Don Pensack

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.

The field stop diameter is the limiting factor for true field at all magnifications/eyepiece focal lengths. Think of field stop as a circular opening in an index card, laid down on a map (the focal plane of your scope). The opening determines how much map you see. The scale of the image on the telescope's focal plane determines how much sky that opening sees. Image scale of a telescope in degrees is (1/telescope focal length) x 57.296. Multiply by 60 to get the image scale in minutes of arc. Exit pupil is not affected by field stop diameter.

Yes. There are multiple reasons: 1) magnification. A 1" separation at 180x is an effective 3' separation. 2) resolution. 1' is about the limit of resolution for the human eye. A 4.5" telescope can resolve a 1" separation! 3) exit pupil. Visual acuity is higher when the pupil used is smaller than the dark adapted pupil. A 2-3mm exit pupil behind the eyepiece will allow your eye to resolve better than the full dark adapted pupil diameter. Far fewer aberrations in the eye can be seen. Many years ago, someone posted a chart I'll try to remember: 1' of arc--the limit of human vision 3' of arc--the typical limit for a person with good, but not superb, vision 4' of arc--the average resolution without strain--fairly easy for most experienced observers. 8' of arc--an easy resolution for the average eye without perfect correction of vision--just about everyone can see this easily. How that translates: a star with a 1" separation will be seen as double by the person with a resolution of: 1' at 60x 3' at 180x 4' at 240x 8' at 480x. Perhaps that will give you an insight into why people need such radically different magnifications to split close double stars. I can see ε Lyrae as double with the naked eye with glasses on. In a scope, that translates to 78x to see all 4, and that is close to accurate--I can see all 4 stars at 83x fairly easily. An easy split by everyone, though, might require 209x. Certainly, seeing them all is much easier at the higher magnification. People's eyes differ a lot, and scope quality and seeing can interfere as well, so there is no set magnification to see any double star.

Seeing the 3' separation of the two pairs in ε Lyrae with the naked eye is not a very stringent test of naked eye acuity. If you cannot see it as double, then you should make an appointment with the eye doctor, because your prescription is out of date. I admit, I was in that camp--I could see them as an elongated single star. I had my eyes tested and got a better prescription and Voilà!, there was a close, but easy, pair. Ironically, those same glasses improved the view through all my eyepieces, too.

In essence, if you want to know the field stop diameter that simply ignores distortion, do a star timing and you'll be very accurate. If you want to know the apparent field, use the flashlight test (or a similarly-measured test) and get a figure that is +/- 0.5°. AFoV won't allow you to calculate TFoV, but the field diameter will. And be a LOT more accurate than TF = AF/M Example: 24mm Panoptic, using the TF = AF/M in my scope = 0.894° 24mm Panoptic, using field stop diameter and telescope focal length TF = (FS/TFL) x 57.2958 = 0.847°, which is 5.3% smaller. [this is the reason the Astronomy Tools field comparator is a bogus tool--it uses the TF=AF/M formula to calculate the field] Neither formula is easier or harder than the other, so starting with field stop is just easier than using the field stop diameter to calculate a fictitious effective apparent field to fit the first formula.

OK, the 27.0mm field stop in the 24mm Panoptic is consistent with an eyepiece with a 64.5° field and zero distortion. I don't see what value that information has, because the eyepiece has a 68° apparent field. Yes, it's due to pincushion distortion, but all eyepieces of that focal length and widest field for 1.25" have distortion. The APM 24mm UFF has a 27.3mm field stop (determined by star timing). That is consistent with a 65.2° apparent field. Is it orthoscopic? No, because people who have measured the apparent field get 63° +/-, so the eyepiece's distortion characteristics modify the apparent field. Of what value is knowing the calculated eAFOV is 65.2°? None. At best, such a measurement might give you a clue as to the amount and type of distortion, but, even knowing that, the eAFOV figure doesn't have any value, because that is not what you see. You see the AFOV and you see the TFOV indicated by the Field diameter. Trying to adjust the AFOV so it can derive a TFOV simply doesn't make sense, because you have to know the actual field stop before you can derive the eAFOV, and if you know that, what's the point of eAFOV?

OK, got it. The inner field lenses turn no matter how the Barlow is configured. So if the Barlow is attached to the 1.25" tube that is attached to the lower section, it doesn't turn, though the field lenses in the zoom are turning above it. But if it's connected to the field lens section itself it will turn with the upper section. In some Zooms, the field lenses (or interior lenses) move up and down but do not turn. In the Baader, obviously the field lenses move up and down by turning. Thank you, by the way, for posting the short video. I appreciate it. I guess I'd always attached the barlow to the 1.25" tube.

I still don't get it. In neither case does the section with the name Hyperion on it rotate. In both cases, the section with the rubber gripper does rotate.

What Louis said and: The 17.5mm needs to be 2.5mm farther in, closer to the CC lens, to become parfocal with all the other focal lengths. All the Morpheus eyepieces have their focal planes at the 1.25" to 2" transition shoulder. Except the 17.5mm, which is 2.5mm higher, in the 2" section, so it needs to be 2.5mm lower to become parfocal with the others.

