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Ben the Ignorant

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About Ben the Ignorant

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  1. From Jean Texereau's How to Make a Telescope: To satisfy the Rayleigh criterion, therefore, the defect on the glass may not exceed 0.14 / 2 micron. That's 0.07 micron. And: If the f number is large enough, i.e., if the mirror is relatively shallow, the difference between the parabola and sphere becomes very small; in fact the uncorrected spherical mirror may satisfy Rayleigh's rule and give a practically perfect star image. BUT: The Rayleigh criterion must be qualified for low-contrast images. The situation here is much less favorable than in viewing a star. Francon, studying planetary detail of minimum visible contrast, found that a wavefront of 1/16 lamba begins to be objectionable. This implies a tolerable error in the mirror surface of 0.02 micron. The difficulty here is not in making the mirror to this precision,... So, separating star dots with a mirror deviating from the ideal parabolic shape by 0.07 micron is possible but solar, lunar and planetary detail is not made of bright round disks separated by a hard-contrast black space. It is more subtle, and continuous so it requires a 0.02 micron accuracy mirror, a 3.5 times smaller tolerance.
  2. From the Cloudy Nights link that johninderby posted: Actually for a parabolic mirror: when tested using a Ronchi grating eyepiece, the image of the lines WILL be straight, as object(star), is at virtually infinite distance, (light is coming in as a cylinder, with parallel sides), so focuses at focal plane, whereas, when using the grating as a bench test, grating and light source, are located at (near), the "Radius of curvature"(2x "focal length"), of the mirror. In this case, the light incident to the mirror, arrives as a diverging cone. Mirror surface modifies this cone.So the returning light beam, forms the reverse identical cone (converging).This cone of light is less steep (1/2) than the cone coming from an infinite light source, hence the image of the lines take on the curved shape of the classic parabola. All telescopes, no matter the complexity and whether aspheric elements are present or not, should present parallel and evenly spaced Ronchi bands on an infinitely distant target. All are interpreted identically, for one is examining only the final wavefront--which is nominally spherical--near the focus. I'm obviously not removing the mirror when I test my scope, so parallel lines it has to be, as in the photo I took. Turbulent but parallel.
  3. Then why are these three parabolic mirrors from the page I already linked showing straight lines? Because professional opticians like Wolfgang Rohr and Carl Zambuto don't know what they're doing?
  4. This is still the case. If it was my 130/900 would show spherical aberration but it shows none. Designations don't change what optics do so there has to be a mistake in the apparently outdated letter designation. Wouldn't be the first time, at some point an FPL-51 scope was labelled as FPL-53 by mistake and TS apologized and corrected the error.
  5. Sky-Watcher surely doesn't make different newtonians for British or German outlets.
  6. The Ronchi test is simply watching an artificial or natural star through the telescope, with the Ronchi grating placed in the focuser as an ordinary eyepiece. What this guy shows is a convoluted method that few people could use because it involves removing the mirror from the telescope, and not testing the secondary, which could make or break the wavefront. He then uses the Ronchi grating as a Foucault apparatus would be used, and claims the light source can be a slit when it HAS TO BE a dot, like a star. This is a Ronchi tester: Simply put it in the focuser and observe the lines. No one removes their mirrors from the scope to make a Ronchi test. That would be terribly inconvenient.
  7. All the Sky-Watcher 130 in TS' page are parabolic, regardless of designation: https://www.teleskop-express.de/shop/index.php/cat/c59_Newton-Reflektoren.html/page/2 They specifically say these are not spherical because sphericals are and outdated idea that disappointed stargazers.
  8. This is not an f/10 mirror but an f/7, it's not a small aperture, small would be 114m or 76mm, and small diameters don't allow spherical aberration in newtonians like they don't allow it in refractors and catadioptrics. Aberration is aberration, nothing hides it when it's there. If hiding it instead of correcting was possible the manufacturers and our wallets would have an easier job. If a Ronchi test is not sensitive enough, why are optics labs using it? https://astro-foren.de/index.php?thread/14862-zambuto-enjoy-your-mirror/ Which they do in addition to the Foucault test, which tells the same result. The "good" sphere "nulling out" means nothing, when a spherical shape is there instead of a parabolic one, the spherical aberration shows, the image is junk. If it didn't all our gear and the lab equipment could go the trash can. And the Hubble space telescope wouldn't have needed a billion-dollar repair.
