Louis D

I found what appears to be a nice used 8" Dob in Cotia outside Sao Paolo. Perhaps you could meet the seller half way near Lorena to personally inspect the scope, avoid shipping costs, and get a quick lesson on how to use it. I was hoping someone would be selling a nice alt-az scope such as the Sky-Watcher Explorer-130PS AZ5 Deluxe at FLO, but I couldn't locate any sellers in Brazil.

Those alt-az mounts are pretty horrendous. I'll parrot the usual advice on here. Get the 8" Dob if your budget will stretch and you have the room to store it and ability to carry it out (it does break down into two manageable pieces). This would be a case of buy it once. Many folks are contented with a lifetime of viewing through such a scope.

Surprisingly, not so much. As long as the flattener is intended for a certain focal ratio range, it doesn't seem to matter all that much. Sort of like Newtonian coma correctors. As long as they're designed for the focal ratio range of your scope, you're pretty much good to go. I'm sure astrophotographers would disagree when going for the absolute best correction, but for visual usage and casual astrophotography, not so much. Getting the spacing exactly right seems to be a much bigger deal.

I've found the Trapezium in Orion to be the best test of edge performance. It's fairly easy to discern if they lose any fidelity as they approach the edge. The moon, being an extended object, is a bit more difficult. Sure, edge color is easy to discern, but field curvature and astigmatism are much less readily apparent. Try looking for very fine details, both high and low contrast, on a relatively featureless mare and see if they are less distinct at the edge.

@Don Pensack Interesting. I was going by the markings on my refractor's focuser. Perhaps they're not in millimeters. I'll have to measure them against a metric ruler to see if they really are. It could be that the two marks difference between the 12mm and 17mm ES-92 were actually 3mm because the 12mm was exactly parfocal with both the 30mm ES-82 (mushroom top) and the 40mm Meade 5000 SWA (ES-68).

@Adam Barnsley Hope you're feeling better soon. My daughter's come down with something as well. Too much NYE partying, I think.

@TioRi Why don't you list some of the Celestron scopes you're interested in so we can give you more targeted feedback?

@JTEC, you might want to read up on CN's Denis's take on various premium Zeiss microscope eyepieces when used for astronomy. Many of them rival the ZAOs in quality. I have a feeling this is where Zeiss is targeting their production capacity due to higher volumes and profit margins. https://www.cloudynights.com/topic/511907-review-carl-zeiss-premium-microscope-eyepieces-on-telescopes/

That's why I went with the TSFLAT2, because it has M48 threads which is the same as 2" filters. There is also no discernible vignetting visually thanks to the large clear aperture. The problem with most T2 threaded flatteners is that they have a 55mm working distance so they can be threaded directly to DSLRs via a T-mount adapter, so they are pretty difficult to use visually. The TSFLAT2 has a much longer working distance, about 110mm to 128mm for short refractors. Thus, it can be attached to the front of a 2" diagonal and be pretty close to the correct separation. That, and it is a unit power flattener. That is, it doesn't have any focal reduction built in. Thus, it pretty much disappears visually.

It's not just with ~400mm telescopes, either. Santa brought me a 600mm focal length refractor, and it also needs some help from the TSFLAT2, though not as much. Technically, it should work best without the 15mm extension, but I can't visually detect any difference in correction with or without it. I'd need to take a photo of a star field to get a definitive answer. However, when I remove the flattener altogether, it's very noticeable that the stars are bloated in the outer parts of the field in very wide true field of view eyepieces when stars in the center are well focused. I'll probably keep the 15mm extension so it can be swapped into the AT72ED unaltered. Without that 15mm extension, stars are noticeably more bloated near the edge than with it in that scope. I did find that stars were ever so slightly tighter at very high powers without the flattener in place in the 600mm scope. This is the same as with my GSO coma corrector in my Dob. In both cases, curvature and coma at high powers matter very little when attention is focused on axis, so I remove them temporarily at the very highest powers.

I'll have to try focusing the moon on a piece of white paper across the 2" eyepiece holder and then stick the 17mm ES-92 to see if it really does focus at the shoulder, within a millimeter let's say. I had thought the 30mm ES-82 and 40mm Meade 5000 SWA focused at the shoulder up until a couple of nights ago. Now, it appears they might focus 2mm below it.

