Don Pensack

If you're going used, see if you can find an APM UltraFlatField 30mm. It is, performance-wise, a high-end eyepiece, priced low, but you might find it used for a very affordable price. It's also sold as an Altair Astro Ultra-Flat.

The observer who wrote that comparison is almost exclusively an observer of planets and double stars and uses exclusively small scopes. That doesn't invalidate his comparison completely, but his results might have been different had he used larger scopes or evaluated the diagonals on ultra-faint objects. Not to mention that some of his comments might have been eyepiece or scope-dependent. He has also reviewed the Baader BBHS silvered star diagonal and never even mentioned the weakness of silver coatings in the violet end of the spectrum. An H-Beta test on the California Nebula at a dark site showed me it was no better than a dielectric diagonal at 1/5 the price. On planets, it's superb, as you would expect from its high % flat transmission from yellow-red. So the evaluations are also target-specific. Most of us cannot afford specific star diagonals for specific objects, or to replace them every couple of years, so take such comparisons with a grain of salt. I put them in the same category as those who claim that multi-coatings scatter more light than single-layer coatings or that gold wires sound better than copper wires in an amplifier. Myself, I'd rather have a star diagonal that can handle a heavy 2" eyepiece and can be canted to the left without unscrewing the barrel. Star diagonals vary in quality a lot, but it's more about the surface accuracy and smoothness than the type of coating or the material the diagonal is made from. They are usually mirrors. Our primary mirrors vary in quality a lot from scope to scope. It's not hard to understand Star diagonals do too. It makes it almost impossible to generalize about a specific brand and model with a sample of one.

Some issues for nighttime observing: --more internal light loss than a conventional prism or mirror --due to the split in the field, there is a phase loss in the field. Critical observers say it results in a loss of contrast and sharpness --there is a vertical line in the field that divides left from right. Because this is not infinitely sharp, there is a flare of light when anything crosses the center line. It's most noticeable with stars, but can also be an issue when the object fills the field, as in the Moon, or looking at the daylight sky, where that line is usually visible. It would be especially apparent when a planet crosses the line. --well made Amici prisms can be very expensive--as much as the high end silver or dielectric ones or more. --long focal ratios have smaller true fields that occupy less of the prism, but that means the center reversal line is even more apparent. --there are some good 2" Amici prism diagonals out there, but I only recommend them for low power viewing during daylight.

I'm an ultra/hyper-wide junkie, but my 2nd scope, a 4" f/7 apo, has such a short focal length, I found I could get adequately-wide true fields with 62-65° eyepieces. So I tried a couple Delite eyepieces, thinking they might be good enough, and was simply amazed at the small pinpoint star images from edge to edge. 62° orthos, essentially, with long eye relief. So I bought the complete set. I wanted one lower low power, so added the APM 24mm Ultra flat field because it can accommodate glasses, which I need with focal lengths of 14mm or longer. I previously had a 24mm Panoptic for a low power, but, alas, it's not glasses-compatible; nor was the ES equivalent. So, did I move down in apparent field? Yes, because I prefer 100-110° fields and treat 82° fields as "narrow" on my dob. I would never use eyepieces narrower than, say, 70-76° in that scope, even for lunar/planetary viewing. So what is the "middle ground"? I think 70-76°--wide enough to feel immersive, sharp enough for critics, and available with long eye relief for glasses. If you had one set for multiple scopes, that would be where I'd go.

Because it's a 4" with an 880mm focal length and ~100x is a great magnification in a 4" scope, 9mm made more sense for a (relatively) high power eyepiece you would use all the time. You can, at some point in the future, go as short as a 4mm eyepiece, but I think 9mm is a logical step from a 13mm and would be used all the time. In my own 4", I have a magnification run of 102x, 143x, 179x, and 238x, but the usage declines with increasing magnification from 102x, which is a 1mm exit pupil. I find in both my scopes that a 1mm exit pupil is one of those "just right" magnifications, because it is not high enough for floaters to interfere with lunar observation, but high enough for some high-resolution images. The reason I asked about an OCA in your binoviewer is that many binoviewers have a 2X OCA in the bottom, so every magnification is doubled, which would turn a 9mm into a 4.5mm, and I don't think you'd use a 4.5mm focal length often.

9mm. You will find it is a great magnification and comfortable exit pupil. If the OCA on your binoviewer applies a 2X magnification factor, then a pair of 13mm might be better as a second choice after the 18.2mm, or, perhaps, 11mm. But if your binoviewers do not have an amplifying OCA, then 9mm for sure.

