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Ok so now you use the Ronchi up to a certain point and then finalize the figure with the caustic if I'm not mistaken – quite right? I have ever considered using a Bath-type interferometric test – although I'm not sure if in practice it can be used for a relationship like this’ pushed.

I can ask you to take a look at this video of a start test of the Otvos telescope:

Is’ a valid test to evaluate the optics?

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Thanks for the info Massimo - in my opinion you should resume part of your sessions and put them on YT or at least here on the site. There aren't many videos out there and even fewer ones that use hand crafting.

Then if I think of such a diameter, the cases are rare.

By the way, there is a new interview with Tom Otvos who completed his 350mm too, 12.5mm f / 3 thick

There is also another Dutch Group that made a 600mm meniscus 18mm I think - completed and aluminized.

Good work we are all with you!

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I am extremely happy to see your progress – what types of passes do you make with the 125mm tool? For curiosity’ how many hours do you think you spent to go from k-0.35 a k-0.7 or from the beginning of December to now?

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Hi Massimo, congratulations for the Ronchi-phone and for the progress.

I've reread almost the entire thread but I'm not sure – you have always worked from the beginning with the tool above (UNTIL – Tool On Top) and on a cell a 9 supports?
You've treated the back of the meniscus in some way? Always hold the mirror in the same places?
Last question: you have always used a tile tool(?)- to a 50%?

I hope to see an update as soon as possible!
Michele

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A preview of the last work session:

https://i.postimg.cc/FsLQ6qpK/IMG-7543.jpg

I usually post updates here as well as on CN and SGL:
Instagram: #800mm_telescope
Facebook https://www.facebook.com/800mm.telescope
YouTube https://www.youtube.com/channel/UCC4…IGmvUwCV9ASGUQ

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Thank you for taking the time to do the analysis – I did a similar thing with PLOP and it was clear from the start that a 25mm e’ tragically too thin.

I repeat – especially to myself ;) – that this should be considered a middle ground between an attempt and an experiment. When I see those who try their hand at uncharted terrain it usually takes longer’ of an attempt. And the second attempt e’ much more’ sent.

A positive result will be’ that we will perfect the processing machine and our master begins to get involved – he definitely wanted to scratch a little’ of glass.

The second / third analysis you do reiterates - as I already have’ seen elsewhere- that x computational analysis a meniscus behaves like a flat disk. The recruitment here and’ that the telescope e’ aimed at the zenith. When it's’ aimed at 45 degrees - just to give an example- the meniscus appears to be slightly higher. In addition, lateral support becomes relevant – until now I have been convinced by the whiffle-tree with Teflon supports on the center of gravity plane.

Ah, in brackets the cell will be’ until 27 points – compared to everything else it is not’ a great deal’ of complexity. I have the pieces laser cut and oh yeah’ took the spherical pads to connect all the triangles.

At Schott Italia I have tried more’ times a few weeks ago but no one answered – thanks anyway for the advice. I try again again

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For our project we are in full evaluation phase – our master of mirrors (which made three 500mm) e’ rather cautious and conservative – how is’ that it is. I am only afraid that this will lead him to not evaluate aspects that are common, they are extreme by the large diameter.

For example, he wants to do all-roughing up to parabolizzazione- with whole tool. There are 10kg of glass to be removed…do you! He never used the half tool’ instead I honestly have never seen anything different used for diameters above 400mm (more’ the name…).

Then:
there is a 53mm blank’ from Reginato ( or at least e’ to catalog) but they are 4000+ Euro. I honestly would have taken my tooth out once with a nice collection – the club though’ for now he doesn't hear us.

Meniscus option (da 25mm) e’ the one more’ desirable – thermally even normal glass is not’ crazy but at that point with 800Euro there is also a disc of borosilicate.
But’ he never made a meniscus – aspe’, it is not’ vero, we tried a 120mm one which is’ it turned out well but we ran aground on a 400mm. Let's say he has no experience.

