Don Pensack

P.S. the 13mm Hyperion shares this issue, just to a somewhat lesser degree.

My problem with the Barsta 70° series (AF70/Redline/Olivon/et.al) 13mm is that it has the worst Edge of Field Brightening (EOFB) of any eyepiece I've ever seen, extending inward from the field edge 50% of the field radius and making the entire field look like you are looking into the center of a planetary nebula as large as the field. It's even been photographed:

Here is a chart showing AMD and RD. Notice both increase as the field gets wider. If AMD is corrected, the uncorrected RD is the span between the curves, or vice versa.

Examples of lower edge magnification than center: Docter 12.5mm, APM Hi-FW 12.5mm, et.al. I have seen many many eyepieces with some AMD, and they ALWAYS have had lower magnification at the edge than in the center. Can you name one eyepiece where the edge has a higher magnification than in the center?

Yes, Morpheus all the way.

An alternative for the Mak is the 40mm 1.25" Plössl. The exit pupil is larger (brighter image). The magnification is lower (~39x), and the eye relief is a bit longer than a 32mm Plössl. Apparent field will be 40-43°, about that of an Abbe orthoscopic. **note: your "127mm Maksutov actually has a clear aperture of 121mm and a focal length with 1.25" diagonal of 1540mm, making the scope f/12.7. That makes the exit pupil of a 40mm only 3.1mm, so a 40mm will be about the brightest image you can get. True field will be 1.00° If the visual back is shortened (the threaded portion on the visual back is removed), the focal length shortens to 1518mm and maximum true field increases. The UFF 24mm actually gives you a bit more true field due to its larger field stop--1.03°, though 64x as a lowest power is a bit high for that aperture. So I'll agree with Louis--a 32mm Plössl is a good compromise--48x and a 1° field.

If the flagpole is a cylinder (likely), then there is some angular magnification distortion in your images. The edge looks more magnified in both eyepieces. I suspect this is in your camera as AMD in eyepieces is almost always the reverse--the edge has a lower magnification. Your images also reveal the edge vignetting in the 16mm (often reported), and a slight brightening of the edge n the 17.5mm (not reported). Also, the 17.5mm has the common edge of field CA--the blue ring, as in most complex negative/positive designs. The 16mm appears to have none--it could simply be the vignetting that reduces its visibility.

The spacer length depends on what eyepiece you use that has the most in-focus, not the f/ratio of the scope. The "working distance" for the lens is 75 +/- 5mm from the focal plane of the eyepiece. But 2" eyepieces have their focal planes from more than an inch down into the lower barrel (below the "shoulder") all the way up to almost an inch above the shoulder. To correctly choose the right spacer between the upper section of the GSO coma corrector and the lens, you first need to do a couple things: 1) determine where the focal plane of your scope is. This can be done by putting translucent scotch tape across the 2" opening of the focuser, pointing the scope at the Moon, then focusing the moon on the tape. That is where your scope's focal plane is. Measure how far out the focuser is and write it down. 2) Insert the eyepiece you own that you know takes the most in-focus (or experiment to see which one it is). Put it in the scope and focus the scope on the Moon. Measure how far out the focuser is and write it down. It will be a shorter distance than the focus on the tape. The in-focus distance that eyepiece requires is the key figure. Write it down. 3) measure the length of the top section of the GSO coma corrector and subtract that from 75mm. Write down that dimension. The spacer length you need will be 75mm minus the length of the top of the coma corrector MINUS the in focus distance for the eyepiece that needed the most in focus. [example: 31mm Nagler needs about a 10mm in travel from the focal plane of the scope. 75-46-10=19mm spacer] If a spacer of that length is inserted in the CC, the focal plane of the eyepiece will be the correct 75mm distance from the lens when inserted in the CC. Then: 4) insert the eyepiece in the CC with the correct spacer added, and focus the scope. Lock the focuser (or just don't move it). For every other eyepiece you own, insert it in the CC and slide it out of the CC until it is in focus. That will be the correct position for that eyepiece relative to the CC lens. All your eyepieces will focus at the same place in the focuser travel when the CC is installed and the eyepieces correctly set. Since that is not easy to do every time, I recommend parfocalizing rings be installed on your eyepieces so each one can be returned to exactly the same place each time it is inserted. You can save time by parfocalizing all your eyepieces with the one that needs the most in-focus before you get the CC. Then, after you use that in-most eyepiece to determine the spacer length in your CC, all your eyepieces will be simple "drop-in" accessories that will already be in focus when inserted (+/- 0.5mm).

