Don Pensack

Or a spectrophotometer, like the type that measure the responses of filters. Here are some graphs once on the web for Pentax transmission curves: https://web.archive.org/web/20141231092400/http://pentaxplus.jp/archives/tech/xo-xw/63.html

It's been 5 years since I saw a refractor at my observing sites that didn't have a camera attached. And Takahashis seem as rare as hen's teeth here. I see AstroPhysics, TEC, and a veritable see of Chinese and Taiwanese refractors. But it's been many years since I actually looked through a Takahashi refractor, and my memory of any tonality is gone. I've never seen an SV140, but if I did, I guarantee it would have a camera attached. So what you find in your comparison will be equal to anyone's opinion. Though I am certain there are many many refractors being used visually in backyards and on patios, those observers do not seem to bring them to my dark sites. I am in that category, too, because when I travel to a dark site, I take my 12.5", not the 4" apo. Most of my friends wouldn't even know I had a 4" refractor, as they've probably never seen it.

When looking at planets, how could you tell what is neutral, warm, or cool except by comparison with other eyepieces?

Many people think the XWs have a "cool" tone that tends to suppress the warmer colors. I've compared them directly in a triplet apo that lacks CA by using them to look at an arctic white building, where you can look directly at the building, then in the eyepiece, then directly at the building. Many, many eyepieces change the tone of the white, and very few see the white as the pure arctic white it is. There is a red wind sock on the top of the building that appears orange or even grey in some eyepieces because their responses fall off in the longer wavelengths. All of these color rendition issues tend to disappear at night except on the Moon or planets, but some show up when looking at carbon stars or red giants. Anyway, the XWs have a strong yellowish tone to them when looking at that white building. I am mystified some see the view as "cool". The red windsock is still red, but a bit less intensely red than the naked eye, which is consistent with the measured steep drop-off they have above 650nm. The TeleVue Delos, on the other hand, had a better color rendition, with the building still arctic white and the red windsock still the same as the naked eye. This can be seen at night when looking at Carbon stars and red giants. I think it may be the lack of fall-off in the red end of the spectrum that makes some observers describe them as "warm" in tone. Maybe that is true in comparison with the XWs, but I found them more neutral in my daylight test. So if you are looking for a 70° field eyepiece for planetary use, I think you would find planetary colors less suppressed by the Delos. You will find differing opinions from different observers. I think that is because color vision sensitivities in the violet and red vary all over the place among observers. However, you do have to pay attention to whether the diagonal is a mirror or prism, how low in the atmosphere you view, and chromatic issues in the scope. You have a reflector scope, which is free of chromatic aberration. Are you planning to get a new scope? Because these eyepieces are expensive and might be hard to justify in your current scope. I guess I should ask what it is about your current eyepieces that has you seeking out others. Because if they are not sharp on axis, it might not be the eyepiece at fault, but the cooling of the optics, or collimation, or simply the seeing conditions. What is it that you don't like in them?

The long f/ratio of your Maksutov means you can pretty much use any eyepiece you want. Chances are likely that there is nothing wrong with the 12mm BST, but that it is merely the magnification that is exceeding the quality of the scope's optics, or the seeing conditions due to the relative high magnification. Your refractor probably has a shorter focal length so the same 12mm yields a lower power in that scope. That the eyepiece works well in the refractor says there is nothing wrong with the eyepiece, the issue is with the magnification it yields in the Maksutov. There may be NO eyepiece at 12mm that will yield a good quality image in the Maksutov all the time. Assuming there is nothing wrong with the Maksutov's optics, then the issue is collimation, cooling, or sky conditions. Have you checked its collimation? Have you let the scope cool down outside for an hour before using it at such a high power? If your scope is an f/15, a 12mm eyepiece will be pushing the limits of the scope (collimation, cooling, optical quality) or seeing conditions. If your scope is f/12, it is still a high power, but not at the limit. Here is a way to look at it: magnification of 3.6 to 10x/inch of aperture is low power. Usable all of the time. magnification of 10-20x/inch of aperture is medium power. Usable most of the time. magnification of 20-30x/inch of aperture is high power. Usable sometimes, but not all or even most the time. magnification of 30-50x/inch of aperture is ultra high power. Usable a very small % of the time. Are there higher quality 12-13mm eyepieces? Yes. But if 12mm doesn't yield a good image quality, a 9mm will be worse. Now, assuming proper collimation, a cooled scope, and excellent seeing, a 12-13mm should work fine. A 13mm Nirvana would work fine, but might not be in your range. Have you considered an Omni Plössl in 12mm? It would have a slightly smaller field than the BST, and a little less eye relief, but you might find it adequate. I don't think it will really improve the image quality, though, because I don't think the problem is the eyepiece.

