Louis D

If you already have a bunch of 2" eyepieces for Dobs and fracs, and 2" diagonals for fracs, it just seems natural to invest in a 2" visual back for your Cats. They're not very expensive at all.

Try the $15 Aspheric first, assuming you're not using it in a f/4 to f/5 system. At 1.4 ounces and tiny in size, it's worth a shot. If you don't like it, use it as a focuser plug.

If the 25mm GSO Plossl is similar to my Taiwan made (GSO most likely) 26mm Orion Sirius Plossl, then it has only 11mm of measured, usable eye relief. This is much too short to take in the entire, measured 53° AFOV with eyeglasses for me. The problem is the highly recessed, 23mm eye lens. Due to edge distortion, it works out to have only a 49° eAFOV. It has a 22.3mm measured field stop. The 25mm Starguider BST (Paradigm) has a true 60° AFOV, a 61° eAFOV, 17mm of usable eye relief, and a 26.7mm field stop. It does fall off quite noticeably in sharpness in the outer 25% of the field at f/6. The now discontinued 25mm Meade HD-60 actually has a 58° AFOV, a 57° eAFOV, 18mm of usable eye relief, and a 24.9mm field stop. It remains sharper to the edge, but the FOV and FS values are less than the BST. The real winner at this focal length for long eye relief and maximum true field of view is the 24mm APM UFF, which is now available in a variety of other brands as well. It has a 63° AFOV, a 66° eAFOV, 17mm of usable eye relief, and a 27.5mm field stop. It does get a bit fuzzy right near the edge due to some vignetting. If you want to stay really cheap, the 23mm 62° Vite/Svbony Aspheric, available on ebay for about $15 direct from China, is really pretty decent. It has a 63° AFOV, a 65° eAFOV, 18mm of usable eye relief with the rubber eye guard removed (it just pulls off), and a 26.2mm field stop. It gets a bit fuzzy toward the edge, but it's not that much worse than the 25mm BST. All of the AFOV, eye relief, and FS numbers were measured by me using a variety of direct and photographic techniques. Below is a comparison image of my 23mm-28mm eyepieces taken through a field flattened AT 72ED. It's pretty clear how well corrected the 27mm Panoptic is compared to the others. I would imagine the 24mm is similarly well corrected.

Never tried it, but I love my 14mm and 5.2mm XLs. It's just that the 28mm XL is generally regarded as the best 1.25" eyepiece in that focal length range ever made.

Here's a review of the Smart Astronomy 27mm EF Eyepiece which I believe is the same eyepiece. Not quite as good as the 28mm Pentax XL, but not bad at all.

Bright stars themselves are slightly bloated as well in Newts. This is the difference I immediately noticed between the Dob and the fracs. It makes splitting close, non-equal brightness doubles easier in the fracs.

No problem. My future son-in-law is also color blind.

Except provide diffraction free, pinpoint stars. For wide field viewing, the difference is noticeable.

Notice the hashed lines go in opposite directions and signify different levels of correction.

The oldest of the designs is the APM UW 30mm 80° which is a Markus Ludes commissioned clone of the 30m KK Wide Scan II (or III, I can't remember) which was a Japanese designed and built UWA of the 90s. It has 5 elements, massive field curvature, excellent central sharpness, long eye relief, and very good edge correction once field flattened. The TMB Paragon came next about 15 years ago. The 30mm and 40mm versions were designed by the late Thomas M. Back. The 35mm version was inspired by his designs. They have very good correction for their size and weight, 6 elements, shorter eye relief, and flat fields. The design was cloned by multiple Chinese optical shops (as were TMB's 58° Planetary line). The APM UFF came last about 4 years ago. They were designed by Mark Ackermann for APM. Since then, they've been sold by KUO to multiple vendors. The designs are shown below:

