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you Mirco, although it must consider that the glass center is obviously thinner, however, the test have made it on a thick glass block 20 mm and there was no difference.
The leading edge and also to output, It is presented with a screen similar to grain irregularities 60-80, appreciably better with a lens, Visually it looked sharp and no chipping, while the touch seemed edgy and slightly’ rough , Instead the walls were very smooth. I gave a finishing the edge with the rounded tip of the cote creating a small chamfer.
after several hours of processing the mirror is now polished, the residual pits are available in limited quantity and acceptable.
For the polishing was carried out a pitch patina “wintry”, as described in: https://www.grattavetro.it/patina-di-pece-invernale/ , tool selection has been conditioned by the central hole, left open without “caps” , which prevents it from working with the full diameter, thus a sub-diameter was used to 40% (120 net mm chamfer) wherein the total surface could be based on the mirror and be pressed for adaptation without interfering with the hole.
Polishing up the ball with the sub-diameter is not easy, complicating things you add the hole which must be treated very carefully,not being able to get over it completely, it is difficult to keep him “stable”, the zone near the edge tends to rise or to lower according to the type of working.
The figure is reached next to the ball, even if it is generated a bad and annoying hillock on the outer edge
which will be absolutely correct before starting parabolizzazione. 
ciao….
What about Max, it is obvious that you are very good on the road…the ball is already very beautiful…
What kind of past you used to make the mirror ball ?
you can use to create subdiametro the ball even on a full-length mirror (without central hole)?
if you what kind of past?hello Mirco, are interesting questions to which there is no need to give a detailed answer
I state that I do not recommend to use the sub-diameter, unless it is not necessary, to build the sphere.Contrary to the full, I do not think there is a technique that allows, as it happens with the past to 1/3 COC, to arrive “naturally” the sphere. The sub by definition always works some areas more than other, the ball then, equal to the parabola must be built for zones, trying to check for errors and acting accordingly.
That's what I did, I started to polish alternating tangential longitudinal past ones trying to cover the whole surface as evenly as possible.
There appeared, therefore, the shape errors, for most predictable and expected, so I acted accordingly to correct them.Working with the sub-diameter , after a while’ it acquires a certain practice that allows to estimate the deformations resulting from the application of a certain technique with respect to another, then try, for example, to deepen before the center and then the periphery up to the uniformity.
Paradoxically there is much difference between building a good dish and a good ball using sub, and I firmly believe that any results can be achieved (on the ball ) with sub, with a full tank would be much better: the sphere is the only form that allows two surfaces , however, the move over one another, to stay in touch. The technique that best approximates this situation is the aforementioned 1/3 COC with full-diameter: if applying this technique, the ball does not come out, It is useless to insist with corrections or other techniques, the problem is not running but the patina setup and work plan !
past 1/3 coc NECESSARILY must lead to a sphere “stable”, if it does not the blame is to be found above all in adapting and then the hardness of the pitch and in the stability of the work plan.
These problems with the sub-diameter are even more pronounced and more, It is added to the pressure element that can radically change the expected result that much more with the full.
The defect to the edges you see in this sphere ( looking beautiful ) It is unacceptable reality, especially in this configuration so thrust, do not allow to build a good parable ( hyperbole) as the extreme peripheral zone would be unusable in the measures. The fix for this defect with the full diameter would be simple and relatively fast, with the sub will be a little’ more complicated…
In conclusion I can say that the sub makes everything more difficult, failing that aspect of symmetry and in the whole application of the technique, but this is also due to the great potential of working that allows to intervene in a targeted and precise, in practice in the two methods, the start of one virtue in overlap of the other limits.
Massimo've made a compelling “radiograph” to your work.
I must say that reading provides many elements to understand the variables that must govern.
I think a “I like it” for my part would be too simplistic for a description so instructive.I agree fully with what was said by Julius…
And since there is no single technique with which to be able to generate a sphere with the sub-diameter but it is necessary to continually correct the errors that are generated, your work and the result certainly deserve further praise… Thank you guys
, aside from the shape still to be improved, the satisfactory result is that the quality of polishing achieved with this patina. The absence of validation roughness good functionality of this mixture for working temperatures around 12-14 degrees. Undoubtedly the total time for polishing has been higher than necessary with the full diameter but this, you knew, is the price to pay for the donuts with a hole…Now, just arranged the sphere will require the precise measurement to the tenth of mm radius of curvature ( tips ?