You are using the term 'backfocus' incorrectly. It may not be intuitive, but the term 'backfocus' refers to the ability to get closer to the scope for accessories requiring more in travel of the focuser (like a camera) by moving the focal plane farther away from the objective. The prism diagonal throws the focal plane farther back, which allows the diagonal to move farther into the light cone from the objective, allowing accessories that require more in-travel to come to focus. In other words, the prism diagonal provides MORE 'backfocus' than a mirror diagonal. An accessory that needs more in-travel of the focuser is said to need more backfocus. I find that understanding of the word very non-intuitive, so I think about back focus as meaning how far back away from the objective the focal plane is. And a prism diagonal moves it back, so an accessory that needs more in-travel can accomplish focus.

You will probably need an optical corrector (GPC) to come to focus. If you do, don't get the one from William Optics. I found it added chromatic and spherical aberration Big Time. There are several other brands that would throw the focal plane far enough back that don't add appreciable aberrations.

It's a little more complicated than that. All positive eyepieces can have a field stop the same diameter as the inside of the barrel, maybe a tad smaller to allow for a field stop. But negative positive designs max out with a field stop that is smaller than the inside diameter of the barrel because the field lens expands the image after passage through the lens. As a result, an all-positive 2" eyepiece can have a field stop of 46-46.5mm, whereas a negative-positive design maxes out at 42-43mm. In a 1.25" eyepiece, field stops of 27-27.5mm are common in all-positive designs, but 24.5mm seems to be the maximum for a negative-positive design. It is the reason why an APM UFF 24mm has a 27.5-27.6mm field stop and is 1.25", while a 20mm T5 Nagler had a 27.4mm field stop and was a 2". Louis points out that a designer can go too far in shoehorning a too-large field stop into a smaller barrel. The Meade series 5000 18mm UWA went from a 1.25" to a 2" diameter to reduce vignetting and it only had a 24.3mm field stop.

That is astigmatism, and it is inherent in the eyepiece. The 31mm and 36mm Hyperions are best used at f/10 or longer focal ratios, and, even then, display residual astigmatism. There are better-corrected eyepieces at those focal lengths.

Supposedly, per the rumor mill, they are the same eyepiece in different housings. Whether anyone has verified that, I can't say.

It may or may not be better than the 21mm. It depends. If your scope is f/4-f/6 it would be an improvment. If your scope is >f/8, probably not, since the 21mm and 17mm are the best in the Hyperion series. However, shop for price. The Celestron is unlikely to be the least expensive label. The same eyepiece is available in several labels: Altair Astro UltraFlat (green) APM Ultra Flat Field Celestron Ultima Edge Meade Series 5000 UHD Orion Ultra Flat Field Sky Rover Ultra Flat Field Stella Lyra (FLO) Ultra Flat Tecnosky UltraFlatField

Rother Valley Optics may have them. All the above are made by Barsta. I have not researched whether there are coatings or internal baffle differences between them. I also note there have been running changes on lower baffles and retaining rings over time.

You might also try the BST Planetary eyepieces (they are identified by a 58° rating). They are sold under at least 8 brand names. And they all have roll-up eyecups. And they come in 6mm, 7mm, 9mm sizes. Here is one of those brands: https://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-uwa-planetary-eyepieces.html

I compared the 17.3mm Delos to the 17.5mm Morpheus and kept the Morpheus, though the fields are identical in size. There was just something about the 17.5m I liked a bit more. I don't remember now just exactly why. But, the 17.3 Delos as a very nice eyepiece.

