Louis D

As far as price, you got that right. My 6" f/5.9 achromat cost 3 times what my 6" f/5 Newt cost (both used).

Funny you should mention that. I'm going to try mounting them on either side of my DSV-2B mount the next chance I get and compare the views for aesthetics as well as for absolute sharpness, contrast, resolution, etc. I'm curious if Sir Patrick was on to something. I figure a 90mm f/6.9 FPL-53 triplet APO should be similar to an old 3" f/15 or longer achromat as far as color correction goes. The focal length will obviously be completely different, though. The Newt is probably faster than the Newts of old as well.

Ultimately, it comes down to what the market will bear. I always laugh when I see ads saying "Final price reduction!" as if that makes any difference to most buyers. Generally, it's sellers who way overprice in the first place on non-in-demand items who do this. I especially chuckle when they're trying to sell at a higher than new current price when the same item goes on sale or the price has come down drastically since new. I've contacted some to point this out, and they get all indignant saying they sticking to their guns asking 75% to 80% (or more) of the price they paid.

Especially when you consider you need to subtract the 20% VAT from the new price. It's highly unlike the seller is going to collect it again, so you really need to work from the pre-tax price to establish the tax-free used price.

Just be aware that type of diagonal often has a restriction to about 23mm clear aperture: If you do buy one, check it immediately to make sure it doesn't have a stopped down clear aperture. I bought one used to use with a 22mm clear aperture binoviewer, so I'm okay with the aperture restriction.

I think the choice was biased by poorly figured, coated, and aligned Newtonians of the day. 3" slow achromats of that era tended to be of very good quality in comparison. A 6" Newt of the day would have been very long and unwieldy. Some old Unitron cartoons along these lines:

I just checked a bunch of recent ads for these vintage black C8s on CN classifieds, and with few to no accessories, they tend to go for $400 to $500, so keep that in mind.

I've been observing with Meade Safari Pro porro-prism 8x42 bins for over 25 years and still love them. I managed to snag a second pair off of ebay a couple of years ago as a backup pair. I like their 65 degree field and 18mm of usable eye relief.

Check your cylinder (CYL) diopter amount for your observing eye in your eyeglass prescription. It is a measure of how much astigmatism you have in each eye. I have around 2.0 diopters in each eye, so I need to wear eyeglasses all the way down to fairly small exit pupils to avoid seeing spiky stars and blurred fine details. Tele Vue has this chart for their DIOPTRX™ line: It's fairly accurate for most observers. For instance, I can get by without correction below 1mm exit pupil. It still improves the sharpness a little bit by putting eyeglasses back on at sub-1mm exit pupils, but not tremendously as at larger exit pupils.

On planets and other small objects, I find it doesn't matter much because I can still see enough of the field of view while wearing eyeglasses to see what I want to see. On the moon, I'll agree, it doesn't work very well unless I'm just focusing in on a single feature to study. On my undriven alt-az mount, I just tilt my head to follow objects as they drift through the FOV from edge to edge.

With my deep set eyes, I have about a 15mm gap between my eyeglass lens and my cornea. This makes 17mm of usable eye relief about the least I can comfortably get by with. Of the BST Starguiders, I've measured 12mm of usable eye relief for all but the 25mm, which has 17mm and is comfortable to use. If 12mm of usable eye relief is enough for an observer wearing eyeglasses, I consider them lucky. It opens up a huge number of eyepieces for them from Nagler T6 to Ethos with sub-17mm usable eye relief.

In my experience, my 6" f/5 Newtonian runs huge circles around my 80mm (3.1") f/5 refractor. To equate them in performance in any way is a complete insult to the Newtonian.

See inline comments in yellow.

Because of this thread, I went out tonight with my 90mm APO and looked the moon, Jupiter, Collinder 70, and the Orion Nebula. It was basically color free compared to my 6" f/5.9 achromat, no surprise. It showed a wealth of detail on the moon with excellent contrast. It showed the main two bands of Jupiter quite clearly along with obvious size differences among its moons. However, I could not discern any detail in the main bands. Collinder 70 was rendered beautifully and in context at low power with pinpoint stars. The Orion Nebula looked great from low power right up to the highest powers. The Trapezium was always obvious as multi-member. However, I could not get even a hint of the E and F components tonight. I loved how light and compact it is compared to my 6" achro. It makes for a terrific grab and go scope.

I forgot to mention that the 90mm APO shows exceptionally nice solar images with an affordable 1.25" Herschel wedge.

I've extensively compared my 6" f/5 Newt to my 6" f/5.9 achromat over the past year, and it is no contest. The Newt runs circles around the achromat. It's color free, half the weight, one third the price, wider field of view, spherical aberration free, holds collimation exceptionally well, and is well corrected with a coma corrector. I have spent countless hours and hundreds of dollars on various filters trying to make the achromat somewhat sharper without cutting out too much of the spectrum. I honestly don't get the hype for large, fast achromats. Either buy an APO refractor, or stick with a fast Newtonian.

I've found that the 12mm of usable eye relief in the 5mm Starguider BST to be too little while wearing eyeglasses to comfortably take in the entire FOV at once. I've found that the 8mm to 9mm of usable eye relief of the Svbony 3-8mm to feel similar in use. I would be concerned about trying to use its zooming action in a helical focuser, though. You'd have to firmly hold the focuser while zooming.