), and then we start with the generation of the hyperbola and related measures, We will be fun with Foucault and test-caustic for a F2.6 
, of course I will need your support to find the right direction in this path yet to be explored…Improved defect on the edge, there for you to fix something, I will make even a touch in the coming days to the edge, but by the time the ball can go, you can start thinking hyperbole
In the last image on your right you can see a “very beautifull” edge retorted in the central hole
. from what I read is a very common defect in perforated mirrors, It is part of the side effects of the machining “open hole”, but in this case it is not important for the purposes of correction on the useful surface , since the hole is 70 mm, but the obstruction of the secondary will be 105 mm, so there is a good margin, also the center for a parabola / hyperbola f 2.6 It will cut a lot '… so I think it would make sense to try to correct it now…I agree with your assessment….
Is, in your place, very intimidated find myself from '….”to invite” convincingly, that beautiful hyperbola “come with me”!!! Those are still and always risks that hold their breath.
Although the curves desirable (“capissci Again)
They are of type “different” and that does not involve our markedly XY chromosomes 
.Giulio, in the meantime I'm organizing for next Foucault test, to get to the end of the caustic test.
In your opinion, a functional Couder mask so “acceptable” for a 300 / F2.6 how many areas would need ?
I thought at least 7 zones, Whereas it will not be measured in the secondary zone of obstruction 110 mm , which is equivalent to about 9 areas on the full mirror.Is’ clear that due to the curvature of the mirror, in theory, the more zones are and the better, but a too high number would make it even more difficult the test, not to mention the corrective actions…
interesting question!
I have tried using “mine” (but it is not mine but, honor on, and Pierre Strock) excel sheet relative to the Foucault test for 8 zones, downloadable from me here translated into Italian:
https://drive.google.com/folderview?id=0B2bzgHnONxwEZzJYalJwc0Z5MFk&usp=drive_web(with windows 53-75-91-106-118-129-140 and 150mm radius).
By placing diamero 300 and focal (300×2,6=) 780mm, I found that the most difficult area 8, He would width 10mm, that from the tester positioning distance of (2 Focal x =) 1560mm, is visible by the operator in a well would say easy way.
However, being down to earth I would say that I am sure that if you use an initial approach to the caustic Foucault, until 7, or even just 6 zones, I think you'll be much better.
This is because with your demonstrated mastery of the demanding subdiametro, you will definitely be able to handle for the better to the course of a greater number of zones, also only limited to the test terminal of the caustic.
Ok, thanks Giulio, go for the seven areas…
Moreover, Today I have withdrawn two discs very noble vetraccio
for the secondary, the size of 115/12 mm e 125/12 mm, in accordance with the sacred texts that indicate the caliber / tool of higher dimensions than the secondary convex, to prevent problems at the edges.This time it was not used by my glassmaker waterjet, but undefined “machining center numerical control” for cutting / shaping the thickness of disks 12 mm.
Look in particular the last image: both the board that the chamfer are perfectly transparent ! the question is: how the hell did they ? It is possible with a cutter work the glass until it is shiny ?
sure that… They are so well finished which is a shame “ruin” to make us a secondary, I almost use it as a paperweight
Is’ a very good job that my old glassmaker done by hand.
The machines work quickly and well especially if you do not have to hold off nanometers
http://www.hobbyland.eu/ita/prod/00000011/00000283/00000286/12210.htmPaquette with glass and cerium oxide
The deepening of the former ball towards the parable-hyperbole proceeds, not very fast to be honest, but there is also to be considered that, for a F2.6 the amount of glass to be removed with the cerium is remarkable !After giving una'altra sprucing up to the edge, I started the approach to the parable-hyperbole.
The surface was virtually divided into three zones (Hole area, median and peripheral zone ) and I began to delve into the center with a sub-diameter 120 mm and then extend the depth at the external zones.Account to arrive with a bit mode’ coarse to a conical constant of at least -0.7 ( the final one will be -1.12 ), then i will change system, using Foucault on 7 areas with sub from 60 mm and reversing the deepening, ie starting from the last two outer zones, carrying them in tolerance and towards the center.
I definitely chose the longest way, taking as a reference the edge and deepening everything else, without using the mode “classical” to a 70% you all know better than I..
I believe that, Having regard to the high curvature of the surface, ( and also the fact that, thanks to the hole “saving” 70 mm glass at the center to be machined ) This mode will give me more guarantees especially the edge, I do not want to risk a shot back edge, I think that trying to “lower” this steep edge is much easier to generate.At the moment we are at a constant conical ( estimated ) of -0.45, the road is still long…
pictured the comparison with the simulation to Ronchi D = 297mm , F=772mm, K=-0.45.
awesome work
but engaging And of thumb I find it very sensible planning you have in mind for your roadmap.Certainly impressive is the fact that a parabolic mirror 300F2,6 would have a central arrow of deep dish well 7.22mm; with a very thin glass thickness in the center area.
In addition to parabolizzazione it would require a “countersink” peripheral, different and finish work well 16.75 microns from the sphere of origin.I have not the slightest doubt that this realization will be a memorable success in its genre.
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