Ben the Ignorant

If you are seeing things like drawing number I here... ...or drawing A here it is only air turbulence, it's not always that strong; you'll learn how and why it changes and how to deal with it.

Yes, no problem. First time I saw Saturn's ring the scope was a 40mm working at 40x.

Yes. Look for the regions of the sky where light pollution and haze are the strongest, avoid those and observe in the other regions. Try to be positioned so direct light from street lamps, windows and such are not in your peripheral vision because even lateral light is an annoyance when stargazing. My microvave oven's screen doesn't shine that bright but when I stargaze from the window (did that last night with a 16x70) close to it the glow bothers me and lessens my dark adaptation. So I cover it and that helps. Don't underestimate stray light, avoid it, and urban or suburban stargazing can be quite enjoyable.

Yes, I did that in a homemade dob, in the alt and az bearings. It worked fine for a while then it lost its ideal slickness; I had to use the traditional materials instead. Before replacing it I tried spraying it with silicone lube, worked for a period of time and then lost its properties the same.

I aimed my Celestron 5 Schmidt-Cass at infinity with the diagonal and basic eyepiece (Explore 24mm/68°, maximal field, minimal power) in place. I switched to all my other eyepieces, getting back to the 24/68 after each eyepiece change. They were Synta Plössl 20/50, Maxvision (Meade clone) 18/82, Hyperion 13/68, Hyperion 10/68, Sky-Watcher Myriad 9/100, Sky-Watcher Panorama 7/82, Sky-Watcher Myriad 5/110, Explore 4.7/82, TS 4/82 and Sky-Watcher Myriad 3.5/110. The focus knob never moved more than 1/8th turn (and always in the same direction) except for the Myriad 9 that needed a 1/4 turn (in the same counterclockwise direction as the others). I've measured the mirror motion for each turn and it's 1/2mm, so it never moved more than 1/8mm. Then I removed the diagonal and put the 24/68 directly in the visual back, the refocus was 4.5 turns, that is 2.25mm, eighteen times more. Obviously vastly more than what optical engineers have allowed for normal use with the diagonal's optical path. It's not about accomodating different eyepieces, it's about observing with or without the diagonal's long optical path. In my test the difference is eighteen-fold.

Precisely, scope makers know what they're doing so the loss in normal use is negligible.

Baseless sarcasm. First, the article in the old Sky&Telescope issue was about Schmidt-Cassegrains which work at f/10, a steeper and less forgiving ratio than the f/12 to f/15 beams of maks. Besides, maks have a diifferent optical design that would probably have other tolerances even if the f/ratio was the same. Optical designers obviously make scopes and eyepieces so their focal planes match with minimal mirror motion, only them have to worry about that, not us users. That remembering the star test thing is nonsense.

Both have the same diameter eyepiece barrels and eyecups, but the APM has about 8mm clearance between the top of the eyecup and the spot where light is focused, that is the real, practical eye relief. TS has about 5mm clearance.

I'm not an imager (I don't have the patience and the money! 😁) but I check the Neue Artikel page of Teleskop Service regularly and spotted this: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11518_TS-Optis-121SDQ-Apo-121-mm-f-5-6-Quintuplet-Flatfield-Apo.html