It makes no difference in the eyepiece. In fact, N2 leaks slower than Ar. The reason Ar is used is related to down time in the factory. The seals in the machinery in which the eyepieces are purged and get final assembly tend to react under pressure with N2. This means every few months the machines have to be shut down and seals changed. That costs the factory in terms of maintenance and also lost production during down time. Ar costs more, but saves the company a lot of money because it is not reactive with the seals in the machinery. Hence, less downtime, lower maintenance costs and higher production. Technically, air and N2 have virtually identical transparencies and indices of refraction. Ar is a little different. Ar is only 1% of the Earth's air, while N2 is 78%. Due to the low density of gas in the eyepiece and the length of the eyepieces, it will make zero optical difference.

They are different designs, with a few more mm of eye relief (still not glasses-compatible, though). But, control of internal light scatter is not as good as the older ones in my testing (in fact, it's quite poor), so I'd still hunt for the originals: 8.8mm, 6.7mm, and 4.7mm instead of opting for the newer 8.5mm, 6.5mm , and 4.5mm.

I would really hunt for one. I think JOC might have discontinued the 8.8mm in favor of the 8.5mm, as they recently discontinued the 6.7mm in favor of the 6.5mm. I would look before the market place is completely dry of them. You might have to look for US as well as EU sources.

In this case, all focal lengths have a long eye relief, so there is no disadvantage to the short focal lengths. Note that almost all binoviewers have a magnifying nosepiece on the front end, so a 10mm eyepiece might be operating as a 5mm eyepiece in the binoviewers. Hence, very short focal lengths might not be necessary.

I'm confused. When I attach the Barlow to the eyepiece and place it in an 1.25" adapter, the thumbscrew tightens on the Barlow. It does not turn with the eyepiece, and neither does the 1.25" barrel on the eyepiece above it. The upper section of the eyepiece turns relative to the bottom, and the zoom works fine. When I attach the 2" skirt and insert the eyepiece and tighten it down, neither the 2" skirt nor the 1.25" barrel turns as the zoom is moved, though the upper section of the eyepiece does turn relative to the bottom. So I have no clue what is being discussed here. If the barlow is attached and the eyepiece tightening in a 1.25" focuser or adapter, the 1.25" barrel of the eyepiece above the Barlow cannot turn. If it turns, it's being unscrewed from the eyepiece. I don't believe the Barlow can be attached to the 2" skirt on the eyepiece. Here it is attached in 1.25" mode.

I start by putting a couple drops of cleaning fluid directly on the Q-Tip (after blowing any dust off the eyepiece) and making a spiral pass from center to edge with the wet end of the Q-Tip. I immediately flip it over and make a couple spiral passes with the dry end of the Q-Tip. I then grab a 2nd Q-Tip and, making a radial motion from just a little past center, to the edge, slowly turn the eyepiece under the moving Q-Tip (maybe a couple turns of the eyepiece) until the lens is clean and free of streaks. This motion is perpendicular to the first Q-Tip motions and seems to eliminate any streaking. I note that when the 2nd Q-tip is moving on the surface of the lens, there is no resistance to motion, as if the lens was a teflon surface. That is because the lens is completely clean and free of anything to cause the Q-Tip to stick. 2 Q-Tips is usually all it takes. If an eyepiece is really dirty and covered with goo, it might take a couple turns with a wet Q-tip to get it all up. I don't have any issue with getting to the edge of the lens. I use regular Q-tips with cotton ends and no additives.

OK, so it is between elements held in with retaining rings. You could dismantle the eyepiece and blow it off, or send it to someone who would do it for you.

Why would that be? Wouldn't the Barlow attached to the eyepiece and tightened into a 1.25" adapter allow the eyepiece to zoom if the combo is used as 1.25?

These: https://www.amazon.com/Alcohol-Wipes-Antiseptic-Sanitizer-Individually/dp/B07C8GXG3V/ref=asc_df_B07C8GXG3V/?tag=hyprod-20&linkCode=df0&hvadid=242002247899&hvpos=&hvnetw=g&hvrand=6054868559722515470&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9030974&hvtargid=pla-450528700298&psc=1 As far as I know every doctor's office and every hospital uses these by the tons.

Ah, you are referring to the adapter that comes with the Barlow, not something that is part of the eyepiece.

Are those Zeiss wipes like the small alcohol-laden pads that come in small envelopes that are used to clean your arm before you get a jab?

It seems to me that breathing on the eyepiece after cleaning it defeats the purpose of cleaning it in the first place. It's impossible, as Covid has taught us, to exhale without some droplets from the mouth, throat, and lungs coming out with your breath. So to finish cleaning with your breath has just returned the eyepiece to an unclean state of affairs. If you've cleaned the eyepiece and left streaks, you either did not clean the eyepiece correctly or are using the wrong cleaning fluid. I've been cleaning eyepieces for just under 6 decades now, and never left streaks if I paid attention to what I was doing. I will say, however, that some eyepieces I've cleaned were so dirty it took 2 to 3 cleanings to actually get them completely clean. A few years back, I cleaned an eyepiece in the shop that looked like a little kid had tried to color the lens with a crayon. That one was really tough to clean! One last note: NEVER use "lens cleaning tissues" as sold in camera stores in a little packet. These have small wood fibers in them and WILL scratch the lenses. Like lens pens (shudder), I don't know why these things are still sold.