Don Pensack

Meade discontinued this filter 10+ years ago. If you bought it new, it is new old stock, which is fine. The issue with it is that it pulls the eyepiece back by a large amount and requires in travel at the focuser equal to its height. Most people don't have that much additional travel available. Most people simply thread a variable polarizing filter onto the eyepiece, but that is not as convenient as the Meade product you have. If it works in your scope, it's a good find.

After your UHC filters, the next filters to get are O-III filters, which are useful on planetary nebulae, Wolf-Rayet excitation nebulae, and supernova remnants. An H-ß filter has an even narrower bandwidth, making the field quite dark. It does help make the large faint nebulae like the California Nebula more visible, but it won't make the horsehead visible in that aperture. I have held one up to the eye (it was mounted in a small tube to block all peripheral light) which allowed me, at a dark site, to see Barnard's Loop and the Lambda Orionis Complex with the naked eye. So I would advise O-III filters first, then the H-ß filters last. You have to be completely dark adapted to use them (45-60 minutes outside, away from lights), and use very low powers, like 3-5x/inch of aperture. One additional comment: your Astronomik UHC filter passes the H-ß line. If you cannot see an H-ß target in that filter at all, an H-ß filter won't make it visible. All H-ß targets are visible in the UHC, albeit with a slightly lower contrast. The H-ß filter helps, but doesn't make visible what was invisible through the UHC.

I might also add to the excellent summary above that the dark-adapted eye at night (scotopic vision) does not see the H-α line in the spectrum with more than a completely minimal sensitivity, if at all. Whereas our photopic, daylight, vison might extend from 400nm to 750nm in the spectrum, our sensitivity at night changes to primarily 425-550nm with very little vision, if any, above 600nm. That makes the 656nm emission of the H-α wavelength pretty much outside the range of our vision. An H-α filter can be useful for a camera, whose sensitivity extends into the infrared, but it is not a visual filter. So H-α is only useful for daylight viewing of the sun through a special solar scope that confines the light coming through to an extremely narrow bandwidth of H-α light. The Cemax eyepieces are, therefore, aimed solely at solar viewing in special H-α solar scopes. They have been marketed as accessories for the Coronado Solar telescopes, hence the gold coloring. However, in the field they perform exactly the same for astronomy as other inexpensive GSO Plössls. For use at night, lots of other eyepieces are their equals.

Well, as I said, a zoom can be a good choice when the narrow apparent fields of the zoom still yield wide true fields in the scope (like your short focal length scopes). Or, I should have added, when narrow fields are preferred (Moon, planets, double stars). It isn't the best choice for the Maksutov in question for general use, which is also on an undriven mount, where width of field is more important still.

Since the scope is a long focal length, narrow field, scope, the zoom just makes it narrower still. I owned one for several years and found either a 32mm Plössl (50°) or a 24mm Widefield (68°) ideal as a low power eyepiece. A great inexpensive set would be a 24mm, 17mm, 12mm, and 8mm 60° to 68° eyepiece set, for 64x, 91x, 128x, 193x It's not that you couldn't use more eyepieces, but that would give you 4 different magnifications, with a low power nearly 1.1°. It's important that the lowest power be a widefield eyepiece for starhopping. Some examples: Baader Hyperion 68° 24, 17. 13, and 8mm Get the fine tuning rings and this gives you focal lengths from 24mm down to 4.3mm I don't recommend adding and subtracting them in the field at night, but preparing the eyepieces to yield particular focal lengths on particular nights is easy. These eyepieces aren't very good at f/5-f/6, but they work great at f/12+. or Celestron X-Cel LX 60° 25, 18, 12, and 9mm or Astrotech/Starguider 60° 25, 18, 12, 8mm or a mixed set, like an APM/Altair 24mm UltraFlatField with a 17mm Hyperion, a 12mm X-Cel LX, and an 8mm Starguider. Many ways to keep the cost down or to have a set compatible with glasses if needed. My feeling is the Zoom works if the scope has a wide enough true field to yield decent size true fields in the zoom, but the 127mm Mak is not such a scope. You need wider fields in the eyepieces to bring out the scope's potential, especially when the mount is not driven. The Baader Zoom, as good an eyepiece as it is, just has too narrow an apparent field in the 24-12mm range.

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My Orion H-ß filter was a surprise--a 12nm bandwidth and a 94+% transmission. It was a pain to thread in, and I had a couple others with even narrower bandwidths, so the Orion went. But, given it is typically at a lower price than other high-end H-ß filters, it is certainly a good option.

It might be useful to know that the actual focal length is around 10.5mm, in case that matters to anyone.

I had one of those zooms around 1970-1974 (can't remember exactly when) and used it in a Unitron 4" f/15 refractor with wooden tripod, hex eyepiece turret, and weight-driven clock drive. I think mine was from Edmund Scientific, but my memory is foggy. By the way, the "Erfle" eyepiece in the picture is not an Erfle design. It's described as a "Wide Angle" here: http://www.quadibloc.com/science/opt04.htm No other design comes close, so who designed it is unknown. I owned one of those, too, but it wasn't all that sharp and had very short eye relief.

Sorry, I can't resist. This is a LongPerm: https://www.hadviser.com/perm-hair/24/ and this is a Long Perng: https://www.longperng.com.tw/goods.php?act=view&no=98 Thanks for the chuckle.😁

The focus mechanism says Long Perng, but it could be their own design.

