Aluminum mirrors

[skround]

I don't know if you already have any’ talked in this forum – alternative materials to glass have long been studied. I. Newton's perspective was not bronze? Or brass?

Anyway, aluminum is also not’ new – if I'm not mistaken in Merate there was a 1.5m which then e’ was replaced like in the 1960s.

Now, there is an interesting work by a group of Brazilians - who are evidently in short supply of quality glass- are repurposing this material.

The thread e’ on CN and they seem to have been able to overcome many of the technical problems in interpreting the processing of optics.

Just to consider a comparison between aluminum and glass (Ca-Na) I report the set of salient physical characteristics (adimensionali car’ e’ the comparison that matters) also to avoid misunderstandings about the thermal aspect:

capacity’ thermal 0.21 Al vs 0.19 glass
thermal expansion coefficient 21 Al vs 9 glass
conducibility’ thermal 200 Al vs 1 glass
density’ and Young's modulus are quite similar

Sure e’ still a lot to probe and validate on a practical level but the preliminary results are encouraging and to date other ATMs outside the original group are trying to prove the new technique.

At the level of practical benefits – aluminum has a relatively low cost and e’ more’ easy to find glass.

Furthermore, for a lightweight solution it is not impossible to put it on CNC to properly dig it and obtain an efficient support grid.

[Massimo Marconi]

There is an interesting article by Mirco about it, where the properties of various materials besides aluminum for mirrors are described and compared.

The telescope mirrors are made all glass ?

[Giulio TiberinI]

Aluminum would be easy to work with, and shares its specific gravity 2,6 with glass. But the problem with aluminum is the very high thermal expansion compared to that of glass, even if at the same time it would be coolable with greater glass efficiency.

Moreover, On the topic of the quality of metal mirrors in general, I am reminded of some reading here and there on books and manufacturing reports for astronomical use, where two problems are highlighted which contribute to limiting optimal quality results, which instead are the prerogative of glass mirrors.

A problem is that the metal mirrors, even if they could consist of one pure metal, however, all physically possess a crystalline structure that glass does not present.
And therefore the crystalline lattice forms molecular stiffeners that make the response to the extremely fine abrasives of the extreme polishing that must be reached to reach the famous ones no more than variable and inconstant and unpredictable. 68,75 nanometer peak / valley error, which characterize the surface of a lambda / 4 mirror for visible light, which is practically the minimum entry level in the definition of "limited by diffraction alone" of an astronomical mirror.

A further problem that also concerns it, and even more the limitation of the optical quality obtainable on a worked metal surface, is that only an electrochemical process takes place in glass (as well as merely mechanical polishing) called glass "molecular transport".

Practically, slowly, that process visually transforms the chipped edges of the craters left by the abrasives up to the grain under the microscope 800 or vice versa 1000, which during polishing are first seen to have rounded edges, and then decrease in size gradually “tappandosi” disappearing completely.

The work of the molecular transport process of glass is due to the electrochemistry that acts simultaneously on the pitch, which blocks Cerium oxide particles (very active rare earth chemical element), with the presence of water and silicates that make up the glass, that Cerium actively interacts with each other.

So much so that you should never leave the polishing by abandoning the tool for the weekend over the mirror, under penalty of appearance on the glass surface of iridescences, which are the visible optical effect of surface defects which, due to wave interference, cancel or intensify some colors / wavelengths, and must be removed with the resumption of processing, but which force an increase in the workload.

If I'm not mistaken, some ancient construction reports on Amateur telescope making volumes, they gave as quality obtained on some metal mirrors as a millionth of an inch … (maybe today we could technologically do better) however, that difference was thousands of nanometers compared to the necessary suns 68,75.

By working with glass, in practice, it is certain that the disappearance of all possible roughnesses, is is a matter of extension in time of work.

