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You don't have to be sorry if you do different than I did. The experimentation that I have not done is still a good thing.
If you see the mirror fully illuminated you are surely at the radius of curvature, regardless of the width of the slit.
But the maximum of vision sensitivity you get with a narrow slit, but not very well, because in that case a whole series of diffraction fringes are created which complicate the clear vision.Ok.
Half a millimeter are 500 microns, which is 25 times more than necessary-
The Ronchi is an indicative test only and the reticle can also be seen with a large and strong light. But that's not how it is done.To do the Ronchi or Foucault tests correctly, the complete illumination of the mirror is however always obtained when the light source on which the slit is installed is installed, and also the eye or the photographic sensor, at an even distance from the mirror (or very close) to the radius of curvature of its surface (which is double the focal length).
I imagine that this too large slit could also be the reason for the fact that you don't see the lattice in detail in the Ronchi.. This is because the maximum sensitivity in optical tests (but also ophthalmic), is given by the maximum subtlety of the slit and such that it does not generate too many fringes that disturb the Foucault test.
This is because to correctly evaluate the optical phenomena at their birth (that is, very close to the limit imposed by diffraction), it would be necessary to use a point light source as is the pinhole. But since the small hole would not let enough light pass through for the eye, a slit is used, that having a width equal to that of the diameter of a pinhole, performs its exact same function, however, letting in much more light, but not exaggerated, and suitable for comfortable sight even with the naked eye.
I'm not saying you can't, but as we know it is very difficult to achieve that precision of the parabolic curve over the entire surface with tools that are not full diameter, especially on large mirror diameters. This is done with professional machinery on large and very large mirrors.
But then… in my possibilist view of my amateur level, I wouldn't even be interested in reaching a very laborious high strehl.
Or at least on the scale of interest, I believe the feasibility of the opening prevails, because I think that the quality of the workmanship would be instructive to check the progress during processing (as I did out of my curiosity), placing the non-aluminized mirror (as it is in the parabolic phase) inside the structure of his telescope, to see on the Moon and Jupiter, with an eye on the progress of work.In that case one would perhaps understand quite well how many enlargements the state of the work in progress can tolerate, because the elevated strehl could be a more not strictly indispensable….I do not know if (as a possibilist) made myself clear
More than reducing the brightness of the led, I have the impression Luca that you are using a too wide slit.
It's not hard to get it close around 20 microns, using a piece of videotape tape as an opening spacer.Well.
Philosophizing, I have always been a possibility by my nature, especially in technical matters, which have always been my job.
I remember that in the factory, for a general and historical assumption, it was believed that neither we nor the competition, a certain product could have been made….until we succeeded by opening up a new market.I therefore believe that an enjoyable use of a large aperture and short focal length telescope cannot be excluded a priori., only because a high Strehl cannot be achieved with manual processing.
it is therefore important to be able to verify “in economics” what limits would arise in visual use; both caused -for the “alti” magnifications- the quality of the surface machined with sub-diameter tools; and both -for low magnification- caused by the curvature of the field given by the considerable depth of the arrow.
As a possibilist “with your feet on the ground”, I could certainly be wrong, but in conclusion I always have in mind the fact that it happened to me, that with a large aperture it is easier to see otherwise invisible objects…. such as happened with the NGC 2359 “Thor’s elmo”, which makes a huge difference regardless of magnification, if you are looking for it with a 300mm or a 600mm.
And! I agree with Mirco.
I have to say once again Massimo that (after all as your usual) you are a very special person in making the most of almost impossible actions scientifically by turning them back to normal, which for many others it is not, and it never will be (starting with me).
The Ronchi testphon is beautiful!
Congratulations on the recovery work that is anything but easy!I agree.
The important thing is that you do not abdicate your aspirations.
In other words, I would not like my advice to be a limitation to your innovative thoughts, towards the limited scope of my specific well-traditional direct experience, but far from innovative in this area.
Hello Carlo.
I see that you are well determined in your intentions on several fronts! And it is a good thing in itself and a harbinger of your sure satisfactions.In fact, the Zeiss machine reads both that and, thanks to its three trees, it is more reliable in freeing the processing from the danger of “precise repeatability”, than in a few “rounds” engine could dig area errors much deeper than those caused by the much less efficient manual work. Errors that are much more difficult to recover than manual ones, especially when you are just starting out and you need to launch into experiments.
Certainly today with inverters powered in single-phase that convert it into truly three-phase current, with a greeting without regret to the old “willing ”but insufficient capacitors used to“ build ”a fictitious second phase, necessary to start the rotation of the motor, with a loss of almost half the nominal power, the task of mechanical construction is brilliantly outclassed by the infinite combinations of speeds and random paths to the tool that can be used in the Zeiss.