They're only $249 in the US, though. You won't get them any cheaper from the EU.

4" f/8.6 = FL of 875mm, I would round off. If it's a decent instrument, and it is, magnifications up to 240x are possible. That is a 3.6mm eyepiece. If you have the scope mounted on a tracking mount, the 3-6mm TeleVue Zoom is a strong possibility. It's not just a single eyepiece, it's a whole set of planetary eyepieces in one because all the in-between magnifications can be used. If you have the mount on a non-tracking mount, then I would not recommend a 50° eyepiece. I would try to go wider--at least to the 62° of the TeleVue Delites (which come in 3mm, 4mm, 5mm, 7mm, etc.) or to even wider apparent fields like 68-80°. You may hear endless comments about how this eyepiece or that eyepiece is a better planetary eyepiece, but if you have to push the mount so often you only have a few seconds when the scope has settled down to yield a sharp steady image before you have to push it again, those super-high magnifications will not be enjoyable to use. It's a reason I use a 3.7mm 110° eyepiece a lot for planetary and lunar observing in my 4" refractor. If the seeing supports such a high magnification, the eyepiece in the focuser will give a great view, even if it's not a Zeiss, or Takahashi. All that presumes you don't need glasses at such small exit pupils, and few observers do. What may be the case, however, is that the really small exit pupils might start showing you floaters in your own eye. My 4", for instance, has great optics and can easily support a 3mm eyepiece, but my vision cannot. Floaters interfere seriously when the exit pupil gets smaller than ~0.6mm (roughly a 4mm eyepiece). I can only go smaller on double stars. So that is the one caveat I would mention for using eyepieces shorter than, say, 4.5mm in your scope. For double stars, the limit is only determined by seeing and telescope optical quality.

The Leica is 8.9-17.8mm, so not as applicable as the APM 7.7-15.4mm It is also a LOT more expensive and needs a special adapter for telescope use. The APM has a wider apparent field in the 15mm range as well.

I compared the 30mm APM UltraFlatField, 31mm Baader Hyperion, 30mm ES 82°, 31mm TeleVue Nagler and 30mm Pentax XW. I kept the 30mm APM. It's available now under several labels. For purchasers in the UK, the StellaLyra UFF is a no-brainer. Highly recommended. It has superb contrast in its focal length range and a truly flat field. It's usable with glasses, the field stop diameter is 36.3mm.

There is a 100mm 1.25" eyepiece from Vixen. I read it has an 11° apparent field, but I think that is overstated on that one. The apparent field in 1.25" eyepieces is directly related to focal length: Here are theoretical maxima in simple eyepiece designs: 24mm--68° 32mm--50° 40mm--43° 55mm--28° 70mm--22° 100mm--15° So what you want is not physically possible in 1.25". If you expand that to 2", 100mm maxes out at 26° If you expand that to 3", 100mm maxes out with 40°. That's about the same as 1.25" in 40mm, but, of course, 3" eyepieces and binoviewers don't match.

A Barlow can be telecentric or telenegative. As far as a 2X magnification is concerned, either type will do the job. Your ES is a telecentric Barlow but will work fine to provide the magnification you need (Focal Extender is merely an advertising name). Morpheus eyepieces work fine in Barlows. Assuming that you have the seeing conditions to support a 3.25mm eyepiece (220x), it will be a good test of whether floaters are a problem for you at that high a magnification. If they aren't, the 3.5mm XW would be a possibility, though there isn't anything wrong with using the Barlow, either.

My advice is to achieve the longer f/ratio with a 25-26mm Plössl and a 2X Barlow instead. You will still have a 45-50° apparent field instead of a 28° apparent field (55mm 1.25" Plössl)

