Louis D

It seems a rather simple effort of trial and error under the stars/planets to find the cutoffs for various achromat f-ratios at both spectrum ends to make them more usable by eliminating most of the unfocused light ruining sharpness. Once these are cutoffs have been established, either dyed or interference filters could be made to pass only the well focused central region tailored to each focal ratio. Sure, the color balance would be way off being a teal blue-green mixed with yellow-orange, but they could be sold as general purpose planetary and double star splitting filters for achieving the best sharpness while retaining as much of the spectrum as possible in fast achromats.

No, the color makes me queasy. I suppose I'll give one a try sometime. I also intend to get a GSO or Meade (Japan) Yellow #8 just to see how it compares to the Yellow #12A. The #11 Yellow-Green is a bit aggressive in the blue, but does let in some yellow and orange while squashing most red:

I've acquired a Hirsch #12A and a spectrograph in the last year and have done some testing of various yellow filters with the spectrograph, an artificial star (not a great one by any means, just a foil pinhole over a Cree LED tactical flashlight), and with my ST80. First the composite image of several yellow filters in increasing violet/blue cutoff order. The named ones with R## at the beginning are Roscolux filters that did a good job of sharply cutting-off violet/blue at a particular wavelength. I didn't bother with the artificial star image of the Minus Violet filter because it wasn't handy when I was doing that particular test. However, it should show plenty of violet. From the above, the Yellow #12A would appear to be a good contender along with the R312 Canary, so I tried them all out in my ST80. In my ST80, everything from Cheap Yellow down to Yellow #12 filtered out practically all visual violet when viewing Jupiter. I'll have to try repeating the test on Venus sometime to be more equivalent to the artificial star. The minus violet did practically nothing to filter visible violet fringing. It also added no noticeable yellow cast. The cheap yellow filter left a slight yellow cast to the image as predicted by the artificial star while leaving a bit of violet. I think poor filter material led to scattering. The Hirsch #12A and R11 Light Straw actually looked the best. Neither one imparted much of any yellow tone while at the same time squashing nearly all visible violet. I was surprised at not seeing violet fringing in the latter despite the artificial star predicting it would be there. The R312 Canary imparted a noticeable yellow cast, as did R310 Daffodil. Both did a good job of eliminating violet despite the artificial star's prediction of a bit of violet for the latter. The GSO Yellow #12 once again imparts a very strong yellow cast while completely squashing all violet and most of the blue. At that point, I could clearly see red fringing, so it was back to light green filters to squash all fringing. Red fringing intruded in all views once violet was diminished or eliminated, so a true anti-fringing filter would need both violet and far red cut-offs. A wide teal-yellow-green filter from deep blue to red-orange would probably work for this purpose. I just don't know of any, even from Roscolux. It's too bad the Hirsch Yellow #12A is so hard to find used as it does a really good, and yet subtle, job of cutting out violet while leaving most blue. The R11 Light Straw would seem a viable, and available, alternative once cut and fit into a filter holder. A large sheet of it is cheap enough that it might be worth covering the entire objective to see how well that works.

In conversation, does the rest of the world always say 10th of March or 10th March? Folks in the US are just used to saying March 10th, and so we write it the same way. The 4th of July and Cinco de Mayo (5th of May) are the main two exceptions that I know of.

Did you use mahogany wood? It looks beautiful.

Try opening both eyes at the same time and merging the red dot in the dark window with the sky view seen by the unobstructed eye not looking through the window. I apologize in advance if you are blind in one eye.

Try a Yellow #8 to lightly filter out the purple haze. It can sharpen up the views a bit. I've found the appearance of lunar mare become more contrasty in my ST80 with a light yellow filter. Try a Green #56 to improve the snappiness of the focus by filtering out both ends of the spectrum that focus at different distances from green, which is what the achromat is optimized for. Even with premium eyepieces and dielectric diagonal, the only way I could make out the phase of Venus in my ST80 was with a light green filter. It could be that there's nothing that can be done to improve the view with the included diagonal and eyepieces, but it might be worth a shot.

The 6mm and 9mm versions certainly do suffer from SAEP. Both also perform fairly well in faster scopes. The reason for both results is the 6 and 9 employ a Smyth lens group to amplify the focal length. This slows down the light cone, improving edge performance. However, the matching of the Smyth and image forming groups was poorly done, leading to SAEP. The 15mm and 20mm versions have no Smyth group. They are purely positive designs which rarely suffer from SAEP. Barlows tend to push out the eye relief and induce SAEP in my experience. Both make holding the exit pupil more difficult. This may be what you're experiencing with the Barlowed 32mm. If you used a telemagnifier like a TV PowerMate instead, you would not experience either issue.

No SAEP in the SVBONY Redline 20mm, if that's what you're alluding to. Notice there's no significant shadowing in it's AFOV image in the third row, far right. It is a poor performer in the outer field below f/8 for certain. However, at f/18 in my binoviewer, they are terrific as a binoviewing pair for eyeglass wearers. Here's my write-up on it:

I've found that the longer the eye relief, the easier it is to see the entire field all at once in a relaxed manner, with or without eyeglasses. My 17mm ES-92 with 16mm of eye usable relief has a much easier to take in AFOV than my 20mm Meade 5000 UWA with its 12mm of eye usable relief even if I take off my eyeglasses. Part of that may be my deep set eyes. To examine the edges, the required head tipping seems more extreme in the Meade despite having a narrower AFOV. The problem is, adding long eye relief to SWA, UWA, and HWA eyepieces generally makes them bigger, heavier, and more expensive than their limited eye relief equivalents.

Due to eye lens recession, it has about 17mm of measured, usable eye relief. This is about perfect for eyeglass wearers with the eye cup flipped down.