Unless you're old like me and have presbyopia that leaves you with fixed focus eyes. I do visual only and couldn't stand the defocused stars at the edge at low powers. I mainly use the AT72ED for low powers, so this wasn't an occasional thing, it was all the time.

Well, have you field flattened it? All of the 70mm class ED scopes have very short focal lengths and thus very curved fields. If you refocus the edges, are the stars sharp? If so, invest in a TSFLAT2 field flattener. I use one with my AT72ED. I swapped out the nosepiece on my GSO 2" diagonal with a 15mm SCT threaded extension that has 48mm threads on the opposite end. I then screw the TSFLAT2 into those 48mm threads. Now I have pinpoint stars across the field.

Yes, good call @DaveL59, there is a picture in post #4 showing the order. I've inserted it in this thread below: Hopefully, it's the same design as the OP's eyepiece.

Since it uses an erecting eyepiece, perhaps this Celestron reassembly page might help. A person on Cloudy Nights disassembled and optimized his Celestron erecting eyepiece in this thread. Perhaps it might help guide you as well.

Well, Santa brought me one of these stools for Christmas and I just got done using it for the first time with a telescope. It had no problems supporting my 200 pound weight at any height. It was quite easy to set the height for various eyepiece heights. So far so good. The down side is that it has a small, hard, 10" diameter seat that makes your rear go numb after an hour or so. I threw a seat cushion on it after two hours due to extreme pain in my coccyx, and that helped immensely. I will have to work on buying/making a larger, conforming cushion for the future. If you limit usage to 30 minutes or less, it's not that big of an issue.

I just checked focus position, and the 12mm ES-92 focuses 2mm further out than the 17mm ES-92. Interestingly enough, the 12mm ES-92 is parfocal with the 30mm ES-82 and 40mm Meade 5000 SWA (ES-68). I had thought it would be the 17mm ES-92 that would be parfocal with them.

Since most 1.25" mirror diagonals have an optical path length between 75 and 95 millimeters, that 80mm extension tube should provide just about the correct distance for focus.

They use an f/4 spherical primary mirror, so only the very center is of any use at all. The rest of the field is a distorted mess of spherical aberration. They're also a royal pain to put on a target due to their short stature and sitting on a table top. Make sure to get a Newtonian with a parabolic primary. It's the most important upgrade on a Newt. You'd be much better off with the SW Heritage 130p. If you can stretch your budget, the SW Explorer 130PS AZ5 would be much more comfortable to use.

I'm sure they're fine, but I went with GSO/Revelation Super Plossls because they were cheaper with a black body. Otherwise, they're pretty similar. In the past, the Omni had a shiny, body colored ring around the eye lens, so make sure to check for this if buying used.

Visually, I find stars are tighter in my refractors versus my Newts. I've always attributed it to the CO pushing light out of the Airy disk. It's one of the main reasons I like them despite the smaller aperture.

I use Battery Tender brand chargers on my car batteries to keep them safely topped off when I won't be driving them for two or more weeks because today's cars have so many electronics that drain the battery even when the car is off. I was going through batteries at a prodigious rate until I figured out what was going on. I went with the name brand that many US car battery stores use to keep their batteries fully charged. The price difference between them and the generics is pretty minor. They pioneered the market here so much so that their brand is in danger of becoming a generic category name like Kleenex or BandAid. I haven't had any issues with dead batteries or sulfation since I started using them.

For travel, I put my eyepieces in a small, padded camera bag with dividers. It worked pretty well for the 2017 eclipse. The telescope was wrapped in bubble wrap inside a duffle bag so as to not draw attention to it in parking lots in the back of the van. Another alternative is small pistol cases such as those from Plano. I would think there's a UK equivalent.

Why do you want them? I have a bunch in storage. I keep my eyepieces in pick-n-pluck foam cases. I never understood the utility of bolt cases except for long term storage of unused eyepieces.

The 17mm ES-92 focuses pretty much right at the shoulder like many other ES and Pentax eyepieces. That matches up with what @Johnsays above because the 17mm Ethos focuses about 10mm above its shoulder. Weirdly enough, the 12mm ES-92 focuses some distance away from its shoulder, though I'm not sure which way. I had kind of expected them to be close to parfocal. Thus, it might not work with your diagonal like the 17mm probably will.