By pinched optics, I mean that bright stars have spikes emanating radially in oddball orientations from the center of each of them. I was panic stricken the first time I looked through mine because of it. However, I noticed it getting better over time, so I went back inside to let it acclimate for 20 or 30 minutes. I haven't noticed color correction changing during acclimatization, but it would be hard to discern among the bright spikes with my scope.

One other observation I made accidentally while observing the moon was thinking I had the LP470 in my fingers when in fact I had the BP500. I was mystified why I was seeing only about 50% of the violet disappearing. I then realized I had the wrong filter. However, I had a much greater epiphany in retrospect. Why was I seeing any violet reduction when the BP500 blocks no violet at all, just orange and red? I went back to the BP500 to try and figure it out by blinking it in and out of the light path from the eyepiece to my eye. I noticed when adding the LP470 that it was attenuating red and orange fringing quite well, but when I removed the LP470 from the stack by blinking it in and out, the red fringing turned to purple/violet! I then tried the really weak Meade #8 Light Yellow by itself. It cut the far violet fringing, but none of the near-blue violet fringing. I then added the BP500 to the stack blinking it in and out. Suddenly, the amount of residual purple/violet fringe dropped way down to near acceptable levels. Near-blue violet fringing I had been seeing with the weak yellow #8 filter was actually red fringing combining with blue that was fairly well focused creating the impression of near-blue violet fringing via purple fringing. It's just that my eye can't discriminate violet fringing from purple fringing. Now I'm thinking a weaker yellow may be sufficient if a strong enough cyan is applied at the same time to avoid red fringing creating the impression of near-blue violet fringing. I'll have to try more experimentation with smaller objects such as the bright planets and stars before drawing any stronger conclusions. I just thought it weird that red fringing could manifest itself as violet fringing on an extended object like the moon.

Calling @Don Pensack for feedback. I believe he uses Ethos extensively at high powers in his large Dob, or at least used to.

My 90mm triplet takes at most 30 minutes to reach equilibrium with a modest temperature differential. Until then, it appears to have pinched optics. My 72ED FPL-51 doublet has no such cool down issues. The issue with the triplet may come down to an under-engineered lens cell. I know AP and other premium APO makers spend a lot of effort on cell stability across temperature changes for imaging reasons.

I have a TS-Optics 90mm f6.6 FPL-53 Triplet APO that I use visually. It's fine on planets and DSOs, but the limited aperture compared to an 8" Dob is really noticeable in resolution and light gathering. It shows the bands and moons of Jupiter, the rings, Cassini division, and brighter moons of Saturn, the phases of Venus, and the largest features of Mars. The moon shows a wealth of details. However, don't expect to see a wealth of details in Jupiter's bands like barges and whorls or fine details on Mars. Lucky imaging with it might reveal some, but the human eye needs more aperture to pick up such details visually. Here's a comparison image of the moon I took through various sub-6" scopes to give you some idea of visible image scale and details: Ignore the peachy color in some images. That was introduced by the eyepiece (12.5mm APM Hi-FW) that I was using for afocal projection into my smartphone camera. As for a diagonal, I like the various dielectrics made by GSO and sold under such many brands including Revelation in the UK.

I've found my 9mm Morpheus to perform almost identically to my 10mm Delos, just wider in AFOV.

I picked up a BP500 Green-Blue (cyan) filter and tried it out last night in the KUO 152mm. It's very similar to the Hirsch #82B Light Blue (light cyan), just with a bit higher transmission in the middle and with a bit longer tail-off in the red. It paired nicely with the LP470 Light Yellow to create a pale yellow-blue-green filter to eliminate pretty much all false color at both ends of the spectrum, at least to my old eyes. I compared it against the new Lumicon #11 Yellow-Green, Vivitar Light Green #11 (more of an X0), and Hirsch #12A Light Yellow stacked with the Hirsch #82B. On the moon, they all worked pretty well. I'd probably be content with any of them over no filtering. It's remarkable how much crispness each of them added to the image. On the Orion Nebula, the LP470/BP500 stack had the highest transmission with the Hirsch combo close behind. It was no surprise that the Lumicon and Vivitar filters dimmed the nebula a bit because they start tailing off before 500nm in the green/blue/violet end of the spectrum. You don't really need much if any filtering on such dim objects anyway, but I was specifically looking for transmission characteristics. On Orion's Belt/Collinder 70, there is a bit of violet and red fringing still visible on the bright stars, but it is quite tame compared to the filterless view. The yellow-green-blue color cast is barely noticeable after a few seconds. I guess my brain was just factoring it out. Lastly, the Newport BG39 cyan glass is way dimmer than either the BP500 or Hirsch #82B with a much sharper cutoff around 600nm instead of 625nm or even a bit further right. I'm considering retiring from my main filter wallet it as I see no practical use for it. By the way, all of the LP, SP, and BP filters were made by MidOpt in the Chicago metropolitan area.

Check out this post of mine linking to direct comparison edge images of the two eyepieces: If that's not enough to sway your decision, I don't know what would do it.