The better question is, can you tell the difference between an ED and an APO visually. In my experience at low powers, you can't. It's only at high planetary powers that you can see the better color correction of the APO. Really long f-ratio achromats can be very good indeed, but mounting them is a pain. If you enjoy wide field views, this means short f-ratios. You'll want at least an ED to avoid excessive violet fringing. The ED you're looking at is made by Sharpstar Optics. They're probably the highest quality Chinese made refractors. I have the TS-Optics 90mm FPL-53 triplet and love the optical and mechanical quality. All Skywatcher products are made by Synta. Optically, they're very good. However, mechanically, they are very basic to hit an introductory price point. Swapping focusers (to MoonLite generally) on the SW 120mm APO doublet is pretty common because of this. Here's a good chart showing the differences in CA across various apertures and f-ratios of achromats: Notice the trend that as you increase aperture, you need longer f-ratios to maintain constant CA correction. This is also true of ED and APO scopes of a constant lens design. If you want good color correction at f/5 and 100mm, you will need an FPL-53 triplet or equivalent. However, if you can live with f/8, an ED or APO doublet will suffice.

The 30mm APM UFF is an entirely different design from the 30mm Aero ED (TMB Paragon originally). And these are not to be confused with the APM UW 30mm 80° which is yet another entirely different design. I think part of the confusion is all are (most likely) made (but not designed) by KUO in China, so they have very similar build qualities.

If you're content with a 72 to 76 degree field, the TV Delos and Baader Morpheus might suit you better. They all have ample eye relief and an easy to take in view.

In my experience, viewing comfort equates to eye relief. If you have to cram your eye into a recessed eye cup to take in the view, how can you relax? The Ethos all advertise 15mm of eye relief, but due to ergonomic factors, it's probably closer to 12mm to 13mm. Most folks find them comfortable to use without eyeglasses, but the view isn't as readily accessible as longer eye relief eyepieces. The ES-92 eyepieces have 20mm of advertised eye relief, but I've measured them at 17mm of usable eye relief. This is just enough to use them comfortably with eyeglasses. Many folks find it too much without eyeglasses, but others find the view more accessible and immersive without eyeglasses than the Ethos due to the longer eye relief. For me, with eyeglasses, I can take in the whole field at once without having to crane my head to the side. I've overhead folks who look into them exclaim "My God, it's full of stars!", and so it is. The flat, aberration free field also helps to contribute to this experience.

Stick with 8x42 or thereabouts for handheld use.

It will also increase the focal length of your SCT by 3mm for each 1mm of increased optical path length. This might amount to 150mm or so moving from a 1.25" to 2" diagonal and visual back. It will also slightly increase spherical aberration by moving the focal point off the design focal length.

Given the large increases in Skywatcher prices for equipment that does come in, that's a great deal.

The SARD Mark 43 6x42mm binoculars are well regarded for long eye relief and wide field views.

The Celestron zoom is $90 in the US while the BHZ is $290 and the matching Barlow is $120. I see they are sold as a combo for $389 here. That's roughly 4x as expensive in the US.

These seem to be discontinued.

Spin it as far as possible in each direction and note when the star spots are at minimum size. That is the point of best focus even if the stars are not pinpoints. Seagull shapes are generally indicative of astigmatism rather than being out of focus. If you have astigmatism in your eyes, you may also be seeing it if you're not wearing eyeglasses while focusing. Try focusing with and without eyeglasses in that case.

It can also massively improve image quality by masking off the objective's edges where figuring errors most often occur.

For cheap, get a 15x70 with 65 degree apparent field and about 17mm of eye relief. They yield about a 4 degree TFOV. They're fairly light and handy for grabbing quick higher powered views. I use them both for astronomy and terrestrial targets.

I've got a few vintage 10 and 11 degree AFOV 7x35mm binoculars. Amazing views, but almost negative eye relief. They used aspheric lensed eyepieces to achieve really decent edge performance. Why ultrawide binoculars in the first place? You could try binoviewers on a refractor or even a binoscope as alternatives.

Merging images in higher powered eyepieces can be problematic for some people, so keep that in mind.