Don Pensack

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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?

The average AF in the Delite line is a hair under 62° The average AF in the XW line is 69° The average AF in the Delos line is 72°.

Yes, calculated field stop = (Apparent field/57.2958) x Focal length But, the accuracy depends on the amount and type of distortion present, and the accuracy of the stated apparent field. Example: APM 30mm Ultra Flat Field (same as Altair Astro): Calculated field stop = (70/57.2958) x 30 = 36.65mm Actual field stop as determined by star timing: 36.3-36.4mm So, is the actual focal length >30mm? The apparent field <70°? Or the distortion modifying the results. Well, measured apparent field is slightly above 70°, so it is the distortion that is modifying the results by 6.8% In contrast, look at the 24mm TeleVue Panoptic Calculated field stop 28.48mm. and actual field stop is 27.0mm. Difference is 5.5% The point is that you cannot trust a calculated field stop, even at 57.2958° unless the eyepiece has 0% distortion, which doesn't occur in real life. BUT, you can use the calculated figure as a reality check. Celestron says the field stop of a 10mm Luminos is 17mm. The calculated figure is 14.3mm. Celestron is blowing smoke. That difference is ~19%. Uh, nope. Simply not believable. And all the other sizes are similarly off. They may be referring to the diameter of the field lens? Or something else. So it's useful to know the calculation so you can check the reality of the MFR's claim. If it's close, discard the calculated figure.

Yes it can. See: https://www.cloudynights.com/topic/527199-spectroscopic-analysis-comparison-of-nebula-filters/?p=7114958 The same is true of dielectric coatings on a mirror. Note that at 7°, the bandwidth shift is minor, the transmission change is minor. The same is true of dielectric mirror coatings.

The dielectric-coated mirror has no metal coating on the glass, just up to 75 (or more) layers of various oxides and fluorides that each reflect at different wavelengths. These coatings interfere both constructively and destructively with each other depending on angle, so the stack of coating layers can result in, say, a 98% reflection at 550nm, but only at one angle. At other angles, the interference between layers changes because the layers are thicker or thinner than they are at the design angle. Hence, the interference can change the spectrum of reflection depending on angle. Properly designed, at the design angle, the reflectivity will be high (say, 95% or more) across the entire 400-750nm visible range (though 425-600nm is OK for nighttime use) and it really doesn't matter what happens outside the visible range. But, at different angles, you might see an enhancement of the blue, or yellow or even red depending on the materials in the stack and the angle of incidence. In a star diagonal, the only thing to be aware of is reflection at non-design angles, so it is critical the interior of the diagonal be as black as possible so no off axis light can bounce around inside the diagonal. Usually, an angle change of +/- 1° won't have any effect on the spectrum except perhaps a very small shift of a few nm, which won't matter in visual use.

Glasses help keep the eyepieces from fogging, I'm noticed. It's always cold where I observe--it can be 32°F in August--so fogging is very common. If you observe without glasses, and use long eye relief eyepieces like the XWs and Delites, it helps to leave the eyecups all the way down, which allows air circulation between the eye and the eyepiece. I also keep a small Japanese fan in my pocket to quickly blow air at the eyepiece to evaporate fog when I see it start forming on the eyepiece. Otherwise it turns to frost. You learn to adapt to conditions, like breathing out of the corner of your mouth away from the eyepiece rather than with your nose, when looking through the eyepiece. Keeping the eyepieces in a foam-lined case when they are not in use also helps keep the eyepieces from getting too cold. Keeping eyepieces in a rack is a sure-fire way to cause them to fog up in use.

The Delites have a shielded eye lens and a smaller eye lens exposed than the XWs. I haven't found either to be prone to dewing, but the Delites are not less resistant to dewing than the XWs. Also, the eyecup locks in place at whatever height you choose, so really no different than the XWs in the field. Also, XWs also have undercuts on the barrels. I have no axe to grind. Pentax XWs are fine eyepieces. As are Delites and Delos eyepieces. As are Baader Morpheus eyepieces. There are some fine choices in the market today.

) + ) = | or curved field scope + curved field eyepiece = flat field image.

It would be very very hard to beat the Delos. You might appreciate this test: https://web.archive.org/web/20130829052725/http://www.cieletespace.fr:80/files/InstrumentTest/201306__6_oculaires_10mm.pdf After reading that, it might not be possible to beat the Delos.

Well, a 650mm focal length has a strongly curved focal plane, so it was likely in that scope. The Mak's 2700mm focal length wouldn't have had such a curve.

Short focal length refractors have strongly curved focal planes. If a flat field eyepiece is used in such a scope, the field curvature seen is due to the scope. Pictographically, it looks like this: ) + | = ) a flat eyepiece in a curved field scope. | + | = | a flat eyepiece in a flat field scope | + ) = ) a curved field eyepiece in a flat field scope ) + ) = | a curved field eyepiece in a curved field scope where the curvatures match. and ) + ( = severe field curvature. The field curvatures of scope and eyepiece don't match.

Jeremy, A clarification: the 24mm Panoptic has a flat field and near zero field curvature. The edge stars are sharply focused at the same time as the center, even at f/4-f/5. What you're probably remarking about is its positive rectilinear distortion (i.e. pincushion distortion), which is noticeable if the eyepiece is panned or if the telescope has a long enough focal length the passage of the field is fairly rapid. It results in straight lines moving across the field as ) | ( That the eyepiece has, in abundance--perhaps stronger than other eyepieces in the size. The 41mm is the same design, just larger and with a wider field stop. If the RD in the 24mm bothers you, the 41 might not be the best choice. Though, the much lower magnification does mean that field drift through the eyepiece is a lot slower, so it largely depends on whether the eyepiece is panned across the sky. It does depend, also, on what you want the eyepiece to do. If you want a good eyepiece for terrestrial use, suppressing rectilinear distortion and allowing some angular magnification distortion may be a better choice (e.g. 24mm APM UFF). Or, for use in an astronomical scope that tracks, the RD would be unnoticeable (just as AMD is unnoticeable (e.g. 12.5mm Docter/Noblex). But, in a scope that doesn't track, many people would prefer a smaller amount of RD. You're not going to beat the sharpness of the 41mm Panoptic at its focal length, though. So it does depend on the observer's preferences.

The design has visible astigmatism at f/10 and probably shouldn't be a top choice below f/8. There are better designs at that focal length, but probably none so inexpensive.