[Bartolomei Mirco]

Hi Michele,
Being able to use aluminum as a blank is something that would please anyone. Its low price, possibility to mechanically remove material from the back of the mirror to lighten it, low weight etc. etc.. Unfortunately, however, there are some limiting factors that are so important that practically very few aluminum mirrors exist. The only ones I know of (in the frequencies of the optician) they are the mirrors of some satellite chambers for terrestrial observation, but they have crazy temperature controls, which keep the entire optical bench within a range of one tenth of a degree centigrade or some experimentation by ESO and the like, who are able to achieve satisfactory results but with processing techniques that go well beyond the reach of an amateur.
There are problems due to both the excessive thermal expansion coefficient, both to the fact that as Giulio says, it is not at all easy to obtain a degree of polishing suitable for an optical telescope and a thousand other problems, low temperature creep etc. etc.…
Maybe for small mirrors it could also be fine, but I still have doubts about it. In any case, I will follow the CN discussion with interest.
Hello and thanks
Mirco

[skround]

Mirco, Giulio, Marco, thanks for the replies – in the end, the reason for the post was to find out when the use of aluminum was plausible.

As for the thermal issue (one of the two fundamental obstacles) I think it is a question more’ academic what else. Indeed and’ possible that the enormous conductivity’ thermal expansion offers advantages that go beyond the unfavorable thermal expansion coefficient

I found this film to be eloquent, even if empirical:

And this with regard to the finishing issue:
https://partnerships.gsfc.nasa.gov/downloads/featured_technologies/optics_photonics/gsc_14147_1_mirrors.pdf

If the Brazilians can achieve something similar then it becomes interesting. After all, the magic that happens on a chemical level between abrasive and glass is not’ said that we can replicate for the elimination.

I too remain an observer of how this trend proceeds until’ the quality’ mirrors will not come’ proven in the field and replicated by some other group.

[Giulio TiberinI]

Interesting experiment that of the video you posted:
I would say that it is an example of localized heating with a fairly intuitive outcome, since with the torch a point on the edge of the aluminum disc under Ronchi's test is heated, to show how the Ronchi is immediately distorted by local expansion, but returns to recompose itself in a short time, extinguishing the phenomenon.

The thing is, in that case, the mass of the aluminum disc good conductor, it has a thermal inertia that quickly dissipates by convection inside, that strong localized heat.

Which would suggest that: if the glass is too slow to acclimate due to its poor heat conduction, but accompanied by a very favorable low coefficient of expansion; conversely, aluminum could be reactive too quickly, at the limit varying its surface shape by very high expansion, too fast, continuously and locally sensitively.

Which it probably could (seen the hand of NASA and Goddard) making aluminum preferable in some optical instruments of artificial satellites intended to travel far away or shielded, from astronomical radiative heat sources.

I would say a finish like that announced by the Goddard system of turning and subsequent polishing (“devilry” which reaches a roughness of 10 angstrom, that is, of only one nanometer) is the result of an exclusive cutting-edge technological patent, which I imagine also at the top of the application costs.

However, a decisively improving process, because it is able to bring the optical processing of aluminum into competition with that of the hitherto unsurpassed glass.

Excluding this innovative process, would remain the too high roughness obtainable on metals with the finest polishing process of all, known as lapping.
Which, however, does not go down as roughness below 100 nanometers high., which are therefore insufficient to obtain the minimum lambda / 4 error, which must count no more than nanometers 68,75.

I don't think any electrochemical transformation is possible on metals (outside the oxidation) with displacement of metal molecules to plug the surface asperities, as occurs in the molecular transport of glass as silicate, caused by the simultaneous presence of water, care, cerium oxide (and aided by heat from machining friction).

[Bartolomei Mirco]

Hi Michele,
However, I remain skeptical about the possibility of building an aluminum mirror. I am aware of several mirrors made of aluminum, many of them mounted on satellites, but on terrestrial optical telescopes, I think there are very few, and those that exist either for particular applications or are experimental.
In almost all the sites I have visited, those who produce and sell aluminum mirrors, propose them for X-ray telescopes, for cryogenic applications, of interferometry, per medical imaging, per laser, IR and more, but for optics of telescopes in the visible, I find practically nothing (this also in the pages you linked).
That said, I'm always ready to change my mind, and like you, I am following with interest the tests that the Brazilians are carrying out, we'll see
hello

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