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Hello Franco. Glad you resumed work!
I believe that with the turntable you will have a nice relief from work and a good result in a short time.You write:
Two weeks ago I started, it was a constant try and try again, then I put the full diameter tool underneath, and the blank above and away, now I have arrived at an arrow of 3 mm, focus 1812; the spherometer measures me 30 cent / mm practically on the entire mirror, I miss the focal length F1500.And, a 305mm diameter mirror requires hollowing out an initial spherical cap with a 3.75mm deep arrow, (that is equal to: Mirror radius squared, divided 4 times the focal length).
Sphere that will then have to be parabolized, perhaps as a precaution not touching the most difficult area at the edge, and deepening with the parable towards the center, with one final difference from the sphere of suns 2,34 microns.
I also believe it can be useful to you (if you haven't done so yet) watch this movie by Gordon Waite,
…using a "Fixed post" turntable machine, similar to yours, but without your alternating arm. But that with only the engine of the plate, in roughing and polishing, that's all it takes to not complicate the consequences of arm work.
The swing arm is more difficult and dangerous to use at the beginning because it is quick to parabolize when it is not yet time. for example while you are still roughing or polishing, knocking down the edge with its uncontrolled exits to the right and left ruining the sphere.
The parameters of using the "fixed post" in that movie, and in general, I am alone 2: Ratio between mirror diameter and tool diameter, and tool decentralization.
That: Diameter tool from 70 to 80% of the mirror (in the movie he uses a diameter tool 230 on the 280mm mirror); and decentralization of the 30% the mirror diameter; which in the video is 100mm which means that the tool "overflows" that is, it comes out of the mirror edge by 25mm, dragged in rotation induced by that of the mirror.All with grain 80! And now the problem arises. I continue with the 80th step to the 120? Still full diameter or I put the tool diameter 240, mirror above or below?
Now I'm working with the full diameter tool. I will continue with grana 80 to switch to 120 when I'm near.Mirror above or below works well when working by hand. You can adjust the depth of digging with a machine like yours with decentralization, because I wouldn't be so sure it looks good with the mirror above that I have never seen used in a similar machine.
I've always worked dry, at what level water is added?
You can use water right from the start to humidify, with the advantage that it does not disperse silica dust in the air; and the abrasive works better because it is "lubricated" by water rather than being low, it also holds it together by capillarity.
Furthermore, re-measuring the mirror I realized that a ring has been created between the edge and the center, that is, the center is less deep than 2 cent/mm. how should I operate to remove it?
I do not know, but being a zone error, I would think that it is the fault of something that is repeated cyclically over time, how could be the difference in abrasion due to the stop due to the inversion of the swinging arm motion.
But the problem could also come from the position of the mirror above, which reverses the roles in the machine between tool and mirror. Given the thin thickness of the mirror that if it is above, it is not laid and supported uniformly over its entire rear surface.. rather.. is loaded with ballast, while moreover it works in a rush.Putting mirror underneath and utensil on top would work in a traditionally recognized safer way.
However, these are just my old-line thoughts
Hi Fulvio.
7 Questions = 7 answers:
I would state that the whole argument “blank glass for telescope”, it fits more into the vast dilemma of his “Availability - Costs – Benefits”, because it is difficult to find the thickness greater than 20mm, and the cost of a blank also increases for this reason alone; furthermore, the benefits have a cost that becomes very important and often unjustified, if not from the question on the part of those who have no means of knowing whether technically the realities in comparison are substantial or “niche “marginal”. And in fact, these benefits of buying an expensive blank are almost always a rip off because they are very often invisible, at least that those who observe in a visual way.So to make a mirror, the greatest commercial thickness is 20mm (for example for a figure around 120 euro, for a pair of round glasses 305mm diameter, one for the mirror and one for the tool,) that thickness does not allow to rise too much with the diameter of the blank, because as the diameter increases, the height of the telescope eyepiece from the ground increases, and therefore to contain it it would be necessary to decrease the focal ratio. But by decreasing it, the parabola to be carved into the glass becomes profound, thinning in the center the already thin raw mirror. Problem that today we try experimentally to get around by letting the blank sag on a parabolic refractory mold base, making a meniscus, with new processing and use problems.
