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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
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 Matteo,
If you could upload your videos here too (or post the various links from time to time) it would be very useful in order not to disperse’ the information.
The 'direct grip (in video)’ the creation of a primary with corrections and suggestions in real time by the most experienced users… would be priceless!!
FulvioBest wishes to all of you for a Merry Christmas from me too, that I am the latest arrival. I hope you will accept me even if I am not yet a “glass scratcher”
. I read the contents of this forum with greed and interest and deeply appreciate the spirit that animates you artisans of sky observation’ !
