Louis D

And by all accounts, they are very well made and fairly innovative. It's a bit of a shame Chinese manufacturers haven't branded their own optics and sold them as such. As a result, there's a lot of confusion in the marketplace about house brands versus clones versus competitive designs. When you put your company brand on something, there's a much higher sense of pride and desire to avoid selling sub-par goods. It kind of reminds me of Japanese optical houses in the post-WWII period when many of them sold their goods in the US under various house brands such as Sears, Wards, Penney, Vivitar, Soligor, Spiratone, Kalimar, Cambron, Quantaray, Phoenix, etc. No one in the US really knew who was making what, and it was a total quality crap-shoot.

I think I have a 2" M&SG filter kicking around somewhere that I picked up for cheap on closeout. I'll have to try putting it on the front of my 50mm RACI to see if it works sometime.

It's probably just got a bit of thread-lock on it to keep it from unscrewing during zooming.

More interestingly, where was the optical design done? Germany? China? Other?

A 2" filter would fit just about perfectly on the front of a 50mm finder scope.

Well, you just changed the subject from your initial question. If you're looking to buy, what is your budget, usage intention of visual vs. astrophotography, distance to carry or wheel-out scope to observing location, your personal weight lifting ability, portability requirements, desired objects to view/photograph, storage room available, manual vs. push-to vs. goto vs. tracking, etc.

Doesn't the moon get tangentially flattened in the Docter at the edge (like it's squished up against the field stop)? I see that in the Meade MWA 26mm. Both forms of distortion are unfortunate but unavoidable in UWAs. Perhaps the "egg" distortion is worse on the moon aesthetically.

Mono mostly because it's a hassle to setup the binoviewer and to keep readjusting the angle as I move around the sky. Binoviewing is excellent for full moon viewing because there's no eye fatigue since both eyes see the same brightness.

How about baking it in the sun to kill off the mold via intense UV? I know our Texas sun works wonders on such things.

Oh, I remembered them, but I didn't know exactly how to classify them among the latest crop of ED/APO scopes coming out of China. 🤔 And yes, @merlin100, they're delicious. 😉

And the various reflector designs to eliminate the central obstruction: Off-axis Newtonian, Yolo, Schiefspiegler, Stevick-Paul, Herschelian, etc. Of course, these are mostly ATM builds.

And I would go further to subdivide ED and APO into four general categories: 1. ED Doublet of FPL-51/FCD-1 glass. 2. ED/APO Doublet of FPL-53/FCD-100 glass. 3. ED/APO Triplet of FPL-51/FCD-1 glass. 4. APO Triplet of FPL-53/FCD-100 glass. As you progress into more elements and more expensive glasses that have lower dispersion, you get better chromatic correction. Design points 2 and 3 above can flip flop depending on the level of design execution. As discussed in another recent thread, the actual design choices for the objective and it's mating element(s) (curves, spacing, etc.) also make a difference in its ED/APO characteristics. Then there's flourite "glass" which has properties slightly better than FPL-53 and is even more expensive. It used to be the only game in town for ultra-low dispersion objectives.

More or less, the original Plossl design was asymmetric while the current usage has mostly come to refer to symmetric designs. I saw mostly because there are the 5 element "super" Plossls out there that are even more remotely related to the original Plossl design. Meade marketed a line of 60 degree, 5 (and in one case, 6) element Plossls (their 5000 series) a few years back just to muddy the Plossl waters even further.

At f/12 in my 127 Mak, the cheap 30mm 80 degree Widefield clones do quite well across the field as is visible in this image (Agena UWA, 2nd from bottom): The ES-82 at the bottom would be equivalent to the 31mm Nagler since it is basically a Chinese clone of the design. The UWA eyepiece has the advantages of no CAEP or SAEP, less magnification distortion across the field, longer eye relief, more compactness, and much lighter weight than the ES-82/Nagler equivalents. For under £70, it might be worth a look.

Hi Stu! 😄 Are these the first 8 steps towards recovery from refractorholism? Only 4 more to go. Question is, what will they be?

You'll really enjoy the Delos 10mm. I've had mine since it was introduced almost a decade ago. It's super sharp across the field and easy to view through.

I'll sometimes leave my GSO 2" ED Barlow with the TV PBI in the focuser to double my scope's focal length all night and use longer focal length eyepieces at higher powers just to change things up a bit on some nights.

And then there's the equally mysterious Antares Speers-Waler Series 4 eyepieces which are also fairly recent, similar in specs and price to the ES-82 LERs, and also have a dearth of reports/reviews of them.

It really depends on the eyepieces. If I've got a 5mm Huygens (not so far fetched, I've got an H6mm) and a 10mm Delos (I do) and a quality 2x Barlow (such as Tele Vue, Meade 140, GSO ED, etc.), then I'd definitely use the 10mm with the Barlow any day over the 5mm Huygens. It will be more comfortable, sharper, etc. However, let's say I've got a 5.2 Pentax XL (I do), I'll use it because the integrated factory solution of positive and negative design elements in it works better than the Delos/Barlow combination, it's more compact, and it will come to focus in all of my scopes. Longish Barlows that I prefer work best in Newtonians and other scopes that don't utilize diagonals. Shorter Barlows that do work well in diagonals have more aberrations than longer ones in my experience. The trick of attaching the Barlow lens element to the bottom of an eyepiece works best when the field lens is well up inside the eyepiece barrel. When they're near the bottom as in your Starguider, the field lens is intercepting the diverging Barlow rays way too soon which leads to a host of aberrations. That's because the Barlow lens has a designed working distance related to its focal length, and you don't want to get too far inside of that distance for best performance. It also doesn't help that the lower elements of the Starguider are acting like a Barlow in their own right, so now you're adding even more divergence to the system with that Barlow lens element. This trick works best with positive only designs in my experience; and even then, it is hit or miss. Often, the field of view is constricted and the outer field is distorted.

That matches up well with this CN comparison of the edge correction of each. I've linked the images below: 30mm Pentax XW: 30mm APM Ultra Flat Field: What I'm wondering is how the Pentax XW 40mm stacks up against the Meade 5000 SWA 40mm that I've been using since the great SWA blowout sale. I picked it up for $125 and am wondering how much better the $400 Pentax 40mm would be.

I'd be interested in hearing your preferences among them.

I think it's clamped to the bottom of one of the truss poles.

Here's a collection of longitudinal aberration plots for various achromatic, semi-APO (ED?), and APO objectives analyzed by Vlad on his optics page linked above. Note that the horizontal scale varies from plot to plot: One of the takeaways is that there is no one singular definition of an achromatic, semi-APO (ED?), or APO objective. Depending on the design and glass choices, the chromatic performance can vary quite a bit across the field of view within each group. However, it is clear that each step up in general correction yields an improvement.

The classic definition of achromatic is 2 crossings and apochromatic is 3 widely spaced crossings. Superapochromatic is 4 or more. However, many APOs have one or more crossings in the infrared rendering them less useful. Here's a diagram from Wikipedia showing the various objective types: Crossings refer to wavelengths of light that focus at the focal plane of the objective. In a reflector, all wavelengths focus at the focal plane by comparison.

I believe this shows the distance in millimeters each wavelength of light actually focuses in front of or behind the focal point as you move away from the optical axis (bottom) to the edge (top). Thus, about 70% out from dead center to edge produces a nearly color free image when in focus.