A reminder of the topic is in order. Paz asked about high-power, narrow-field, high resolution visual observing without a diagonal, and all the respondants talked about planetary viewing because they understand that is the subject. Widefield imaging has nothing to do with that. Backfocus was not an issue either because if it was that kind of observing would not be possible to begin with. I don't have the book you mention so I can't see that for myself, and I'm not sure what the image blur exceeding the Airy disk means. Does it mean the smallest detectable loss of contrast/resolution or does it mean a big loss? I don't know. What I do know, and is recognized by everyone is, taking the traditional meaning of diffraction-limited optics as 80% Strehl factor, that is already too lax. If a top-brand, say LZOS scope had an 80% Strehl it would be rejected as badly defective, the norm is 95%. Even moderately priced Explore refractors are guaranteed 93%, so a drop to 80% is unacceptable, and vastly worse than any loss of quality a good diagonal could cause. And that was the question. As a general rule, the slightest change in the optimal arrangement of optical elements always lessens contrast and resolution. That's why so many threads are written about collimation, centering and spacing of optics and mechanical components. No one tells posters to tolerate miscollimation in their newtonian or their Schmidt-Cass. No one wants the slightest decentering in their triplets (or doublets for that matter), and telescopes are returned or sent to the optical bench to be adjusted. Wellenform and Teleskop Austria optimize triplets by respacing the lenses by only a few 1/100ths of a millimeter. Out of specs is never good. Catadioptrics are also designed with certain accurate specs, they are optimized to focus at infinity with a diagonal in place so it's probably an error to use them when the mirror spacing has changed because the diagonal is removed.

That's a discussion about backfocus, not main mirror position relative to the secondary mirror/plate assembly. The backfocus is behind the main mirror and outside the telescope, but the main mirror's position is inside the telescope. Backfocus is where the focal plane is, but I was talking about where the main mirror is, different things. The spacing between optical elements is critical, you can't change the distance between a doublet or a triplet's lenses without damaging the image. Same for catadioptrics, they are designed with a certain spacing between the main mirror and secondary mirror/plate or meniscus. Changing the distance only a little reduces performance. For example, messing with the spacing in a triplet refractor reduces the Strehl factor by about 1% for every 1/100 of a millimeter of error. Ideally, a Schmidt-Cassegrain or a Maksutov should have fixed optics and an outside rack-and-pinion or Crayford focuser. But then two problems arise: one, the scope becomes less compact and loses the ability to screw accessories firmly onto the rear metal plate. Second, focusing on close objects pulls the eyepiece or camera a long way to the rear and allows flexure and play. Moreover, the Cloudy Nights discussion is about widefield imaging where losing a little resolution doesn't show. If Mars was in a widefield image it would not look larger than a star because it is not magnified enough.. But magnify Mars as they do in high res and the loss of contrast/resolution shows if the mirrors are not spaced right. Schmidt-Cass and Maks are designed for infinity focus so that doesn't happen but focusing on closer targets lessens performance. Say the last thing you looked at with your Schmid-Cass was a close tree, and then you switch to an aftermarket external focuser. You can still focus on anything at any distance but because the mirrors were left out of the ideal spacing the scope will underperform all the time.

The formula to calculate the loss of resolving power according to the mirror's displacement was in an old issue of Sky&Telescope, I don't remember which but it was in the late eighties or early ninities, I believe. The margin to keep the scope diffraction-limited was less than half a millimeter on each side of the ideal spot.

Maksutovs and Schmidt-Cassegrains have a sliding main mirror to adjust focus but when it is a large fraction of a millimeter away from the ideal spot the telescope is no longer diffraction-limited. Removing the diagonal requires displacing the main mirror a lot to reach focus and there lies the problem. The sliding mirror is very convenient, I can use my Celestron 5 as a long-range microscope looking at insects 2.5 meters away and magnifying them 250x like seeing them to naked eye at a distance of 1 cm. The image is still quite sharp but not optimal. Especially when viewing planets, all the resolution has to be used so moving the mirror away from the optimized position (focus on the infinite) will deny what little you could gain by removing the diagonal. Excellent dielectrics like those from GSO (I bought four of them, all are superb and consistent, same for my StarGuider) are common and not expensive so there is no reason to remove them except as an experiment. Besides, if optics makers couldn't make diagonals that respect the main mirror's performance every newtonian would be disappointing but that's not the case.