When purchasing a more expensive blank to make a mirror, let's say up to a diameter of 300mm, equally achievable with normal glass discs in 19mm thickness, we are therefore in the same parallel, mostly emotional conditions, of those who must necessarily buy something that makes them feel psychologically "good" and protected. in making an important environmental investment, for example, which in reality in that case of equally difficult decision, would satisfy the seller, but over time it would make the investment very cheap, or it would make it less and less profitable than agreed (see who did it years ago in photovoltaics, or those who buy the fully electric car that ends up favoring the nuclear power plants of the adjacent states, built close to the "customer" border).
To answer you it is convenient for me to do so by reporting your questions:
Question 1. The advantage of using a black with a low coefficient of expansion is only for those who make optics on an industrial level, not having to wait for the cooling and stabilization of the glass to perform measurements following processing. For the amateur self-builder it changes little, since time is not money but "fun".
Question 2. the mirror, of whatever material it is made of (from calcium-sodium to glass ceramic), however, it suffers from tube turbulence. I found this perplexity of yours on a par on the site of the manufacturer Nauris. Here is the translation:
“If you want to use glass ceramic instead of borosilicate glass due to its low coefficient of thermal expansion, keep in mind that: small and medium-sized mirrors are not limited by the deformation of the surface as the temperature changes, but by the turbulence immediately in front of the mirror surface. This turbulence is fueled by the large thermal mass of the primary mirror and is maintained by the falling temperature during the night. In fact, Zerodur has this turbulence at the same level as any other glass of this size. What initially seems advantageous - no thermal expansion of Zerodur - is in practice nullified by the much more damaging release of heat in the form of turbulence due to the primary mirror as a heat reservoir. As a top priority, a mirror that is as thin as possible should be chosen. Only then should you think about using a better substrate quality than that provided by borosilicate glass. "
But then,, why use a "precious" glass? Net of the replies already received, it's not clear to me.
There are some advantages for the end user?
Some considerations that I have read or that I have drawn from myself. I would like your opinion, and everything you want / want to add.Answer 1 e 2: A big immediate benefit is the industrial time savings, but on a professional level, or in any case for an amateur astronomer with high magnification planetary vision needs, who has learned to achieve a quick acclimatization of the mirror, there is the advantage that acclimatized optics remain less sensitive to changes in the temperature of the observation site.
Question 3 “A glass contracts when it goes from hot to cold and does so unevenly from the center to the edge. The larger the glass, the easier it is to notice the temperature variation because it takes longer.
But it doesn't end there.
It is easy for glass to "follow" changes in temperature and to produce crap (visible better) in intra ed extra”These are not my considerations. If true, however, a mirror with low coefficient glass would return "less deteriorated" images as the temperature varies during the observing night.
Answer 3: I would defy any human eye with all its own flaws, to distinguish, looking at the same object through the eyepiece of two identical telescopes side by side, which of two acclimatized mirrors is the one in normal glass, and which one is borosilicate.
Question 4. A "noble" glass has a better amalgam, right at the material level. So it wouldn't have bubbles, stripes. And maybe it would reduce the possibility of the presence of deaf scales, tensionature… etc. Definitely, a "better" material would guarantee a better final result.
Answer 4: I would say that better glass could lead to a better result, but not always visible by eye.
In the sense that a well-kept glass is of better quality, which does spend a little more is certainly fine. But all that extra care doesn't have to lead to an exorbitant price gap, considering that glass is a molten mass which, having no crystalline structure, cannot present a predisposition to form deaf flakes during processing, because it is the coarse abrasive that creates those fractures in any glass. And why glass is physically speaking, in fact a liquid with almost infinite viscosity at room temperature. Hence the vitreous mass, regardless of replacing one or two ingredients such as boron to replace calcium-sodium, the physical substance of the vitreous mass does not change.
The problem if anything, in a very ancient time, it could have been that of tensions in cooling that do not exist today, or of inclusions of air bubbles in the non-vacuum melted glass mass, which in solidification could not reach the rising surface, and that in the process they could leave a hole in the scratched surface. But with the common float glass production technology (whose laminate plate cools slowly by floating on a bed of molten tin at approx 230 degrees) it is a disappeared problem, that remains present are in the blank of desired thickness greater than the standard commercial one of 19mm, if it is the private individual who melts a blank in his home, starting from normal glass cullet, or boron silicate
Question 5: A noble glass (therefore "harder") facilitates some stages of processing (like polishing) and allows a polishing not obtainable with poor materials. This implies the achievement of better optical standards.