So in parallel I am looking for borosilicate between China , Germany, UK, Canada etc…

That said’ , since he has the 'fregola’ or itchy hands since we kicked it off’ taken 2 25mm discs of normal glass and want to start working shortly. I know, I know. For now he is modifying the processing machine.

For me and’ extremely important that you gain confidence because’ in the end e’ he who will spend’ hours and hours on that piece of glass.
Even if he wants to make a 15mm arrow in a 25mm disc! At best we will end up with a tool already’ bent…

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Maximum, sorry if I still disturb you – I tried to google 'pisa nautical glassworks’ and Inglas and Roberglass turned out. I was wondering if it was either of these two that you used.
Did you get / made the mold yourself or did they think about it? It was refractory, it wasn't a mold they already had’ in home?

Thanks in advance
Michele

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Sorry if I keep breaking – do you remember which descent profile you used?

You had checked the residual voltages with a polarized filter?

Is’ just to understand how the process can be strengthened – next time starting from a borosilicate slab I guess it will protect more from these tensions. It costs a little’ more’ but it would give more’ certainties of reaching the goal.

Last thing – you had thought of doing everything in one go or meniscus and structure? The failure could be partially considered in the geometry of the support parts. Then maybe you have access to the oven you used and it isn't’ a problem to make two passes.

For your info – maybe you already knew him:
http://rokoszoptical.yolasite.com/double-arch-mirror.php

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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.

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The approach you describe does not make a turn. At this juncture what you risk is’ at most’ to come back a little’ back with the workings and resume figure.

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Hi Mirko, even if this topic is’ now old of 3 years I still want to write .

I find your approach very interesting and obviously’ a pity that it broke early.

Is’ one of those experiments that have the intuition and courage you need – and that I am a refreshing wind. And this applies regardless of the fact that with this method it is possible to obtain an adequate dimensional precision.

It reminds me a little’ -even if with different execution- the Hubble Optics approach.

Is’ a brilliant way to overcome the high cost of a standard approach through multiple use’ effective material. Is’ true that requires an oven but the potential is’ high.

Congratulations again – together with Marco e's 600mm f / 2.2’ one of the most projects’ interesting I've seen recently.

Regards,
Michele

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In our group we did some slumping experiments up to 400mm (I think it was an f / 4) with refractory shape – e’ been a process full of…teachings (also read failures) but in the end we did it. It was a normal and fairly thin glass.
In my opinion 0.5mm is an excellent result – for better precision I think you have to switch to a metal mold.
Here you can find a work of the INAF about it:
http://www.elettra.trieste.it/actop0…s/Main/p21.pdf

As I see it, the generation of curvature in the oven, apart from the thin thicknesses, it saves a good part of the roughing phase.

A note – if I am not mistaken I am’ I seemed to understand that you are using an 'x' cell’ points to support the glass during processing.
Personally I did not find anything explanatory as a support in the processing of the menisci but I and’ I seemed to understand that the cells are not adequate – The reason is’ which is’ easy to print out in the final works.
Personally I find that it makes sense since the cells are designed to support the weight of the disc itself while in working the tool pressure would bring a significant contribution to the deformation of the disc – and since here we are dealing with thin disks the risk is’ maybe even more’ high.

Then that the alternative isn't easy’ e’ a whole other matter.

Forgive me for the note, I will not allow myself to give you indications on your project – e’ rather a consideration that I bring you in this field’ which is’ still experimental. And one thing I have to’ I should also consider thoroughly when and if we intend to do a meniscus.

Keep us updated on how it goes with the grain 800

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Hi Massimo, I follow with 'interested’ pay attention to your project.

In the first post you leave already’ from a meniscus glass – I can ask you if you made a topic dedicated to that phase?

As for the meniscus, you are aware of Tom Otvos' work in Canada?

I hope to see your next posts soon.

Regards,
Michele

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