  9. According to the FLO blog page, 130M means M for motorized, 130 without additional letter is the f/5, but the 130P with a P should mean parabolic. There seems to be some confusion here. It was always my understanding that the P in 130P meant "parabolic", and the 130/900 I bought IS parabolic. Here is a reminder of what a spherical mirror's Ronchi pattern looks like: The lines of a spherical newtonian are not parallel, they form either a crescent or a barrel. And so they are thicker in some places and thinner in some places. This is my 130/900 Ronchi (10 lines/mm) pattern, sorry but complete cloud cover so I used a very remote road lamp as an artificial star, and ground turbulence was horrible. The unevenness is the lines is not due to zones or whatnot, just turbulence. What matters in this discussion about spherical aberration is the lines are parallel, no crescent, no barrel. They have the same width throughout despite the troubled atmosphere. My Sky-Watcher had the thicker spider vanes that I amputated and replaced, but it is a parabolic, the Ronchi test shows it. I tried to photograph the defocus test but air conditions are too bad. Visually the defocus test is excellent, intrafocal and extrafocal figures are the same. It looks like the descriptions of these telescopes need to be checked.
  10. The 82's focuser drawtube seems to be quite large relative to the main tube, probably to illuminate large sensors, which suggests the thing can be fitted with reducers/correctors. Very large focuser lock and dual finder/guidescope holder suggests imaging, too. If Sky-Watcher bothers with a new scope they probably have improved the doublet objective, with aspherical surfaces maybe and/or a better glass formula? The very long dewshield is a good thing, condensation and stray light are such stupid things to have to deal with. I'm happy with my take-anywhere 80mm semi-apo and my stay-at-home 80mm triplet, so no personal interest in these, but the 62mm is smallish for my taste anyway. I was never satisfied with the brightness and resolution of scopes smaller than 70mm.
  11. Beware that very old 130/900's might be spherical, but those of recent years are parabolic. The parabola is just a requirement of the newtonian, like having salt in the kitchen, if it's not there the whole kitchen is unusable.
  12. Small or large, the spherical aberration will be the same for a given f/number. A spherical 130mm f/5 has the same aberration as a spherical 300 f/5, but fortunately Sky-Watcher seems to have stopped making spherical newtonians. The info about them has become too easy to find with the Internet, so the old claim that spherical mirrors of a certain f/ratio or a certain diameter are acceptable doesn't catch on anymore.
  13. Sorry to contradict but I own a 130/900 and it is parabolic. https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4778_Skywatcher-Explorer-130---130-900-mm-Newton---optischer-Tubus.html I would never have bought one otherwise, and upon receiving it I did a star test which was one of the very best I've ever seen in any scope of any type. I still remember the orange star's diffraction patterns that were a mirror image of each other on either side of focus.
  14. It's not a matter of taste, film solar filters that give an orange image are made of an inferior material (mylar) that makes the image less sharp and contrasty. Only Baader's AstroSolar film retains most of the telesope's sharpness and contrast. Some makers used to offer (maybe they still do) high-grade glass solar filters flat and polished to the same degree as fine telescope objectives but they cost 6 to 8 times as much as a standard-grade glass filter. I used to own a standard-grade Thousand Oaks glass filter with nickel-chrome plating, it did produce a nice "easy on the eye" orange Sun but some sharpness and contrast were lost. Buying the higher-grade glass filter was out of the question but switching to AstroSolar solved both the price and quality problem. I made three Baader filters for my scopes, the largest being 300mm across, and several more for club members; all make a bright white Sun with very strong contrast and resolution that remains there at high powers. By the way, 5-inch is the threshold at which planetary, solar and lunar observation becomes really rewarding, I found.
  15. Not always. Some modding can save money: And the AZ-4 head has more potential than its price suggests but it also takes some modifying. This is what I call a Z-arm, it carries a standard counterweight at the front and to the left of the head to balance the scope whose weight is at the rear and to the right. It's bolted to the baseplate but it could also be clamped between the baseplase and the arm without drilling any extra holes. This allows to drive the azimut axle with barely any friction so the motion is more fluid and accurate. Made with stuff from any tool store, L-brackets bolted together to make a Z-arm. And speaking of balance, the elevation axle needs quite a bit of friction if the tube is not balanced but a system like this... ...allows much less friction, so the cheap AZ-4 performs like a higher-grade mount. The tube tilt-lock counterweight, as I called it, simply slides along the tube and locks itself in place because there is play between the tube and counterweight, whose weight is all on one side. So it tilts and that forces the counterweight to stick to the tube. Doesn't scratch the tube and doesn't move one millimeter after one hour when the tube points vertically, I tested that. As you see, spending a lot is not mandatory.
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