Don Pensack

Almost every "Orthoscopic" eyepiece has some rectilinear distortion. The question is whether it can be seen or not. People seem to like the Baader Classic Orthos, yet these have quite a bit of RD. I really don't know why people object to a small amount of RD yet tolerate horrible astigmatism or field curvature. It might not be orthoscopic, but give me an eyepiece without astigmatism and field curvature first, THEN think about how the distortion is viewed. Plus, we see up to about a 7% level of RD as distortionless because of the nature of the eye. By the way, it is usually the control of distortion that identifies the eyepiece as orthoscopic, i.e. no distortion of a square grid across the field. Since that means near zero angular magnification distortion AND near zero rectilinear distortion, it means 40° fields or less. Even 42° is pushing into distortion territory. See: Note that there will be no visible distortion if the curves are together and do not separate.

Read this and the thread that follows: https://www.cloudynights.com/topic/700422-starbase-orthos-first-light/ They are distributed by Takahashi in the US, which is why the association.

It's a small flake of the black paint used to reduce light scatter inside the eyepiece. Try Louis's remedy. If that doesn't work, turn the eyepiece over and unscrew the chrome barrel. That should expose the internal lens surfaces close to the focal plane, which is where the flake probably is. Blow it off with your blower bulb and screw the chrome barrel back on. Easy. Successful surgery completed. LOL.

Not clear on what you mean by this. The 1.25" section of eyepiece is never removed. I presume you mean the 1.25" threaded adapter that comes with the zoom lens?

If the glasses touch the eyepieces, the first thing they touch is the folded down rubber eyecup, which always sticks up a mm or two above the eye lens of the eyepiece.

Only if it's done wrong. After well more than 100 cleanings, most of my eyepieces show no scratches and a perfectly clean surface with a 10x loupe inspecting the surface. Multicoatings are hard. I don't recommend the use of microfiber cloths in general, though the little ones that feel like silk can be very good. I prefer Q-Tips because they are 1 pass and discarded--always perfectly clean.

I have both the 30mm APM (61x) and the 22mm Nagler (83x). The 22 gets 90% of the eyepiece time. Field sizes are 36.4mm and 31.1mm respectively. In my scope, 1.14° and 0.98° respectively. That makes the 30mm 17% wider in true field. I use it when the wider field and lower magnification are needed, which isn't often. Normally, I prefer the darker background of the 22mm, and that is in a site with a 21.4+ sky (~mag.6.9NELM).

I look at my eyepieces between each session and clean if necessary. The ones I use with glasses rarely need cleaning. The ones I use without glasses sometimes get cleaned each time, sometimes every other time. I have had eyepieces 20+ years old, cleaned >100x, which still looked new.

I lent the 12.5mm APM to a friend and he commented, unsolicited, "the edge is brighter". He noticed it right away. I have seen a lot of eyepieces with EOFB, and on a scale of 10. 10 being the worst, this one is a 4--noticeable, but not so bad the eyepiece is unusable. It is comfortable to use and quite sharp, with well-controlled field curvature and chromatic aberration. I suggest soliciting comments from users to develop a consensus on the eyepiece.

Acetone won't harm glass or metal, but it will dissolve plastics and rubber, so be sure to remove any rubber around the cleaning area (like an eyecup). It won't harm coatings at all. In fact, it is the cleaning material of choice at TeleVue.

No, The 19mm and 24mm Panoptics, and the 10mm and 15mm APM Ultra Flat Field are not compatible with glasses. The 18mm and longer in the Ultra Flat Field are compatible, as are 27mm and longer Panoptics. Other Glasses-compatible eyepieces from TeleVue: Delites, Delos, 32mm and 40mm Plössl, 22 and 31mm Nagler Other glasses-compatible eyepieces from APM: 12.5mm Hi-FW 84° Some others: Baader Hyperions, Baader Morpheus, Pentax XW to name a few. But, no eyeglasses required, then great binoview eyepieces include the 24mm and 19mm Panoptics, the 16mm Nagler, The APM Ultra Flat Field 18mm, The Type 6 Naglers, to name a few.

It will be. And, thusly, you start down the road to finding eyepieces compatible with glasses. As the focal lengths get shorter, you will need glasses less and less until, with some particular focal length and shorter, you will no longer need to use glasses. For me, that is 9mm (1.57mm exit pupil), but you will likely be at a longer focal length if you see only a minor improvement in the stars with the naked eye. For me, naked eye stars look like stick men with long hair, without glasses, and tiny points, with glasses. Your astigmatism is minor if you only see a small bit of improvement with distance vision glasses.

Very good! You didn't quite get it all, but since the eyepiece will need cleaning again, you can wait till next time.

If it is fungus, you must remove it immediately or it will spread. Try acetone on a Q-Tip first, something stronger if acetone doesn't work.

It's difficult to know if the 18mm is a negative/positive design without dismantling one. That information is not published anywhere.