They had to design the Paracorr so that eyepieces could get closer to the lenses than in the Paracorr I. And the adapter had to support the eyepieces that had a combination 1.25" and 2" skirt which needed a longer support for the 1.25" barrel that goes up inside the 2" skirt. So a flat top adapter would require redesigning their dual-sized eyepieces. A Hi-Hat style adapter is taller (some people do need a taller adapter), AND is compatible with 1.25" eyepieces with 2" skirts. I would emphasize the In-Travel adapter was only designed to parfocalize the 17.3mm and 14mm Delos with the other sizes. That adapter is not necessary to use the 17.3mm and 14mm in the Paracorr. Only one 1.25" adapter works with all TeleVue (and nearly all other brands)--it's the one that comes with the Paracorr. As a company that offers almost 50 different eyepieces, TeleVue is also under no obligation to design their equipment to work with everyone else's eyepieces. Yet, by and large, their equipment does.

That looks a lot like the color black anodizing ends up looking like when faded by the sun.

There were multiple versions of the S4000 "Super Plössls". The exact dates are a bit confused because there was store stock later than Meade actually brought them into the US market or the European market. Version 1 was a smooth side, no rubber eyecup, 5 element, made in Japan, mustard color lettering. Production seemed to end ~1990 +/-. Nicknamed the "pseudo-Masuyama" version. Version 2 had a rubber eyecup, 5 element, made in Japan, mustard color lettering. Production definitely ended in 1994 for these. Version 3 had a rubber eyecup, made in Japan, 4 element, mustard color lettering. There may have only been one production batch ~1994 Version 4 had a rubber eyecup, made in Taiwan, 4 element, mustard color lettering. It appears there may have been only one production batch of these, as they are rare. Version 5 had a rubber eyecup, made in China, 4 element, mustard color lettering. Version 6, 7, 8, and no one knows how many more, had a rubber eyecup, 4 element, made in China, white lettering. To add confusion to the mix, Meade, apparently, with their last production, went back to mustard color lettering. How many years ago that was I can't find the answer to. You cannot trust that is what you will get, though, because many retailers have very old pictures on their sites and may have stock of the white lettering versions. One industry person mentioned several years ago that Meade had had at least 6 different manufacturers make these, and possibly more.

Louis makes a point. I was unimpressed with the quality of the star images in the APM 30mm Ultra Flat Field when I first reviewed one several years ago. A couple years later, I decided to try wearing my glasses at the eyepiece because it was obvious I needed them to see naked eye stars as points instead of little stick men. Eureka! Tiny pinpoint star images from edge to edge in the 30mm. Why I hadn't realized it was in my eye was because the star images in the center of the field were no worse than the 36mm or 31mm Baader Hyperions and I just figured it was the same level of eyepiece quality as the Hyperions. Nope. I went back to wearing glasses and tried the two Hyperions again--still bad. But the 30mm APM became world class.

Louis, There is method to their madness. The 31mm Nagler, 21mm Ethos, 17mm Ethos, 17.3 Delos and 14mm Delos all use setting A, with the Delos both using the 10.5mm tall adapter. There was no setting that far in for those eyepieces in the Type 1 Paracorr. Setting B works for the 22mm Nagler and the 6mm and 8mm Ethos (the Ethos are used as 1.25" eyepieces). A Hi-hat style adapter was necessary for safety on the 6mm and 8mm Ethos--a flat top is less safe--but the 16.5mm tall Hi-Hat adapter wouldn't have worked for the 6mm and 8mm Ethos or the 2 longer Delos eyepieces. The Ethos 6 and 8 could have used an adapter 2.5mm taller, but the 17.3 and 14mm Delos could not. Hence, 10.5mm and it works with all TeleVue 1.25" eyepieces, even the 40mm Plössl, at the other extreme setting of the Paracorr. Virtually all the rest of the 1.25" TeleVue eyepieces use setting D with the 10.5mm tall adapter. They would all focus closer to the 2" eyepieces with the 16.5mm tall High-hat adapter, but then the 17.3mm and 14mm Delos would not have been able to come to focus, nor the 6mm and 8mm Ethos. The In Travel adapter allows the 17.3mm and 14mm Delos to be parfocal with all the other 1.25" eyepieces. It isn't necessary if you don't care about parfocality. But, a 10.5mm tall adapter allowed all the TeleVue 1.25" eyepieces to focus in the range of the Paracorr's travel, and no other height would have.