The VIP Barlow is long out of production, but if you can find one, it is one of the best Barlows ever made. Beautiful polish. But, like all negative-only (telenegative) Barlows, it will add to the eye relief of the eyepiece. If not adding eye relief to the eyepiece is important, then you should be looking at telecentric Barlows like the TeleVue PowerMate or Explore Scientific Focal Extender.

You could add the 4.7mm APM XWA (they call it 5mm). It's a different enough experience from the Pentax that they might have different uses. But 110° is plenty wide if wider is what you want. And less costly than the TeleVue Nagler T6 in 5mm. With such a small exit pupil, you probably won't need glasses, so extra long eye relief isn't critical.

Bearing-wise, the Moonlite is a step up from the Long Perng (Lacerta), but Moonlite no longer makes visual-only focusers--they're all motorized now. There are very few high-end visual focusers left in production. There is the Starlight Instruments Feather Touch (with long back orders) and little else. JMI is out of production, Moonlite is gone as a visual-only focuser, Baader SteelTrack is another Chinese focuser. GSO and Long Perng seem to be cornering the market. Which is OK, I guess, as some of their focusers are quite nice.

Whoa! Was it one of the prototypes of the new barrel design? If that is the 20mm, replace the bottom cap with a hard plastic cap--the rubber caps get pressed in to contact the bottom lens and leave a smear there.

These 100° eyepieces are made by United Optics and are or have been sold under the names APM Antares Tecnosky Telescope Service William Optics Stellarvue Skywatcher Lunt Astrotech Sky Rover Think of them as all the same man wearing different suits.

One thing to know is that the difference between an inexpensive eyepiece and an expensive eyepiece, on axis, doesn't amount to much. Any difference there will be related to seeing more than the eyepiece. But that does not mean there are no differences. You should review this, because it will give you a lot of things to look at to compare eyepieces. https://www.cloudynights.com/topic/795988-how-to-evaluate-or-compare-eyepieces/?p=11463451 One simple rule is this: Well corrected to the edge of the field in short f/ratio scopes......Wide to ultrawide field.....Inexpensive: Pick any two.

I agree, perhaps the focal length is not exactly 24mm, or the field stop is not exactly 27mm, or the apparent field is not 68°. However, those calculations seem to be assuming orthoscopy, and distortion can alter the figures. I just measured a new one, and the field stop is 27.15mm +/-0.02mm

Yes. Magnification can stay the same and the object be distorted in shape. Take the example of a double star, which, with AMD, changes its apparent separation as the star nears the edge of the field. With RD, it does not, though the position angle may change.

Normally, I would ask the same question. But TeleVue's stated design philosophy is to reduce angular magnification distortion to as close to zero as possible, leaving RD wherever it falls. So in this particular case, the extra field is due to pincushion distortion, not a change in focal length/magnification.

Lumicon no longer makes an H-ß filter. TeleVue or Astronomik, if available, are your best choice. Just remember, this will be a low power filter. If your focuser uses a 2" eyepiece, don't buy a 1.25" filter. You will be using this below 10x/inch of aperture, and 2" filters fit the bottoms of many 1.25" adapters so are usable on 1.25" eyepieces.

You could just buy direct from APM......

Have you tried overhauling or adjusting the stock focuser? The 2" and 1.25" adapters are not the ideal way to do it, but almost any replacement focuser will be a low profile focuser, which will require a fairly thick stand-off block adapter between the tube and focuser. You used to be able to get these from Moonlite in the US, but I'm uncertain if that is still true. Plus, Moonlite no longer sells visual-use focusers. Perhaps Starlight Instruments (long lead time)? Maybe Baader makes such blocks for its SteelTrack focusers. They don't advertise them, however, so such a tall block with specific mounting holes might be a custom piece from a local machine shop. You see, other focusers don't use the dual adapters setup, so the tops of any other focuser would automatically be a lot lower than your current focuser with one of the 2 adapters. It's a good reason to see about adjusting your current focuser to be smoother and more accurate in its movement.

Positive rectilinear distortion stretched the field radially. It allows a certain field stop to hold a larger apparent field than its size indicates. As an example, the TeleVue 24mm Panoptic has a 27mm field stop and a 68° apparent field. With zero RD, the apparent field would be 58.7° With a 15.8% distortion, the field becomes 68°. Observers have noted for years that the design has a lot of RD and many observers remark that it is quite noticeable. In contrast, take the 24mm APM Ultra Flat field, which has a 27.3mm field stop (from timing) and a 63° apparent field (from the flashlight test). The zero RD figure for that focal length and field stop would be 59.3°. A 6.3% radial distortion gets to the 63° measured. Both eyepieces have pincushion distortion, but at very different levels. Tests of visibility for distortion in a moving field shows that roughly a 7% RD is seen by the eye as distortionless in a moving field, which is why many reviewers of the 24mm APM point to seeing almost no distortion. Alas, the eyepiece is not completely without lateral astigmatism. With zero RD, any widefield eyepiece will have significant AMD, and this leads to "globe" or "rolling ball" distortion when panning a field. In practice, in the hundreds of pairs of binoculars I've looked through, no designer opts for zero RD (straight lines always seem to curve very slightly near the edge), but RD is minimized while AMD is usually the predominant issue.

You're right. I was referring to rectilinear distortion. I should have specified. I edited.