Don Pensack

Differences of note: --slight tint difference: 11T6 slightly warm. Apollo 11 neutral. --better presentation of color in the Apollo. Red stars in clusters stand out better. --longer eye relief in Apollo--usable with glasses. --T6 11mm is 78° (measured). Apollo is 85° (measured). --very slight SAEP in 11mm T6 (slight). None in Apollo 11. --thin blue ring at the field stop is thicker in the 11mm T6. In the Apollo, it's right at the very edge--as thin as I've seen in a complex eyepiece. --the 11mm Apollo has a 2" threaded-on adapter provided. Eyepiece is usable as 1.25" or 2". --the Apollo is larger and heavier by several ounces. --If looking at a very bright star or planet, some corneal reflection is visible in bounce back from eyepiece in the 11mm T6. None in the Apollo 11 --vignetting--none in either --edge of field astigmatism--none noticed at f/5 in either. I can't say about f/4, though the Apollo 11 was sharp to the edge in a friend's f/3.45 32". --Contrast and image brightness appears better in the Apollo, resembling the 10mm Ethos. Given the almost 4:1 difference in price, it's not hard to see which is the best value. That extra few % cost a lot of money!

One manufacturer still making 0.965" eyepieces and Barlows--good glass and all aluminum construction barrels--is Harry Siebert of Siebert Optics. He's in the US, but I think he will ship to the UK. His website is a bit primitive, but I'm sure you can handle it.

Hard to define but: --easier to use, i.e. to find the exit pupil and stay there. --much thinner ring of color at the very edge by the field stop --somewhat better contrast--feels less like a Nagler and more like an Ethos. Is it worth all that money? Maybe not, but certainly one of the best eyepieces ever made with that wide a field.

That eyepiece wouldn't get an Oscar from me, but I have to agree it is quite sharp at f/5-f/6, especially in a coma corrector. I have yet to see an eyepiece that bests the TeleVue Apollo 11, though there would be many contenders that are all quite satisfying to use.

Should be decent at >f/12, but also will have a quite narrow field of view. Just be aware Huygens eyepieces are not designed for chromatic correction. https://www.telescope-optics.net/eyepiece_raytrace.htm

If you're in the UK, look at the diagonals page on First Light Optics' website. If you're in the US, I agree with Louis.

I just looked, and the 30mm doesn't have an S after it, just an R, for Ricoh. There's no -S on the boxes, either. https://www.ricoh-imaging.co.jp/english/products/binoculars/scope/xw/index.html No changes, either, per the national sales manager for Pentax Sport Optics.

Eye relief. Given they all have 82° apparent fields, the diameter of the eye lens determines the maximum eye relief obtainable from the eyepiece. It has less eye relief than the other two. Eye relief (maximum) = 0.5 eyelens diameter / Tan 0.5 Apparent field angle. As you can see from the formula, the larger the eye lens diameter, the longer the eye relief.

But made by another company, likely Barsta, since it is a duplicate of the one I used to sell under the Olivon label, which was Barsta's house brand label.

The focal length is so short, you can get nice big fields of view with 50° Plössl eyepieces, which you can find for very little money. And because the instrument is f/4, you'll be better off with them than something wider. You want magnifications from, say, 20x to 75x. Higher magnifications will be difficult to use because the scope won't move smoothly and the field would be dark and narrow. So, I'll suggest a 15mm and a 10mm with a 2X Barlow, to give 20x, 30x, 40x, 60x. If it turns out that you want a higher magnification, though the scope will be a bit more difficult to use, try a 2.5x Barlow instead, to give you 20x, 30x, 50x, and 75x. If you have to wear glasses when you use the scope, Plössls will not work. In which case, you'll need long eye relief eyepieces, and I'll defer to others' recommendations for inexpensive eyepieces with long eye relief. Most I'm familiar with will cost more than your scope, each.

+1. What Louis said.

See: https://www.telescope-optics.net/maksutov_cassegrain_telescope.htm https://www.telescope-optics.net/Mak_spherical.htm Short answer: YES. It can be compensated for in the design. Is it? Probably not. Read the 2nd paragraph above the section "Maksutov-Cassegrain Star Test "in the 2nd link.