Answer 5: A borosilicate glass is less "hard" than calcium sodium glass, in the sense that it is more easily "scratched" and attacked by abrasives. But it is a completely irrelevant difference in the processing and on the final quality
Question 6: It is not clear to me if a glass with a low coefficient of expansion, first reaches thermal equilibrium with the environment. If so, the boundary layer would be reduced.
Answer 6: The heat transmissibility power of glass, unfortunately it is very low and undifferentiated by its almost equal chemical composition.
Question 7: A low coefficient blank (or in any case specific for optics) it deforms less. Therefore, in making a mirror, with the same diameter, a lower thickness could be used without losing stiffness.
Answer 7: No. Glass of any type remains a liquid with practically infinite viscosity, and therefore substantially subject in any case to deformations imposed by the force of gravity due to its lack of crystalline structure. Is’ the mirror cell which must provide the necessary support, and which once justified a thickness of 1/7 the diameter , then reduced to 1/15 diameter in modern times with the cells that can be designed with Gui-Plop. Ma Mel Bartel (see link) has created mirrors in large diameters and even much lower thicknesses, even if only tested with the terrible, poor Ronchi or with the inadequate Foucault,and therefore "good for those who are satisfied".
https://www.bbastrodesigns.com/largthin.htmlin conclusion, a little free thoughts. I'd like to understand something.
Definitely, it pays to invest in a better blank?Concluding answer:
In my opinion, at the beginning it is not worth looking for a nit.. And you understand that if you see the John Dobson video, how he built the 400F6 telescope. You thus realize that any economic choice within your reach is good to start without risk. https://www.grattavetro.it/auto-costruzione-di-specchio-e-telescopio-o400mm-f6-con-video-tutorial-di-john-dobson-parlato-in-italiano/Hi Fulvio. Welcome here!
YOUR QUESTION 1: It is correct to say that a glass with a low expansion coefficient, as it keeps its shape better as the ambient temperature varies, it retains its parabolic shape and therefore “better” respects Couder's criterion than ordinary glass?YOUR QUESTION 2:This is the advantage of using high-quality glass with a low expansion coefficient? make sure that when the temperature changes, the rays still fall into the diffraction notch?
THE ANSWER 1 e 2 and yes; A low coeff. of thermal expansion, as the temperature changes, it maintains the shape imprinted on it during construction. Good or bad it may be, it will remain unchanged.
YOUR QUESTION 3:Otherwise what would be the benefit of less deformation? Avoid various aberrations (astigmatism type)? Or both?
REPLY 3: No different advantages, because if in building the mirror one makes it astigmatic or badly aberrated, it will maintain those characteristics regardless of the ambient temperature.
An average grid line width of 0.125mm creates a lattice with four lines per millimeter, and it is already a good result for a DIY lattice.
Two years ago I ordered some from 20 lines per inch at the US publishing house Willmann Bell, but they returned the money with the communication that they no longer sent it, causes frequent loss by postal services….A stupid excuse like the lady who owns the company. But today I understand that the cause could well have been the one that made her close the business.
Even I used a double frequency grating, I have no idea it wouldn't make your job appreciably better
Thanks Massimo for reminding me.
The fact is that I forget the notions (…and where things are from) which I haven't used in a long time.
I think it is normally a geriatric fact for those over 70…Unfortunately!It already looks beautiful though.
So if the parabola has K = 1, with -0,25 you're already in a lot of hyperbole .. or am I wrong? (…Not using them for a long time, I get confused with the meaning of positive and negative K's!)And. Now the central area seems to me only slightly deeper, with the narrowing slightly more marked in the connection with the periphery.
well Massimo.
my surviving neuron did not allow me to remember that I had already mentioned the use of Mel Bartel's Ronchi matching.I also believe that to achieve such a large and accentuated surface curvature while respecting tolerances, the best way to test is the classic one, than with Ronchi purely visual, flanked to Foucault leads to the sphere and beyond but only up to the proximity of the lambda / 4 parabola, to continue with the caustic test to destination, putting in place the external areas of the mirror, notoriously more extensive and steep and impossible to bring to the correct destination even by the Foucault test.