The rubber eyecup is attached to a metal ring that threads onto the top of the eyepiece. Included in the box is a small ring that can thread onto the eyepiece UNDER the threaded eyecup, raising the eyecup several millimeters. You may find that makes the effective eye relief *just right* for avoiding the blackouts that come from getting too close to the eye lens.

It seems to be the whole line.

I just took apart a 15mm SLV from a fairly recent batch. The spacer underneath the eye lens is a very light grey color and almost white at a low angle. There is also a retaining ring holding in the upper set of lenses that is an anodized grey color, also almost white at low angle. The field stop had a shiny knife-edge as well. The bottom group of lenses is excellently-housed with dark threads and good baffling. What happens in use, I found, is that the image of the Moon appears outside the field stop in reflection, upside down relative to the Moon inside the field stop, as if a mirror image with the field stop being the edge of the field and the edge of the mirror. When the scope drifts over the Moon, a reverse image of the moon outside the field stop drifts in the opposite direction than the image of the Moon inside the field stop. I think this might not be much of an issue in a tracking scope or if only the center of the field were paid attention to. My sample also had a lot of debris inside the eyepiece, which I took care of when reassembling. It was 7 elements in 4 groups, with 8 air-to-glass surfaces. It's always possible that the internal light scatter has been taken care of in the most recent production, but if so, I haven't seen one yet. Except for removing the field stop, which is glued in with a few drops of glue (but which dissolves with a tiny tiny drop of acetone), the eyepiece is easy to disassemble and reassemble, so an industrious owner could easily solve all reflection issues and end up with a fine long eye relief eyepiece.

Just a note: the Vixen SLV is fine in all regards save one: it is a lousy eyepiece for Moon viewing. There is a bright, shiny, spacer directly under the eye lens that reflects the light of the Moon upside down outside the field stop of the eyepiece. If you're up to dismantling the eyepiece and blackening that spacer, then never mind. But if you plan to use it for the Moon, look elsewhere. On the other hand, for deep sky it is fine and very comfortable to use.

The eyepieces fit in the pouches with eyecaps on. You might not want to uncap them in a case or a pouch lest the lenses get scratched.

Yes, this chromatic aberration of the exit pupil, or CAEP, is common in longer focal length ultrawide eyepieces, making them less appropriate for daylight or lunar observing. Fortunately, those are two uses less likely with long focal length eyepieces in astronomical telescopes.

Boy, has there been a lot written on how to choose magnifications for a scope! One way is to have a range of exit pupils, but which? Another way is a % step between magnifications. Another is to have even steps from a low power up. I side with this latter approach for a couple reasons: --it makes the highest powers closer together, %-wise, which is useful when bumping up against the ceiling of your seeing conditions --a smaller % jump at the high end is necessary to not make the magnification jumps too large. a 40-60x jump at low power is very close together. The same % could be 400-600x and that is a huge jump So, what is the even magnification jump to favor so that low powers aren't too close together and high powers aren't too far apart? I think it varies by scope size. If you have a 20", it wouldn't be outside the realm of utility to have a 100x jump in between magnifications, i.e. 100/200/300/400x and so on That wouldn't work for a 4" refractor, where jumps of 30x might be more rational. So, it seems to me, from owning 31 scopes so far, that a 1X/2X/3X/4X sequence is reasonable, where x = a number appropriate for a certain size of scope. Perhaps, 50 for an 8", 60 for a 10", 70 for a 12.5", and so on. And if the jump at some particular place in the sequence seems too large, then fill in with a 0.5x in the middle, like 1x, 2x, 2.5x, 3x, 4x, etc. So, in answer to your question, and having owned an 8" scope for 11 years, I think you do not need a magnification in between 50 and 100x, but each observer has a favorite power that just feels right. If you are constantly finding 50x too low and 100x too high for some preferred targets, you might definitely add one in between. What i would do, though, is just to get a wider apparent field at 100x. The objects would stay in the field longer, and the desire to have a lower power might evaporate. I think having jumps of 50x on your scope is pretty close to perfect, and the wider field may make the 100x eyepiece more usable and desirable. Make your 9mm a 100° eyepiece and the 20mm might get a lot less use.

The SSWs should be OK. Just be aware of the fact they have a lot more spherical aberration of the exit pupil than the Type 6 Naglers, which are their direct competitors. And slightly less well corrected for lateral field astigmatism than the Naglers, though this varies across the line. There is a good review of the SSWs here on SL. Over here in the US, the SSWs are actually more expensive than the Naglers, but that may be different in the UK.