I therefore think that the car could be the preferential initial target for self-construction and experimentation on a (for example) 300F5 in 19mm thick normal glass (saw that the two mirror discs and tool, they cost a hundred euros, or even less, in industrial glass factories that produce furniture, walkable floors, etc. in glass).
Instead with regard to the rest 3 self-constructive points (measuring bench, for example of very precise focal length; Adjustable slit in different openings; sferometro) I would refer them to your future evaluation of usefulness resulting from the use of the machine, because each of them would certainly provide high precision performances, which, however, is not required by normal mirror construction technology, not just amateur (and name, to create binoscopes where the two optics of the instrument must necessarily be identical).
The reasons for the postponement to later, would be the following:
The measurement of the exact focal length, normally never needed, because in the technical practice the one deriving from the final arrow obtained / evolved in the working is accepted, which may not even be known with a rough accuracy better than a centimeter, due to the practical fact that in the final construction phase of the telescope the distances of the mirrors are finally adjusted, until a star aimed for the purpose is in focus.Ditto for the construction of a slit with variable opening, which is not strictly necessary as a slit from 20 micron is universal, and it is very easy to do by bringing two half razor blades together, the cutting edges have been smoothed out to make them straight.
("Smoothing out" its sharpness and its inevitable microscopic undulations, rubbing the cutting edges on a piece of glass, and then fixing the two blades with double-sided tape on a common base plate, with interposed spacer to the facing cutting edges, consisting of a piece of magnetic tape formerly a videotape, which has a standard thickness of approx 20 microns).Finally, as for the spherometer; It would also be quick and easy to build using a thick multilayer board (like i did) or an old pulley with three holes for three screw-feet arranged at 120 °; in whose central hole is fixed a micrometer unscrewed from the bow of a palmer purchased on market stalls, or scrapped by ISO9000 company metrological procedure.
The spherometer, however, I have found that it is not necessary to make parabolic mirrors with focal ratio equal to or greater than F5., because a full diameter mirror tool is used in machining, which by its nature produces by wet abrasion, between tool and mirror, two complementary spherical surfaces, which reach their equal spherical radius of curvature, as soon as it is seen to disappear in transparency between them, the air bubble which denounces the non-uniformity of the common curvature of the two surfaces in contact, present since the creation of the initial arrow.
On the other hand, when working with sub diameter tools (mandatory for mirrors with a focal ratio lower than F5) it is of ambiguous utility, because the Ronchi i di Foucault tests are much safer and more eloquent.
(the Foucault test records the same draft on a spherical cap both in the center and at the edge of the mirror; and Ronchi shows the perfectly straight lines of the lattice of a sphere).Because I say of ambiguous utility??
Since it is necessary to consider that with the spherometer a significant difference is measured between the plane identified by the three peripheral feet and the central micrometer, from which the radius of curvature of the hypothetical sphere represented by that measure…But with that measure, difference in height between a floor and a point, there is not the slightest certainty that the surface measured with that difference in height (which is the only sure thing) has the course and curvature of a sphere.Hi Alberto.
As an indispensable premise to all my comments to your many questions, I must remember that each car is different from the others, and the processing on each of them always presupposes accurate and specific experimental control tests for all the activities assigned to it.
This is because the manufacture of a glass mirror, as known, occurs by abrasion, with a very high quantity of mechanical actions (for example of rotation or "to and fro" of a tool) each of which unitarily removes a very small quantity of glass.
Hence the work of building an astronomical mirror, it is governed more by statistics than mechanics.
In the mechanical actions of “and fro”, machines with arms have the defect of being “too precise” in repeating them at the same point of attack or inversion of the motion. And so if there is no immediate corrective action, in a few passes there is a deep local error that is difficult to correct.
While instead working freehand, the natural manual imprecision of the attacks and of the reversal points becomes an advantage, because statistically the errors of opposite sign can automatically cancel each other out.You write 1): Mirror smoothing (I think this is what I do).
REPLY 1: Ok
You write 2): In case of meniscus, regularization of edges and convex surface (and I do it myself with a full diameter concave tool).
REPLY 2: Ok it's possible. But about the making of menisci, the works that produce a better approach to the spherical cap, occur by dropping the reflective face of the future mirror onto a good driving surface, spherical convex.
You can find an interesting work by searching on youtube for the movie entitled “Slumping a 16 inch diameter telescope mirror”, where a mold is concave milled to create a convex base in refractory gypsum, to be put in the oven to guide the "Slumping" of a 16 "dF3.3 in a sheet of common glass 19mm thick (3/4″).You write 3): Mirror edges smoothing.