Well that explains the prices! Ort! Ort! Ort!

I recall from reading about the SCT design that if the spacing between the primary and secondary mirrors differs by more than 3mm from the design spacing, it adds 1/4 wave of spherical aberration. Whether you could see that at low power is a different issue. On my older 8" SCT, the focal reducer flattened the field and reduced edge of field astigmatism in the scope as well. I thought it improved the image quality, so I used it 95% of the time. As for having a large field to find things, a good 8x50 finder, piggy-backed on the scope, would give you a nice big field for star hopping. At a dark site a huge number of DSOs are visible in the finder as well.

The OE eyecup sacrifices a lot of effective eye relief. I was not able to see the entire view with glasses on. A conventional eyecup replacement that folds down would allow easy glasses use. Its position would have to be carefully adjusted though or the glasses wouldn't be protected from the wide aluminum top.

For example: the 8" SCT. Every mm of additional rear focus you add adds 3.1mm to the focal length of the scope by moving the primary mirror toward the corrector. If you add 100mm of back focus to accommodate the binoviewer, the focal length of the scope is longer by 310mm. The 2032mm focal length becomes 2342mm and the f/ratio is now f/11.5 and has a narrower true field of view as a result. In a moving mirror scope, like an SCT or MCT, a binoviewer is a great thing for lunar and planetary viewing. But for deep sky, a single eyepiece and diagonal is better. Plus, there is the additional light loss in the binoviewer, the additional chromatic aberration, the extra weight on the back, and clearance issue if the scope is fork-mounted. Not to mention the large amount of spherical aberration caused by moving the mirror. The SCT is only "diffraction limited" in a very narrow range of focus.

It is known that JOC sells these eyepieces with seals (Explore Scientific) and without (many direct-from-China providers). Opticstar 82s are claimed to be sealed.

Nice theory, but the focal reducer requires less than an inch of back focus adjustment, and many eyepieces can fall into that category, too. I don't think a reduction in aperture will be as important as a big increase in spherical aberration.

Use the buds (Q-tips in the States) with pure isopropyl alcohol. You may have to go over it 2 or 3 times to be sure you left no smears.

Most affordable option with decent quality: Baader Hyperion Zoom 8-24mm. If you have a widefield low power eyepiece already, perhaps the APM Super Zoom 7.7-15.4mm. The innumerable zooms at £100 or lower have many issues (some have all these issues, most have a few): --poor baffling (lots of internal light scatter) --excessively narrow fields of view in the long focal length half of the range. --poor control of edge of field aberrations at f/6 or shorter f/ratios --short eye reliefs --lots of variation in the zoom mechanism: how easy to turn, how well it deals with cold temperatures --more internal debris. A truly minimal set of eyepieces could be with the zoom plus one low power 2" widefield eyepiece (APM 30mm Ultra Flat Field as an example) and possibly a 2X Barlow for high powers (your scope is usable with eyepieces down to 3mm under superlative conditions, though usually 5mm is about as short as you'll go)

The standard SCT reducer/corrector on a C6 shouldn't reduce the aperture at all because its clear aperture exceeds the diameter of the rear port. The "choke points" are the rear port I.D. and the I.D. of the front tube on the 1.25" diagonal (and any internal stops in that diagonal). What it does do is reduce the field illumination diameter by the ratio it reduces the f/ratio of the scope. So if the rear port allows a 50% illumination at the edge of a 27mm field, then with the focal reducer that same illumination is at the edge of a 17mm field. Note that a 17mm field stop at f/6.3 yields the same true field as a 27mm field stop at f/10. The white papers released many years ago point to the SCT scopes being designed to illuminate a 1° field (plus a little), so technically we are exceeding the original design intent to see wider true fields. As I mentioned, though, I personally found a 1.2° field was fine, and that is a field stop of 19.8mm at f/6.3 in the C6. All these discussions of trying to get wide fields out of the SCT scopes just indicates a misunderstanding of the scopes. SCTs are narrow field, high power scopes that can be used for most large DSOs at low powers. Think about it. How many objects are really bigger than a degree? And of those, how many are going to be viewed in a C6? As I see it, an SCT is a really nice general use transportable scope and for the occasions when you really need a much wider field of view, a refractor makes a nice companion. An 8" SCT and an 80mm refractor make a really nice pairing and can often be used on the same mount. A 60mm refractor could even be piggy-backed on an 8" SCT. You don't use a hammer to install a screw, and you don't try to squeak a 1.6° field out of an SCT.