At the same exit pupil, the contrast should look the same. It could simply be that your skies are incapable of supporting the Dob's higher powers necessary to equalize the exit pupil with that of the smaller refractor. If you're looking for contrast on big objects like M31 that require lower powers to frame it properly as a whole; then yes, the smaller aperture instrument is the better choice.

At the same exit pupil size, the refractor's star image will be tighter because there's no central obstruction or spider vanes reducing the central peak intensity of the Airy disk. The following diagram from TelescopeOptics.net shows how as the central obstruction grows from 0% to 50% of the clear aperture diameter, so grows the bloat of the Airy disk. This manifests itself at the eyepiece as bloated instead of pinpoint stars. I can confirm that my ED and APO refractors show much tighter stars than my Dob or Mak at a given exit pupil. It is entirely possible that it is impossible as you say to separate a given double star pair due to insufficient resolution as a result of insufficient aperture in a smallish refractor versus a large reflector. It's just that the unresolved pair will look nice and tight in the refractor relative to the larger reflector's view at the same exit pupil even if resolved into two slightly bloated stars.

I looked through an 8" Celestron EdgeHD with a 10mm Delos at Jupiter a few years back at a star party, and it was astonishingly sharp. Apparently, all that extra glass in the corrector puts all the errant SCT light rays where they belong. The 12.5" Mag-1 Portaball I viewed through about 10 or 15 years ago was a lot easier to move around the skies. The Zambuto primary made Jupiter look like a photograph. Being on a Osypowski EQ platform made extended studying of the image a breeze. Eyepiece balance can be an issue, though. The owner was using fairly lightweight, but high quality, Plossls.

I had the opposite experience 20+ years ago at a local star party. The Obsession, Starmaster, Star Splitter, Tectron, and Mag-1 Dobs with their premium, hand figured mirrors and hand built mounts were blowing away the Tak and AP refractors on Jupiter, to say nothing of SCTs. I bought a Dob later that year and have enjoyed it ever since. I only recently bought a 90mm APO refractor and quite enjoy it as well. Yes, stars are more pinpoint in the APO, but aperture limits resolution under our steady Texas skies. It's not unusual to push a 12" to 15" premium Dob to 350x and higher on planets and GCs revealing far more detail than a 4" APO at 200x.

Here's what Ernest in Russia had to say (after Google Translate) when running the 24mm APM UFF through his testing: Direct comparison in the focuser of a fast (1:5) 24 mm telescopeES68 and UFF showed that the quality of correction of field aberrations in UFF is better than in ES68. In fact, in the image that UFF builds, at 90% of the field of view there are only signs of small focus variations (higher-order curvature) - you can choose the focus at which the field of view looks very uniform and only at the very edge of the field diaphragm image quality drops sharply. But in ES68, visible manifestations of curvature and astigmatism start from about half of the field of view and gradually increase towards the edge of the field of view. Both eyepieces are excellently corrected in terms of chromatism. Straight line distortion is excellently corrected in UFF, angular distortion is better corrected in ES68. UFF is more massive and overall. 24 mm Panoptic lighter and more compact, in terms of monochromatic aberrations the field is corrected better than in UFF. But UFF and ES68 build an image more free from chromatism. All three eyepieces have virtually the same effective field stop (TFOV) diameter. Both Panoptic and ES68 lose a lot to this UFF in terms of eye relief.

Sounds like a hard pass for me on the 23mm XW85. The 22mm Nagler T4 was working well for me Saturday night despite not being the sharpest tool in the case. Sure, it would be nice to have an improved version like a 23mm ES-92, but it doesn't seem like that is ever going to happen. The XW85 could have been a contender were it not for the eye relief. I will say I was enjoying the view through my 26mm (really 25mm) Meade MWA Saturday night. Backed off to a comfortable eye relief distance, SAEP is not an issue. AFOV is very similar to the 14mm Morpheus. Sharpness is as well. Moving your head side to side allows for seeing more field obscured by SAEP, just not all at once. The Trapezium became difficult to split at 24x only in the last 15% of the field, which I consider a pretty decent showing.

Finally compared the 3mm (really 3.5mm) setting to my 3.5mm Pentax XW. There is no color fringing anywhere in the field with the Pentax, but there is in the Svbony. After comparing images with my premium eyepieces, I'd say from 5mm to 8mm, the Svbony hangs with the best of them. At 4mm and 3mm (really 3.5mm), image quality becomes second tier. So, to get premium level performance from 5mm to 8mm at it's current price is a pretty good deal. I'd consider the sub-5mm settings as nice to have, but not really showing any additional detail due to aberrations.

All well and good as long as you don't need moderately long eye relief.

However, once VAT is removed, shipping to US added, and GBP to USD converted, it comes out to within $10 of your 18mm APM UFF price. The GBP is well off of its September 2022 lows.

Nothing more than what I've read on CN. Probably best in slower f-ratio scopes based on that.

I just recalled another 18mm step-up eyepiece that had poor performance relative to its stablemates. It was the 18mm 55° LER made by Long Perng and sold under many different names. Again, it lacked a Smyth lens group and had really poor edge correction even in modest f-ratio scopes. The rest of the line all had Smyth lenses and were well regarded.

Make sure the triplet is fully acclimated. My 90mm triplet takes over 30 minutes to acclimate just 11 degrees C. Until then, I see all sorts of star spikes.

Here's a comparison of premium scopes of different designs which sought to answer this very question.

The 18mm Meade HD-60 is similarly the weakest link in that line as well. The problem is that all of these step-up 18mm eyepieces are positive-only designs with no negative Smyth lens group to improve correction in faster scopes. The 18mm APM UFF and its brand-mates would probably be a good bet at that focal length since it is a negative-positive design. Yes, it is considerably more expensive.