REPLY 3: The bevel of the mirror edge must always be done anyway, in order to avoid chipping during processing. If the 45 ° plane of the bevel will be extended over a 4mm wide surface, it is likely that it will last until the end of processing, saving the risk of chipping while you will have to redo it at an advanced stage of processing
You write 4):Rough excavation of the sphere, if blank flat or even if meniscus but "quite" deformed with respect to the sphere
until . Macchina Fixed Post. With toroidal tool, extremely rigid, not consumable (for example steel disc, pulley and so on) with a diameter equal to half the diameter of the blank, positioned so that the edges are tangent, being careful not to exceed the desired sagitta. In this case, I suppose considerable pressure can be exerted, perhaps limited only by the power of the motor which may possibly not be able to rotate the blank-tool pair.
b. Macchina Fixed Post. With consumable tool with radius r equal to 70% of the blank radius R, offset by a certain amount (R-r) + 10%R[mm]REPLY 4: From the essay said "Do not bandage your head before having broken it" It is much better to produce a good "spherical cap" immediately, experimenting properly, to avoid the problems you describe.
Its a Fixed post, playing and checking the result of the tool decentralization, I would prefer choice b, to get closer, or even center the spherical curvature leading to the desired arrow depth.You write 5): C. Machine with eccentric. With sub diameter tool, style diamond cup cutter or similar. Full diameter center-to-center pass, minimum transshipment. Again, I suppose considerable pressure can be exerted, perhaps with lesser limitations than those in point a as the contact surface is smaller. Perhaps it is the fastest way to excavate the approximate sphere. Be careful not to exceed the sagitta.
REPLY 5: It is the rudimentary method used by the experienced craftsmen of the sector, who have the experience that makes up for the roughness. But that method would leave newbies in the middle of the ford of sub-diameter tools…
You write 6): Refinement of the spherical excavation. In the event that a non-consumable tool was used in the previous activity, or the already built tool has worn out too much, the tool will have to be built using the spherical face of the blank as a "mold". Otherwise, the point tool will be used 3)b. During the refinement, the tool will be positioned on the mirror in the manner described in 3)b, taking care to loosen the blank stops to a minimum to allow micro-displacements (it should help prevent astigmatism).
5) Sphere polishing. Tool construction in pitch, starting from the mold of the concave face of the blank, pressure - heating - techniques to reconfigure the surface of the tool after the polishing session? Tool dimensions: full diameter of the blank or slightly less (70%)? Little pressure, reduced speeds, frequent checks.
until . In case of Fixed post, see point 3)b.
b. In case of machine with eccentric, as random as possible but without exceeding the edges too much.
This is a bit of the operation that I imagined after reading here and there, but please, if I'm wrong somewhere - or anywhere - feel free and be kind enough to correct me and above all extend each point with new content (my m is not laziness I'm afraid of writing unspeakable filth).REPLY 6) The technical basis of the conclusions you arrive at, would require rewriting here, of the existing technique, because the correct or intuitive answers would derive from your knowledge of the manual processing of mirrors, whose actions must be "translated" into mechanical action, to get some with the machine, the same results obtained manually.
About this, and for this reason, usually it is advisable to make at least one mirror by hand “school”. So I think it would be helpful if you download and study well the manual processing described and illustrated very well by Jean Texereau in his book from the 1930s., still and always valid, downloadable in whole or in chapters in PDF format here: http://www.astrosurf.com/texereau/You write 7): Last thing (unfortunately or fortunately you can find everything online): the rotating table of the machine, what I do? I have read from steel to aluminum to Corian, to marble to MDF ... If it goes well, the latter is clearly the fastest and least expensive thing, maybe properly painted etc etc ... . In the end, I don't think I'll open a parabolic mirror shop, so it must last in time to create my ...
REPLY 7):Chipboard made of particles is not suitable for the construction of the rotating table, nor the MDF made with sawdust (which are not cross-referenced, they both flex, but you need a rigid base of a composite material, as is plywood, even if only poplar; gluing two discs from 12 or vice versa 15 glued overlapping with vinyl, and then painted to make them waterproof.
In various projects of rotating table for mirror processing that you find online, you will see that most are then supported by three wheels of "Rollerblade" skates arranged at 120 degrees very close to the radius of the table, under it.Your "half pendulum" reasoning is correct, because with that system you would get (even digging maybe even just a straight line with an inclination of 2.39 ° from the center to the mirror edge) as excavation a course very close to the desired radius of curvature. The difficulty lies in “curvature” approximate, which may not help right from the start.
On that same way of roughing there are interesting works of realization on Youtube , by Gordon Waite of Waite Research, which I would advise you to watch, if you don't already know them.
For jobs of that heaviness (like making a 20″F3) I believe that the minimum is to have a revolving table, which offers many advantages to learn how to use it.
