Don Pensack

If you stare at the center of the field, no one with normal vision would have a problem seeing the edge of a 100 or 110° eyepiece in peripheral vision. Each eye has about a 145° lateral and 125° vertical field of vision. But, you are generally correct about 68-70° fields when you want to look at one edge and still see the other edge with peripheral vision. Turning you head on your neck to look directly at the edge becomes second nature when you do it for quite a few years. The fact peripheral vision cannot see the opposite edge 100° away never bothered me one iota. It's like having a stage too wide for the spotlight to see it all at one time. It's still nice to have the rest of the stage there when you want to look at it.

It's convenient to have your eyepieces at your fingertips, but in many places having them out like that, they'd be covered in dew or frost and very slippery to pick up. Not to mention they'd be a lot colder than if kept in a case, so your warm moist eye would fog the eyepiece up almost immediately.

1) Dielectric coatings on diagonal mirrors never need replacing, as they do not deteriorate with age, only cleaning. 2) you should remove the mirror for cleaning periodically and clean it just like you would clean a telescope primary mirror. Use only distilled water, and this method works great: https://www.youtube.com/watch?v=9Y8xFnXFVGQ

Though it kind of makes the wives depart quickly from the room.

I would rate the APM XWA line (and all the other labels they are sold under) as better, optically, than the ES 100s. Try one focal length first to see if you like the large apparent field. If you do, they come in 20mm, 13mm, 9mm, 7mm, 4.8mm (labeled 5), and 3.5mm. The latter 2 are 110°.

The term "Focal Extender" was first used commercially, I believe, by Meade or Celestron back in the '70s. It was a tube you added to the back of your SCT to extend backwards the focal point of the SCT, and it had camera T-Threads on the rear end so you could attach an SLR camera to the piece for prime focus imaging. I think the purpose for the device was to lengthen the focal length of the scope and increase the image scale so the Moon and planets could be imaged. Otherwise, I could see no reason not to make it quite short. Meade later introduced a line of 4-element Barlows they termed "Focal Extenders" to differentiate them from others in the market. They were made in China by Jing Hua Optical (JOC), and still are to this day, though they sell under other labels now, like Explore Scientific. Technically, ALL Barlows are focal extenders. That's what they do--extend the focal length. And, nearly every one moves the focal plane out as well, though how this impacts the focal point of an eyepiece is determined by the focal length of the Barlow and the length of the upper barrel above the lens. So the term "Focal Extender" does not necessarily mean the Barlow is telecentric. Telenegative Barlows are focal extenders too.

As they say, "there is a method to your madness". It sounds OK to me.

As long as the sky is quite dark. The ES Focal Extenders have more light scatter than the TeleVue PowerMates. If used for deep-sky objects and not planets and Moon, the ES and TeleVues are pretty interchangeable. The PowerMates are arfocal with the eyepieces used without the PowerMates. The ES require additional refocusing. That may or may not matter.

In a Maksutov, the long f/ratio means either a prism or mirror can be used. Pay attention to clear aperture diameter if using long focal length eyepieces with large field stops. In the refractor, it depends on the scope. If the scope is a triplet, or a doublet longer than f/12 (at 4") or f/9 (at 3") or f/18 (at 6"), then either type of diagonal. If the doublet is shorter than that, and the scope is not a triplet, then I'd use a mirror. You don't want to add any CA to what the scope already has.

One Zoom that gives sharp images, excellent contrast, and adequate eye relief PLUS a constant 66-67° over its range is the APM 7.7-15.4mm "Super Zoom". You can run into in-focus issues if it's used as a 1.25" eyepiece, but used as a 2" one shouldn't have focus issues. I liked the optics a bit better than the Baader over the same range (8-16mm on the Baader). Maybe it doesn't qualify as cheap, though.

1) the lower (black) caps on Delites require too much pressure to install and remove. I suggest changing the lower caps to a type that fits looser. 2) Remove the collet and rings and make sure the eyepiece is cleaned of all grease or oil. I used a small alcohol pad. 3) always use two hands to loosen and tighten the collet in the eyecup. I was able to tighten the Delite eyecups sufficiently it would take a hammer to move the eyecup. 4) When removing the lower cap, grab the eyepiece below the shoulder so that pulling the bottom cap off doesn't put upward pressure on the eyecup. The top cap comes of so easily you needn't worry about that.

They do. That eyecup style originated with the Delos.

I just looked at one, and it appears the outer surfaces are both concave, but the lens facing the eyepiece is more concave than the one facing away from the eyepiece. I did not dismantle the lens assembly to see the orientation internally, however, if there is more than one lens.

That drawing is correct if the negative lens is flat on one side. If it is bi-concave, it could be in either direction, though only one direction would be correct.

Louis, You might find these measurements useful if considering other focal lengths of Morpheus eyepieces: I found I could use down to an effective eye relief of 14.3mm with glasses and still see the entire field. The Morpheus 6.5mm had a little more effective eye relief than the 14mm. Eyepiece............eye relief/depth of lens/effective eye relief from rubber(folded down) up to exit pupil: APM UFF 30.0 22.0 -4.90 17.10 NAGLER 22.0 19.0 -3.00 16.00 MORPHEUS 17.5 23.0 -2.30 20.70 MORPHEUS 14.0 18.5 -2.30 16.20 MORPHEUS 12.5 20.0 -2.30 17.70 APOLLO 11.0 18.0 -3.70 14.30 MORPHEUS 9.0 21.0 -1.90 19.10 ETHOS 8.0 15.0 -3.20 11.80 APM XWA 7.0 13.0 -4.00 9.00 MORPHEUS 6.5 18.5 -1.70 16.80 ETHOS 6.0 15.0 -3.10 11.90 ETHOS SX 4.7 15.0 -5.25 9.75 MORPHEUS 4.5 17.5 -2.10 15.40 ETHOS SX 3.7 15.0 -5.20 9.80

Very interesting. Nearly everyone who uses the 24mm Hyperion in a scope of f/6 or shorter eventually sells it because only the center 50% of the field is sharp. It does have more eye relief than the Panoptic, though. Then there is the APM UFF 24mm, which has the eye relief and better sharpness at f/6 and lower.