Some notes. The first link lists the rear port aperture on different sizes of SCTs and Maks. What is generally not known is that the field illumination at the edge of this port is only about 50%. We are really poor with seeing vignetting, so no one typically comments they can see the vignetting with an eyepiece that has a field stop of this diameter. But if you add to that vignetting, at some point you will see it, so it's probably desirable to not go larger than the port using eyepieces with larger field stops. It can be photographed with a full-size chip camera (say, a 38mm chip) at full aperture, with short images. Experimentation, if you can afford it, will teach you where you can see vignetting. For example: on an 8" SCT of 2032mm focal length, I could see vignetting easily when the field size exceeded 1.2°, even though an eyepiece with a 37mm field stop yields 1.04°. So I could go a bit larger than the 50% illuminated field and still get away with it. About focal reducers: they do two things: reduce the size of the 50% illuminated field and reduce the focal length more the farther you are back from the lens. An SCT with an f/6.3 focal reduce and a 2" diagonal is actually operating at f/5-f/5.5 depending on the length of the 2" diagonal and visual back. It is typically only f/6.3 with the 1.25" visual back and diagonal that comes with the scope. So, the 50% illuminated field of 37mm is reduced to a 50% illuminated field of 23.3mm when the f/6.3 reducer is used. Though a 32mm Plössl yields a true field of 1.21° on the 8" SCT when used with the 1.25" diagonal, visual back, and f/6.3 reducer, it has a reduced brightness at the edge. That may be OK for a low power (40x) on the C8, but you might be able to see the vignetting. It's easy to see when pointed at a daytime sky to look for edge illumination. The above does point out, though, that focal reducers and 2" eyepieces don't go together. Achieving larger fields of view is either a 1.25" + focal reducer thing, OR a 2" thing. The measurements provided by Celestron also point out that the C6 and smaller catadioptric scopes are 1.25" scopes. The only reason one would use a 2" visual back and diagonal on such a scope is to support large heavy 1.25" eyepieces, like a 13mm APM XWA, i.e. large eyepieces with field stops small enough for then to be 1.25" eyepieces.

Yes. OCA = Glass Path corrector (GPC) = effectively a barlow, but often compensating for the CA of the binoviewer itself. In the case of the Norin binoviewer (William Optics et.al.) the 2x OCA ADDS CA instead of solving for it. Pure garbage, that one. And yes, the multiplying effect of an OCA will act just like a Barlow.

Don't forget the eyepieces will act like eyepieces with a shorter focal length, based on the magnification factor of the optical corrector assembly (OCA) on the binoviewers. An 18mm eyepiece may function as a 9mm. If the binoviewers are being used for lunar and planetary observing, that may be what you want. The most popular pairs of eyepieces I see used in most binoviewers, though, are 32mm 50° and 24mm 68° because of magnification. Those correctors come in many magnifications, though, and what eyepieces would be appropriate should be based on that.

Many years ago, I was looking for the best quality 1.25" diagonal I could find for a 5" Maksutov, and tried the Celestron prism star diagonal, along with TeleVue, Lumicon, Takahashi, and GSO offerings, all 1.25". It was a fairly recent version, with a multi-coated prism and a full clear aperture all the way through. I was surprised to learn it was the optical equal of the very best dielectric mirror diagonals--indistinguishable in brightness, sharpness, or contrast. Yet, it was 1/3 to 1/6 the price of the others. I didn't go with it only because some of my eyepieces wouldn't fully insert in the diagonal--they were blocked by the prism. But, in terms of optics, that one was a real bargain. I'm not sure whether I could trust the plastic prism housing to hold a 1-1/2 pound eyepiece without breaking in the long run, but for lighter eyepieces, it is STILL a bargain. Is the one from FLO threaded for filters?