The advantages are that only one economical three-phase gearmotor is used 220 was the 4 poli, powered in three-phase converted from single-phase with an equally economical Toshiba inverter, with speed regulation from zero to 60 rpm.
The method of dragging the tool resting and ballasted on the rotating mirror, is given by the differential speed resulting from the greater or lesser decentralization of the tool rotation axis with respect to the mirror rotation axis.For this reason, the resulting "machine" is called "Fixed Post" or "fixed pole" of the tool rotation pin that enters a “glass” center of the tool diameter 12mm.
Moving the tool towards the center of the mirror digs into the edge, making the mirror convex; while vice versa by moving the tool towards the edge, the center is excavated making the mirror concave. It goes without saying, therefore, that there is an internal media decentralization measure that realizes a plan, and that there is another that creates the desired spherical cap, always with only one engine.
The correct measurement to obtain a spherical cap is to use a tool with a diameter of 70% mirror, decentralizing it by a certain amount which is described in a dedicated tutorial movie by Waite “Rough Grinding a 20″ Quartz Telescope Mirror on the Fixed-Post Machine”; such as the Building a 20 series″ F/3.3.It may also be useful to play around with a fixed post machine simulator, by clicking on this link; https://www.grattavetro.it/simulatore-di-non-solo-lucidatura-specchi-parabolici-software-polsim/
But also read here: https://www.grattavetro.it/forums/topic/posto-di-lavoro-piano-rotante/
where you will find more precise information on the Fixed post and on the decentralization settings.Getting a good ball is therefore not difficult. The rest is different.
Hi Alberto and welcome.
You say right: A parabolic mirror 20″ (508mm) F3, assumes a 10.58mm deep excavation arrow in the center of the initial spherical cap; and therefore of discreet commitment if it can be recommended by machine, with one of the known methods, such as pendulum grinding with a radius of 3048mm (equal to twice the focal length). Or with an edger on a lathe carriage or a CNC milling machine, or "in copy" (on a sheet metal template cut with that radius). Or again forming the curve in the meniscus, by softening in the oven at 500 ° C).But the problems would become great in polishing, necessarily to be done with sub-diameter tools "healthy carriers" of astigmatism. Then going up in difficulty to very big problems in the parabolization flaring, which would differ at the edge of the parabola with respect to the edge of the sphere, was i 18,43 microns (that are 18430 nanometers high.), that is an enormity of excavation to do a “free hand” with the maximum tolerance of deviation error of that parabolic surface of only 68,75 nanometers with respect to the theoretical parabola taken as a constructive reference, to obtain final accuracy “minimum wage” lambda / 4 error between maximum peak and valley.
In conclusion, I realize that I have played the part of the devil's advocate a little. But the purpose, before embarking on a journey, is to collect all the information necessary to plan the route, evaluating the degree of difficulty to be overcome to reach the goal, putting at stake availability of time and patience “in reasonable quantities”.
HELLO Frankq.
You say:I understand that in order to draw the optical scheme of a telescope I need to know the focuser that I'm going to mount?
To draw it yes. But from the practical point of view of construction, not necessarily. Because you will settle everything in the end, when as a last step, you will cut the tubes of the trellis to make sure that the eyepiece of those you have is in focus, which requires the greater insertion into the focuser.Hi guys:
Regarding cerium oxide, it must be said that in fact only what does not work “rolls” on the glass, but it becomes encrusted in pitch, whose compliance also has this purpose of correct retention / replacement of the abrasive granules, facilitated already starting with the adaptation compression that is frequently done with the brushed tool resting on the mirror.Then using the part (mirror or utensil) that you have on the table, brushed with creamy diluted oxide, it gets lost by overflowing and pouring but not a lot; and it is not convenient to recover it.
(for example the term “creamy” it basically means diluting from session to polishing session, one tablespoon of oxide at a time, with two, or max three tablespoons of water).
Equally it is better to consume all the spoonful of product that has been diluted in the jar where the brush is dipped, and repeat the new dilution at each exhaustion or at the beginning of the work session.
These tips apply to manual processing. As with the turntable the dilution is much greater (having a consistency like whey) because the mechanical work of rotation, much more marked than simple “and e vieni” manual, makes up for efficiency, consuming, however, perhaps the same amount of oxide over time, given the higher dilution.
Well well Carlo!!
Since you already have the abrasives 60 e 80 for roughing with the “cord races”, I can advise you (if you have not already done) to read this article to you:
which will help you avoid the common mistake of struggling to reach the spherical cap with the desired depth of arrow in the center of the mirror (which is 3.12mm for a 250F5 mirror):
During the reading, you will also find the link to the article on chordal passages that are used to efficiently excavate the spherical cap.
You will see that you will get there soon.