There is more to focal length than AFoV. Longer focal lengths yield larger exit pupils and brighter images. The 21mm also has a 4mm wider field stop than the 17mm, so yields a larger true field, regardless of apparent field. That's what makes a 40mm Plössl a good eyepiece for a small, long f/ratio, Maksutov. Yes, it has the same TFoV as a 32mm Plössl, but it has a larger exit pupil and a brighter image, as well as being lower power, important for some objects. I doubt anyone would buy both the 21mm and 17mm Hyperion anyway, as the magnifications would be too close together.

Just got an updated list from Baader in Germany. They haven't updated their site yet. They say: 4.5mm--358g 6.5mm--326g 9mm--328g 12.5mm--315g 14mm--363g 17.5mm--305g. That is very close to JOC's weights. Excellent.

The fields of view are wider, but though that might be enticing, they are not all equal. The 24 is poorly corrected in an f/5 scope. The 21mm and 17mm are good. The 13mm suffers from edge of field brightening. The 10mm is fine. The 8mm is just OK, while the 5mm is a bit better. I don't have experience with the 3.5mm. I could see buying the 21mm, 17mm, and 10mm, but I'd skip the others. Your Zoom has the flaw of having a wide field below about 14mm, but a narrow field above that. Were I to suggest supplementing the Zoom, I'd say add a long focal length with a wide field. If 24mm is enough, try an APM 24mm UltraFlat field for glasses, or an Explore Scientific 24mm 68° if you don't wear glasses. If you'd like a maximum field eyepiece, then go for a 30mm 2" design, like the APM UFF 30mm, or the same from Altair.

OK, I will have to re-weigh them all again. I just weighed my personal 17.5mm at 305g, the same as JOC. And I got 320g for the 12.5mm. For now, trust his figures. They are likely to be accurate for current production. I'll have to ask Baader for clarification.

Odd. I measured them on my gram scale only about a month ago and got: 4.5mm--370g 6.5mm--350g 9mm--360g 12.5mm--345g 14mm--360g 17.5mm--312g. I rounded off, but I was within a gram of Baader's quoted spec sheet weights. I wonder if they have thinned the barrels and lightened the eyepieces?

Yes, on a gram scale. The amount of astigmatism in the SSWs depends on the f/ratio of the scope. At f/5, they are just OK, not perfect. At f/8, they're nearly perfect. The SSWs also all have spherical aberration of the exit pupil, but its effects diminish with focal length. By the shortest focal length, it would be hard to notice.

12.5mm is 345g. Nope, the latest one weighs 320g.

Try a 30mm APM UltraFlat field eyepiece to round out the set. Then your training will be complete.

For low power, a 30mm APM UltraFlat Field eyepiece is a winner. It's lighter and smaller and as sharp as the 31mm Nagler and 30mm Pentax XW, with which it competes. And compatible with glasses. The next one needed is something like the 17.5 Morpheus. Wider, which befits a higher power, and also compatible with glasses. If you don't need glasses, the TeleVue Nagler Type 6 eyepieces could round out the higher powers (down to a 3.5mm eyepiece). The APM 100s are a good value, too, and the shortest focal lengths are the best-corrected, I've found. It makes sense to go wider in apparent field as the magnification goes up, to keep the field from shrinking too much. Especially in an undriven dob, where things can go whizzing through the field and a wider eyepiece gives you more time between nudges. Russ, though the Morpheus can be used as 2" eyepieces, you should think of them as 1.25", because if used as 2", they require an enormous amount of out-travel at the focuser. They are closer to parfocal with a lot of other eyepieces if used as 1.25". There is no harm in using them that way. All the eyepieces I've mentioned except the 30mm APM can be used as 1.25" eyepieces.

The primary thing to do in an urban LP environment is use a bit higher powers than normally used at a dark site. Yes, this will dim the object as well as the background, but the larger size and darker background will make most extended objects a bit easier to see. For any star or star cluster (open, globular), higher power yields improved contrast and allows you to see fainter stars. Many if not most beginners use too low a magnification for most objects and that's OK in dark skies, even though they'd see more at higher power. With most DSOs (not the largest objects) observed in an urban setting, you probably shouldn't go lower in power to view them than 0.5D in millimeters, i.e. a 10" scope at 127x, or a 6" scope at 76x. The lower powers can be quite nice, but not particularly when the skies are light-polluted.

I seem to recall some early Questars, Bushnells, and even a Celestron with threaded-in eyepieces. But I haven't seen one since the '80s.