Clean the outside of the lens first. Better get used to it, as all eyepieces need frequent cleaning. It's not hard--just use cotton Q-Tips and isopropyl alcohol or a lens cleaner fluid from Nikon, Zeiss, Baader, or ROR. That looks to me as if it is all on the outside surface and would clean off immediately. THEN look at it again to see if anything is inside. Till the outside is cleaned, you won't be able to tell. If the spots are inside (and it is unlikely), they can also be cleaned, but the eyepiece will need dismantling to do so, which requires a bit of skill and a couple tools. I would, since you're a newbie, then send it directly to TeleVue. They'll clean it inside and out for a fee. Their phone number is on their website.

The 18mm has one fewer lens than the 15mm. The 15mm is 6 elements, the 18mm is 5. Either the negative lens is gone, and the correction is applied in the upper positive design, or a very weak negative lens is place close to the upper section.

This eyepiece is excellent, and recommended. It's available under the names APM Ultra Flat Field, Meade UHD, Celestron Ultima Edge, Altair Ultra Flat Field, Tecnosky Ultra Flat, and Orion (US) Ultra Flat. Some differences: APM--all aluminum, 1.25", smooth barrel, no undercuts Meade--all aluminum, 1.25", undercut on barrel Celestron--all aluminum, 1.25", undercut on barrel Altair--Stainless steel bottom barrel, 1.25", safety "kerfs" on lower barrel (accts like a smooth barrel when inserting) Tecnosky--all aluminum, 1.25", smooth barrel, no undercuts Orion(US)--all aluminum, 1.25" with removable 2" skirt, undercuts on barrel.

The 21mm and 17mm are the best in the set, and work at a slightly faster f/ratio than the others. The 24mm is not the equal of the others, optically. The 13mm has significant edge of field brightening. You can do better at each focal length. Careful if buying used, as I recall the 17mm was revised at some point, and the new one is better optically. But I do not know how to tell them apart.

I recall it being measured at 74° +/- with the flashlight test. With the other focal lengths at 78°, and just the 6.5mm at 79°

I wouldn't recommend O-III filters as a "first" nebula filter purchase, because they don't work as well on the large hydrogen gas clouds like M42/43, M8, M17, M20, M16, etc. as a narrowband filter. They do, however, work a bit better on planetary nebulae, some supernova remnants (though I think a narrowband works better on M1), and Wolf-Rayet excitation nebulae. Ultimately, I think astronomers should have a set of at least 2 nebula filters--one good narrowband, and one good O-III. But, since the narrowband does pass the O-III lines and is usable on more different nebulae, I'd start with that one first.

A 40% jump between magnifications, starting with the 17.5mm runs: 17.5...12.5...9...6.5...4.5 Now you know where the focal lengths came from. So why a 14mm? Because it was the longest focal length possible in the original formulation of the design. It really wasn't in sequence with the other focal lengths. They tried 3 different designs for the 17.5mm until they landed on a good design, but the 17.5mm is different internally than the others, and has different eye relief (longer), a different apparent field (slightly smaller), and one note: the Baader data is incorrect--the field stop in the 17.5mm is 21.7mm, not the 23.5mm Baader quotes. I think this was changed somewhere in between the 3 prototypes they tried.

A 40% jump between magnifications, starting with the 17.5mm runs: 17.5...12.5...9...6.5...4.5 Now you know where the focal lengths came from. So why a 14mm? Because it was the longest focal length possible in the original formulation of the design. It really wasn't in sequence with the other focal lengths. They tried 3 different designs for the 17.5mm until they landed on a good design, but the 17.5mm is different internally than the others, and has different eye relief (longer), a different apparent field (slightly smaller), and one note: the Baader data is incorrect--the field stop in the 17.5mm is 21.7mm, not the 23.5mm Baader quotes. I think this was changed somewhere in between the 3 prototypes they tried.

Frankly, I don't know where Ernest's figures come from. Take his "essentially perfect" 10'. In a 100° eyepiece, that would be 1/600 the width of the field if talking linear field size in an angular sense. Would 1/600 the width of the field appear perfect? No way. A 10' star would be a blob. So I view his figures as "relative", meaning an eyepiece yielding a 20' spot size would be a spot 2x as wide as one yielding a 10' spot size. One eyepiece I know has a design 1µ spot size on axis and 1.5µ spot size just inside the field stop, and shows on his list as <5' on axis and 11' at field edge. 1.5µ is a lot smaller than the Airy disc in an f/5 scope, so, correcting for coma (many times as large), the eyepiece should yield stars at the edge indistinguishable from perfect. And his f/10 numbers are perfect, so f/4 just falls below perfect only at the very edge. That is consistent with the design specs. So, however the numbers are derived, they do have some basis in comparative reality. I do think choosing f/4 and f/10 is somewhat arbitrary, though. Few scopes are as long as f/10 these days, and those that are have other significant aberrations affecting the star images. And it is largely the very big scopes (say, 20" and larger) that are commonly as short as f/3-4. And really big scopes have a "spot of bother" yielding tiny star images for a variety of reasons. I would have picked f/4.5 and f/9 to more closely correspond to more of today's scopes. One thing the numbers do tell, and that is if f/4 is too fast for the eyepieces to yield excellent star images across the field. It's unfortunate his list doesn't contain more of the contemporary eyepieces.

Ironically, this happens with TeleVue 1.25" eyepieces in TeleVue 1.25" star diagonals. You have to press inward on the eyepiece as you tighten or the thumbscrew side lifts slightly.