1.6x in front of the binoviewer. As an experiment, try timing the eyepiece in the diagonal without the binoviewer to see what the transit time is, then time in the binoviewer with the necessary GPC. I think you'll be surprised at the magnification factor.

One of the main points is: observe often to catch good seeing conditions. And 11 or 12 of those things I mentioned can be done if you live in a city. Traveling to get to better seeing may not be likely, but most of those things people pay little attention to, yet they affect seeing.

A Barlow only magnifies by the set amount at one position back from its lens. Increasing that distance increases the magnification, decreasing that distance decreases the magnification. Since focal planes in eyepieces are not all at the shoulder, the magnification of a particular barlow will vary according to which eyepiece is in it. Complicating that even more is the fact that not all barlows have their rated power exactly at the end of the eyepiece tube. Eek. So there is a very easy way to tell the magnification of a barlow with a particular eyepiece: --time the passage of a star on the celestial equator across the field with the eyepiece alone. Convert that time to digital minutes (i.e. 2min.12sec. is 2.2min.) --time the passage of the star from edge to edge with the barlow or GPC in place. Convert that to digital minutes. --divide #1 by #2. Voilà! the magnification of that Barlow or GPC with that eyepiece (or whatever else happens to be in the focuser), regardless of where the GPC is placed. Some Good news: the 2" PowerMate 2X only changes magnification by 0.1x over 4 inches of travel away from the lens! So, to all intents an purposes, just assume it is 2X (remember that timing might yield 2.02 or something like that). The Glass Path Corrector (OCA) in the binoviewer has a set magnification when used with the binoviewer. But it is designed to be used attached directly to the binoviewer, not in front of the star diagonal. When used in front of the star diagonal, it magnifies by more, just like a Barlow. You will have to do the star timing trick to discover exactly by how much more. If you have 2 GPCs (and I would not advise it), one by the binoviewer and one a few inches farther away, then the magnification facor may be somewhat unpredictable since the internal light path of diagonals are NOT all the same in a given size of diagonal. Whatever that magnification is, just multiply it by 2 when used in a PowerMate, whether the PowerMate is used in front of a diagonal or in front of the binoviewer. The star timing trick doesn't even have to be a star on the celestial equator, but the star moves across the field fastest at that location, meaning you'll spend less time timing the passage.

In my 12.5" f/5 scope (f/5.75 with Paracorr), I see it this way in terms of performance: 21mm Ethos > 20mm APM XWA > ES 20mm 100° So if you're looking for a 100° eyepiece in 20mm +/-, get the Ethos. And if it's not affordable, get the APM (or Stellarvue Optimus). You mentioned the 17mm ES 92°. I recommend, instead, the 17mm Ethos, which I think slightly outperforms the 21mm Ethos. And unlike the ES 17mm 92°, it's not a boat anchor.

I owned both, and currently use the 22mm since I wear glasses at that exit pupil. Advantages of the 21mm: --wider apparent field and true field --distortion in field a little less apparent(mostly because it's farther out) --slightly better star sharpness than the 22 --slightly better contrast than the 22 (could be simply the magnification). Advantages of the 22mm: --longer eye relief --feels more immersive (hard to describe, but noticeable) --smaller, lighter --much less expensive --fewer elements internally --doesn't focus in as far as the 21mm --less rolling of the head to look directly at the edge of the field.

I did 3 timings of a star in the 24mm UFF and got a field diameter of 27.3mm +/-0.05mm (level of uncertainty caused by very slight fade out at the edge), for an eAFoV of 65.2° You measure using a ruler, correct? And in your case the uncertainty is from distance to the target and magnification error possibilities? Interestingly, timings of stars also gave me 36.4mm +/- 0.05mm for the 30mm field diameter. Yet, in daylight use, both eyepieces have visible rectilinear distortion in the form of pincushion. Why the 24mm has a larger eAFoV than its measured AFoV is a mystery. It should have barrel distortion. I did the same with all the UFF and got eAFoV approximately the figures claimed. I wonder if the actual field diameters were ever measured at the factory.

Louis, you're right. But I can't tolerate eyepieces with edge compression, or barrel distortion. It makes me feel the field is rolling over a ball or that I'm looking at the surface of a globe. It's why I don't own the 12.5mm Docter or the Nikon NAV-SWs. The good news is that eyepieces with edge compression are very rare among astronomy-oriented eyepieces. There are no Pentax, TeleVue, Explore Scientific, Stellarvue, or Baader eyepieces with it. The APM XWAs don't, but I'm not sure about the UFFs. It looks like the AFoVs claimed for the UFFs might be calculated eAFoVs. And since almost all eyepieces have varying degrees of radial stretching at the edge, I just don't find it objectionable or even noticeable, regardless of percentage. Judging from comments on this and many other forums, I think my point of view is a common one.

The 7mm Nirvana, which lab tests have shown to have a focal length of 7.5mm+ and an apparent field of 84°, and made by United Optics, is available under other labels: Meade Series 5000 PWA 7 Stellarvue UWA 82° 8 and used under William Optics UWA 7 Other companies are marketing re-badged versions of the Celestron Luminos 82° line, e.g. Omegon, Astrotech

To see whether your hypothesis is true, let's take a 13mm Ethos. The field stop is 22.3mm. Using the apparent field to calculate true field in my scope, TF = 0.712° Using the field stop, TF = 0.70°, a difference of 0.012° or only 1.7%, or 0.7' of arc. That's nothing. Using a 31mm Nagler, the AF calculation yields 1.392°, while the field stop calculation yields 1.318°, a difference of 0.074°, or 5.3%, and 4.44' So, you might be right--the smaller the field stop, the less the discrepancy between the true field calculated by apparent field versus field stop.

Distortion does not determine the true field seen, it only determines the apparent field. So comparing field stop diameters will definitely tell you which eyepiece has the wider true field. eAFOV is a calculation based on a poor formula, TF = AF/M, while field stop is a physical characteristic of the eyepiece. eAFOV is not a physical characteristic of the eyepiece, unlike AFoV. So in my Eyepieces Buyer's Guide, here is what the true field calculation is based on: IF the mfr's field stop diameter is listed (known), the true field is calculated by TF = (EPFS/TFL) x 57.296, where EPFS is eyepiece field stop and TFL is telescope focal length If the mfr's field stop is not known, it defaults to the calculated field stop using the formula (AF/57.296) x EPFL where AF is apparent field and EPFL is eyepiece focal length If the field stop cannot be calculated (like in a zoom), the true field shows as N/A. Using the spreadsheet, you can directly compare true fields of one eyepiece to another. A link to the spreadsheet (current as of April. I have a much more updated version but it is not posted anywhere): https://www.cloudynights.com/topic/758306-2021-eyepieces-buyers-guide/?p=10917573

It is a rare eyepiece where the field stop is the inside diameter of the eyepiece. Usually, there is a small iris in there which makes the field stop smaller than the barrel. The "usual" limits are 46mm for 2" eyepieces and 27mm for 1.25" eyepieces, though a few eyepieces sneak another 0.2-0.3mm onto that. Your Celestron 40mm has a 27mm field stop for true field calculations. Field stop is ALWAYS the limiting factor for true field of view because it ignores all distortion. What IS true is that field stop won't tell you the apparent field. The field stop of the Zoom changes as the internal lenses move. that is a result of the field lens moving. So there will not be a constant field stop diameter at all focal lengths. If it was constant, the field at 8mm would be 3x as wide as at 24mm instead of ~1.5x. But, the field stop is still the limiting factor for true field, even in a zoom.