No, the Astrosystems ultra-low adapter is not an option. Here is why: The eyepieces in question are not small enough to insert into the recessed section of the Ultra-Low adapter. They will rest on the top of the adapter, just like they would in the TeleVue in-Travel adapter. However, the opening of the 1.25" bore in the TeleVue adapter sits only 5.5mm below the surface the eyepiece rests on, which is no problem--the setscrew will grab. In the Astrosystems Ultra-low adapter, on the other hand, the 1.25" bore starts 14.7mm below the surface the eyepiece rests on and the setscrew is many mm below that. The setscrew will likely miss the eyepiece entirely, or, at best, grab only the bottom couple mm of the barrel. It might work, but I could not guarantee it. That Ultra Low adapter works great with eyepieces small enough to fit into the recess, but not with eyepieces with shoulders too wide to fit into the recess. The TeleVue In-Travel adapter, on the other hand, has a shorter recess, so a wider eyepiece can rest on its top and still have enough barrel inserted to have it grab the eyepiece's barrel. I use the In-Travel adapter in the Paracorr with a 17.5mm Morpheus in my 12.5" because otherwise it cannot come to focus in the Paracorr, and the 17.5mm also rests on the top of the adapter, yet the 1.25" barrel inserts almost to the bottom of the adapter. I use all the Morpheus in the In-travel adapter in my refractor to make them closer to parfocal with other 2" eyepieces that require more out travel.

You should know the factory field stop dimensions on the Ultra Flat Field eyepieces are the actual physical field stops, not the virtual field stops that you see in the eyepiece. Without another repeat of a long-winded explanation, these are the actual field stops you see in the Ultra Flat Field eyepieces: 30mm --36.3 That is 33.5% wider than the 24mm ES 24x68 (2.1° becomes 2.8°) 24mm --27.5 18mm --21.7 15mm --18.2 10mm --11.2 The 30mm is narrower than the Aero ED, but not that narrow. It's a real 70°. By the way, there is a way to tell whether an eyepiece has pincushion distortion or barrel distortion at the edge. If you calculate a field stop assuming 0% distortion, it is = (apparent field ÷ 57.2958) x FL. 1) Example: the 24mm TeleVue Panoptic. A calculated field stop based on 68° is 28.5mm. Its actual field stop is 27.0mm. That means the 68° apparent field is created by stretching the field stop radially by 5.6% to yield a larger apparent field, so the eyepiece has pincushion distortion. If the calculated field stop had been smaller than the actual field stop, it would mean the edge of the field was compressed to yield a smaller apparent field than the field stop implied. 2) Example: 12.5mm Docter/Noblex 84° eyepiece with a calculated field stop of 18.3mm and an actual field stop of 19.2mm. That means the edge is compressed by 4.7% to yield only an 84° field, so the eyepiece has barrel distortion. The 30mm UFF 70° eyepiece has a calculated field stop of 36.65mm and an actual field stop of 36.3mm. That's pincushion distortion. The field stop is stretched by ~1% to yield the apparent field measured. That is very low distortion.

The small setscrew can be lightly tightened so you can simply rotate the eyepiece slightly to insert it. Not loose, but not tight. Then, eyepieces can be inserted and removed by slightly rotating them in and out. It sounds harder than it is. I've been doing it for years. It helps the setscrew has a nylon tip, so low friction. Now, if it turns out that your eyepiece barrels vary enough in size that some are loose and some are tight, then you may have to come up with another option. And if the eyepiece gets too loose when the setscrew hits the undercut on the barrel it could be a problem (all my 1.25" have smooth barrels). In that case, you may have to fill the undercuts on the barrels with metal tape to have them slide in and out easily. Or use the 2" Pentaxes in setting 3 and use the 1.25s in setting 5 and just refocus the scope, knowing the coma correction won't be ideal, but still a lot better than no correction.

Not all. Here are specs from Vixen Japan:

Yes.

If you can afford it, the 30mmUFF is a far better eyepiece.