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#9939 Giulio Tiberini wrote:
…Those glasses , although entry-level of boron-silicates, they certainly have the advantage of a lower expansion coefficient compared to sodium calcium when the temperature changes.
And this has a modest positive aspect in observation, but very large especially during the manufacturing / parabolic process…Is’ it is correct to say that a glass with low expansion coefficient, as it keeps its shape better as the ambient temperature varies, retains the parabolic shape and therefore respects “better” Couder's criterion of ordinary glass?
Is’ 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?
Otherwise what would be the benefit of less deformation? Avoid various aberrations (astigmatism type)? Or both?
Thank you
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.
hello Giulio.
Thanks for the reply.
In fact I am writing about this “forum” for some years. So I didn't show up, I already did.
I have read your concerns about the use of glass “nobles” in the production of Dobsonian mirrors. Perplexities that I could summarize in the following two points:A. 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, as time is not money but’ “fun”.
B. 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 – it is effectively nullified by the much more harmful 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 glass “valuable”? 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.1. “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.
Is’ easy than glass “follow” sudden changes in temperature and that it produces crap (visible better) in intra ed extra”
These are not my considerations. If true, however, a mirror with low coefficient glass would return images “meno deteriorate” as the temperature changes during the observing night.2. A glass “noble” it 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 material “better ” it would guarantee a better final result.
3. A noble glass (then “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.
4. 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.
5. A low coefficient black (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.
in conclusion, a little free thoughts.
I'd like to understand something.
Definitely, it pays to invest in a better blank?I'm sorry I polluted this thread. However, it has stopped for a while and I have seen that it is possible to open new discussions only if related to a new self-construction project.
Mine are simple curiosities of an enthusiast / interested in the subject.
thank youHi Fulvio, you can open as many threads as you want, it is always preferable to open a new one for better visibility of the topic when it is different from the discussion in which it is inserted. Moreover this topic, it seems to me full of interesting ideas and therefore we move it to a dedicated thread
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/Thank you for answering me, comprehensively, and point by point!
Definitely, I understand that the exorbitant cost of certain glasses does not go hand in hand with a corresponding optical performance gain.
I deduce that, more than the type of glass used, extreme care should be taken in the design and construction of a suitable cell. In order to reduce the thickness and, Consequently, the boundary layer (due to a lower vitreous mass)!
I had already seen the video of J. Dobson, and also the works of Mel Bartel. Thanks anyway for the links.
I intend to study the cells a bit, starting with the article on Gui-Plop present here on the site.
Sometimes wise and disinterested advice can give an indication of the path to follow.
Thanks again
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