The use of Ronchi throughout the processing of such open and short focal lengths, it is a shortcut to demonstrate satisfactory personal impressions of sufficiency with only one, that it would be easy to achieve what is actually not easy, because the resulting work is only indefinitely approximated.
Unbelievable because the Ronchi test notoriously does not provide any descriptive number of the quality of the curvature obtained, indicating the deviations from the proper numbers describing the curvature taken as a constructive reference.I realize that I am just inadequate…..
I couldn't figure out how the hell to go about it “upload to Browser” an image of me Ronchigram, as Mel Bartel suggests in the description of his Matching Ronchi test, as if it were the simplest thing in this world
.I understand the difficulty.
I don't know if you already know what follows, in case parabolization could help you…since always you “what you touch better”, always managing to turn the result in your favor
.A while ago wandering around Mel Bartel's papers I had seen “on the fly”, the existence of a method that facilitated the evaluation of the curvature of the Ronchi test.
The method consists in superimposing an image taken on the rhonchigram of the mirror under examination, to another image “negative” of the equivalent ronchigram simulated by the Ronchi Matching program, for a common offset distance” (obviously understood as the same positioning distance of the real and simulated gratings, from the center of curvature).
http://www.bbastrodesigns.com/ronchi.htmlthe following image shows what I am saying:
http://bbastrodesigns.com/JoyOfMirrorMaking/25%20inch%20quarter%20wavefront%20undercorrected%20at%20center.jpg.
And it is taken from the article “making a mirror of 25″ F2,6” by Mel.
http://bbastrodesigns.com/JoyOfMirrorMaking/25%20inch%20f2.6%20mirror.htmlThe image shows the overlap of the two images, of which the one with the black bands should be relative to the ronchigram actually detected on the mirror under examination, while the one with a pink background is generated by the simulator “in negative” ie with white bands on a pink background, works , due to the error present, they do not completely overlap the black ones, but the white residue remains visible, the part of the error present.
In fact the black bars adhere to the internal curvature of the white ones generated by the computer.I understand that it would also be possible to print two simulations with a certain value of lambda / x apart from each other, to see at what distance from acceptability the mirror being processed currently resides.
However, another thing remains the difficulties related to the corrections to be applied to ensure that the black curves fill the spaces of the white ones….
Already, I'm sorry…I calculated the tenths which instead are hundredths!!
On Ronchi you catch me unprepared but curious:
I need you to explain to me the reason for Ronchi's next useful illegibility, as the parable deepens.
Tell me if and where am I wrong, but unprepared I would seem to guess that the lines with which Ronchi shows the sphere, gradually becoming the curves of the parabola being excavated, they might show up (maybe maybe) with local accentuations, for the correction of which it becomes difficult to estimate the quantities of work?
Thank you.
Anyway congratulations!Bravo Massimo! I agree with the compliments and the excellent idea of parabolising starting from the center, despite the 700 thousand nanometers to be removed on a smaller area with a more peaceful processing.
And, I also believe that to leave the old manual road for the new one, the saying is put into practice that we know what we lose but not what we find.
Especially since a sub diameter with new experimental technologies of a proportional type to that described in the article for telescope by 8,5 meters, in any case, it would certainly have a dimension too large to work amateur telescopes even of important aperture.
Of course, of that application is also fantastic the way of controlling the processing, independently guided by software based on the position of the tool on the surface, compared to a previous mapping of its nanometric asperities. Mapping made by counting the fringe hundredths, reflections from the surface hit with a red laser from 630 nanometers high..
If I'm not mistaken, the accounts, a penny of that light's fringe, measure 6,3 nanometers high.. Which on the glass would damage the wave of light that passes over it twice, in incidence and after reflection, introducing an error on the reflected wave of 12,6 nanometers high., which with respect to the wavelength of the yellow-green visible light of 550 nanometers used for normal telescope, correspond to a terrifying optical precision of the Lambda / 43.6 Large Magellan telescope.
Before reading the article, I had never encountered the practical use of the properties of non-Newtonian liquids, nor a read of the methodology (ingenious) to map the mirror surface with diffraction fringes, to determine where, and how many fringes is the defect to correct, to be able to maneuver the tool up to literally zero out the error.