When you say "good eye relief", do you mean compatible with glasses? If so, there are some possibilities with medium to wide apparent fields: TeleVue Delite 4mm (179x) or 3mm (238x) TeleVue Delos 3.5mm (204x) Pentax XW 3.5mm (204x) Stellalyra 80° 4mm (179x) If you mean just comfortable eye relief lone enough to avoid accidental eyelash contact with the eyepiece, I would add: TeleVue Ethos 3.7mm (193x) APM XWA 3.5mm (204x) TeleVue Nagler Type 6 3.5mm (204x) There are others, of course, at lower price points. And, of course, adding a 2X Barlow lens to anything from 6-8mm would put you there as well. Anything narrower that 60° of field will require a lot of pushing of the scope and smooth mount movements. With a tracking mount, that doesn't matter. I own a 102mm refractor of the same focal ratio/focal length. I would caution you about small exit pupils and floaters. You will experience no issues with floaters in the high power if it is for double stars. But if it is for the Moon or planets, floaters may interfere strongly. I cannot use a 3mm eyepiece on the Moon or planets not because the scope or seeing cannot support it but because my eye has too much junk in it to yield an image without visible floaters. I max out at 193x (a 3.7mm eyepiece), with its 0.53mm exit pupil, and some of the time max out at 159x (0.64mm exit pupil) because of that. I have a scope with >2.5x the focal length for when I want really high powers without floaters. In that scope, 493x is a 0.64mm exit pupil. You know your own eyes, but were I to advise an approach, I would try Barlowing a lower power eyepiece first to see if floaters interfere instead of getting an expensive wider field eyepiece with adequate eye relief. Then you'll know if you can use such a small exit pupil without the issue of floaters.

A wide variety of emission nebulae, you mean. It serves no useful function for anything with stars, or dark nebulae, or reflection nebulae.

As they say, "Nostalgia clouds your vision". I have re-bought eyepieces many times, and they never were as good the second time. You are not the same observer the 2nd time--more experienced, more knowledgeable about the performance of the eyepieces and how to evaluate them, different scopes, different eyes. Also you develop different preferences. I was excited when I got my two highest-power eyepieces because they were 110°. Now I wish they were 100° instead. Of course, I'd never have known that if I hadn't tried them. They're still good eyepieces, but......

You wrote this post about me: glasses, vision, eyepieces, regrets not getting some scopes earlier in life. Parallel paths.

Agreed. Too bad the filters in those boxes weren't very good. Well, at least they're cheap.

Favorite Broadband: Baader UHC-S. Best view of M20 in a dark sky. Does well on reflection nebulae, which is uncommon in filters with this much contrast. Honorable mention: Lumicon Deep Sky. Favorite Narrowband filter: DGM NPB. Best view of M8/M17/M16/etc. Honorable mention: TeleVue BandMate II Nebustar, Lumicon Gen.3 UHC, Astronomik UHC Visual, Orion Ultrablock (if you get lucky with QC) Favorite O-III filter: TeleVue BandMate II O-III (my highest transmission O-III filter for BOTH O-III lines). Honorable mention: Astronomik O-III Visual 12nm Favorite H-ß filter: TeleVue BandMate II H-ß (highest transmission H-ß filter I've owned). Honorable mention: Astronomik H-ß Favorite planetary filter: Baader Contrast Booster Honorable mention: Baader Moon & Sky Glow filter. Note: Baader does not have visual filters in these categories except the UHC-S The other filters they make are really oriented to astrophotography, not visual. There are some discontinued filters I would add to the honorable mention lists, but since they aren't serial numbered, I could not recommend buying a used one because it could be from a different year. What you're looking for: >90% transmission at: 60-70nm for broadband 22-27nm for narrowband 11-13nm for O-III (dual band 495.9nm and 500.7nm). 8-10nm for H-ß

One thing to remember about field curvature, whether it comes from the scope or eyepiece, and that is, "always focus on a star 50% of the way from center to edge". If you have any accommodation left in your eyes, you will be able to focus the entire field that way. If you focus solely on the center, the edge will be out of focus or vice versa.

Printed on the eyepieces by people who don't read English. Vixen's Japanese specs state the 2.5mm, 4mm, 5mm, and 6mm SLV eyepieces have 45° fields, while the longer focal lengths are 50°.

I don't see a test for the 4.7, but here is the 3.7mm: Tele Vue, Ethos SX 3.7 112 <4 6-8 6-9 diffr! diffr. diffr?? Personally, I don't think the 4.7mm quite measures up to the 6mm or 3.7mm. It's closer to the performance of the 8mm--a whisker below the 6mm. Note that the eye lenses on the 3.7mm and 4.7mm are recessed more than the longer focal lengths. The eye relief feels a lot tighter.

Agreed, I think few would spend that much. But how many of those small narrowfield eyepieces can also be used for deep sky purposes on a non-tracking mount? And have you looked at the prices of used Zeiss Abbe orthos or TMB SuperMonocentrics?