I received them and will make a comparo with the TS 10x50 MX. But not to leave tico waiting, the APM proved itself better in ALL respects immediately, handling, weight, focusing, edge and center sharpness, chromatism, transparency, contrast, flare control and vividness of the colors. However I can't write a complete review before I use them for at least one week so my eyes get used to them (won't be difficult). The testing I've done so far was in the daytime and at dusk, then clouds invaded everything so I judged edge sharpness and chromatism on street lamps. They are much brighter than stars but the aberrations were very reduced and they will be even more so with stars.

I know and I don't use the cheaper ones for that reason, my broadband filter is an Astronomik and my O-III is a Baader. But I'm a lifelong observer who has developed an acute sense of very small visual differences while Mike_S is a beginner. So I don't want him to give up on his equipment because of preconceived ideas on price. Whether we want it or not, financially speaking, examining many faint and small things makes us more able to discern imperfections, and whether we like it or not, financially speaking, we feel bound to get the better intruments in time. I am replacing my achromatic binoculars by apo's for that motive. Will cost more, a casual or beginner observer might not see the need, but I do.

Cheap filters have only a few percent less transmission than expensive ones, they will work well. But focused attention and observing experience have to complete the job, as well as averted vision. Some use a black observing hood (like 19th century photographers), an eye patch or other tricks to better their night vision. Try them and see what works for you. Also, try seeing the fainter things first on an observing night and the bright easy ones last, so it won't end on a disappointment if you don't succeed at first.

I always view M15 and M2 one after the other, M2 is nearly its twin, and very simple to find. It's a dozen degrees south of M15, or another way to find it is to climb five degrees north of the bright naked-eye star Beta Aquarii. M2 shows itself in a 10x50 binoc from a city so you should see it in your finder. NGC 7662 in Andromeda is small but exceedingly bright as planetaries go, very low power might disguise it as a star blurred by turbulence but 60x or more reveals what it really is. Gamma Andromedae is Albireo's little sister, and later in the night, Castor is a spectacular double that doesn't even require star-hopping. Same for Mizar in Ursa Major. Gamma Arietis is another quickly found and rewarding double that doesn't even require high power. However the list of things to see would be long so you should get a star map like Sky & Telescope's Pocket Sky Atlas. For 20€ it contains everything a largish binoc or largish beginner scope can show.

If it's between the lenses this might help.

I thought it would take longer to budget for it, but I had forgotten a box of coins behind the toaster that contained 120€. A courteous store manager agreed to exchange them for paper money that went straight to the bank account. I'm ordering a Ronchi eyepiece with the binoc, and together with the shipping fee the sum amounts to more than 500€ so it couldn't be done at a moment's notice, or so I believed. I'll receive it in a few days and you'll have one more opinion to read.

Impressive!

Yeah, reducing light pollution is one of the next big challenges. Welcome, Kanita.

Wow, so many folks want a lighter big dob! TS is offering a customer return 8" Sky-Watcher for 300€. https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11503_Gebraucht--Skywatcher-Skyliner-200P-Dobson-Teleskop-mit-hochwertiger-Optik.html

I'm going to order the 10x50 ED within the next couple of weeks, if no one else reviews it, I will.

Some older achromatic objectives were made of glass that was slightly tinted yellow in the mass to absorb some blue, indigo and violet light. That was a ploy to reduce the chromatic problem. Some old Vixen achromat telescopes resort to that trick, too, which is not unfair, if the offending colors can't be controlled it's better to suppress them. The reduction in brightnes is less troublesome than the loss of contrast and clarity due to the unfocused light spread over the focused image. By the way, my chinese 80mm FPL-53 triplet has a very pale yellow cast on a side-by-side comparison with my 80mm achromat which is pure white in overall tint. Of course the colored fringing is no match, the triplet has none whatsoever but to achieve the apo goal, the chemical makeup of the glass has to be such that it has that very pale yellow shade. The chinese 90mm triplets have it, too. (I was comparing resolution when I saw that, I didn't expect to see a change in overall tint between the two scopes).

If they were made in the Soviet Union they were made by Kommiez, obviously. 😁