Fill the undercuts with metal tape (copper and aluminum both easily acquired), and then the eyepieces can be inserted and removed without catching. Interesting. I found the 31 Nagler sharp to the edge and the 35 Panoptic had field curvature. That's in a dob of 1825mm focal length, however.

The Delos eyepieces are made in Taiwan of Japanese glass.

And you would be right. The best eyepiece in a line is heavily dependent on the scope and the skies and expectations of the observer. And, what is best, anyway? Best sharpness at the edge? Least chromatic aberration? Flattest field? Most in-focus field stop? Best contrast? Longest eye relief? Best eyecup design? Lightest weight? Smallest size? Best external polish and plating? Best rendition of colors? etc. No one eyepiece is likely to be the best in all categories of evaluation, and there is no perfect eyepiece. Still, if you live with a complete line of eyepieces and use them over time, you will form an opinion about which one in the series is best. There may not be just one best, either, or one might be sharpest but another yield the best contrast. How then would you identify the best in the line? Still, this is a fun thread and it will be interesting to see what people come up with. Will there be a consensus on any eyepiece?

A particular design may be close to perfect in one focal length, but once the lens diameters and spacing start changing, differences crop up. And if the lineup is not scaled, but tries to maintain a constant eye relief among all the focal lengths, then each eyepiece in the line will have to be different internally to accomplish that. And one focal length will then be the one closer to the ideal configuration for the eye relief and lens configuration, with small compromises in the other focal lengths. Plus, in any lineup, the number of elements may change as you go from long to short focal lengths. Now, each eyepiece focal length could be completely independently designed, but no designer is likely to do that when another focal length can be created merely by lengthening the distance between negative field lens and upper section of the eyepiece. And, people come and go in companies, so not every focal length may have been designed by the same optical designer. There is also a variation from sample to sample in the same model and focal length, but those differences are a magnitude smaller than the differences between focal lengths. Nonetheless, if an observer were completely fussy about getting a particular eyepiece as close to perfection as possible, it might be desirable to buy 5 of a particular eyepiece and compare all five in the scope on the same night. I've done so with 3 samples of the same eyepiece or star diagonal and seen noticeable differences, albeit quite small.

TeleVue Delos 8mm was sharper than the 8mm Ethos in my scope on the same night.

Interesting post. My scope is a 12.5" f/5 (with Paracorr f/5.75). These are the best in series to my eye, from over the years: TeleVue: Plössl--32mm; used--21mm original Delite--18.2mm Panoptic--24mm; used--22mm Delos--10mm Nagler--22mm T4 and 13mm/5mm T6 tie Apollo 11--only used, but perhaps the best eyepiece TeleVue ever produced. Ethos--6mm Baader: Hyperion--17mm Morpheus--3-way tie among 12.5mm, 9mm, 4.5mm Pentax: XW 70--7mm XW 85--16.5mm APM: UFF--30mm XWA--7mm Explore Scientific: 52°--no favorite 62°--26mm 68°--20mm 82°--6.7mm 92°--17mm 100°--5.5mm Now I wear glasses at the eyepiece and my favorites have changed, but the above were from my tests of complete sets.

Understood, but that's unlikely to ever happen. 4.5mm, increased by 40% is ~6.5mm. 6.5mm becomes 9mm 9mm becomes 12.5mm. And 12.5mm becomes 17.5mm. So there was a rationale for the focal lengths. Pentax XWs follow the same % progression, but start smaller: 3.5mm, 5mm, 7mm, 10mm, 14mm, 20mm Whence comes the 14mm, then? Well, in the original internal design, it looks like 14mm was about the longest focal length that could be made, so we got a 14mm to complete the set. But, 3 years+ later, the 17.5mm came out, after some unsuccessful prototypes. I think it is different internally to the other focal lengths, which is one reason it has a slightly narrower apparent field (72° versus 78-79°) The first 17.5mm prototype (maybe the design on paper before the prototypes), which didn't work, is the data in Baader's spec chart. They never updated the chart when the final version came out. If you read Ernest Maratovich's charts and reviews, you'll also see they actually missed the exact focal lengths as well. He measured: 17.2mm, 13.9mm, 12.4mm, 8.9mm, 6.7mm, and 4.8mm. This is far from the only line of eyepieces where all the focal lengths are off, and some other eyepieces are in error by more than 0.5mm. If we start measuring actual focal lengths and apparent fields, though, the market becomes filled with chaos and lots of exotic focal lengths. How much does it matter in the field if your 13mm 100° eyepiece is actually 13.3mm and 101°? It really doesn't.

I don't know, and the eyepiece has not appeared under other brand labels. Maybe someone else has seen the eyepiece in China or on AliExpress from someone else?

It's the same. The manufacturer is KunMing United Optics (KUO), and Sky Rover is their "house brand". They also sell eyepieces private label to at least 10 companies, maybe more.