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The Ronchi testphon is beautiful!
Congratulations on the recovery work that is anything but easy!Thanks Giulio, especially for pointing me out ( as always ) the right strategy
In fact, correcting astigmatism is not easy at all, the sectors must be worked independently and for a long time, without touching the mirror center and therefore it is unlikely to perform a perfectly symmetrical machining…
These are the images of last night, this time done with the tester “classic”:
is seen as the border on the southwest quadrant ( the sector subject to correction together with the north-east ) is still high but not only, the corresponding edge on the opposite side is also high but slightly less, symptom of non-symmetrical processing.
therefore at this point, I don't think it's convenient to insist on zone correction but, given the slight extent of the defect, it is preferable to work to standardize the figure with classical methods, to avoid the emergence of other local defects, which is most likely when working with a sub-diameter in small portions of the surface.
So I did a few sessions ( a couple ) as if you were to treat a raised edge over the entire surface, con ran a bit’ longer and with more overflow.
And even if that may temporarily take me away from the sphere, however, it will allow to arrive at a symmetrical figure, as the whole surface is worked in the same way, albeit with sub-diameter.
In the video that follows ( done just now ) you can clearly see the residual entity of the defect on the southwestern edge when only three bands are visible.
Hi Massimo,
sorry if I intrude on your 3-D, I just wanted to know how many minutes you spend on each work session (indicatively).
I just need to see if my progress is progressing as it should
Greetings again for your project
Thank you
hi Luca, imagine, you are welcome!
20 minutes for each of the last sessions I was talking about, ( with a few sprinkles of water each 5 me) which then is the limited time beyond which cerium completely loses its abrasive effectiveness, or at least that's what happens to me…
At that point to continue on the same session, I need another “charging” there cerium oxide.
Rarely, except in the final touches, when there are only a few imperceptible nanometers of glass to remove, i have done multiple sessions’ short. Ehilààààà Massimo,
what a job you are doing
You can see that there is a lot of method in your work and the results are starting to show…considering mostly, the size and small thickness of the glass…
Really good
See you soon
Mirco And! I agree with Mirco.
I have to say once again Massimo that (after all as your usual) you are a very special person in making the most of almost impossible actions scientifically by turning them back to normal, which for many others it is not, and it never will be (starting with me). Giulio, Thanks Mirco, Too good
In reality, I believe that the method and understanding of many aspects of processing need to be deepened and improved, I still feel far from one “standard” effective and reliable for what concerns the prevention and correction of all anomalies, of the unexpected events that occur along the path that leads to a finite optic, especially when stepping out of the safe territory of a mirror “classic” of do-it-yourself, with its proven shape, thickness and diameter. Often the action is linked to the intuition of the moment or the choice of the best probability rather than the awareness and certainty of the result.The more I get on with these unlikely social misfit jobs
and the more I seem to have “discovered” just the tip of the iceberg… and even if nothing is invented in this field, everything has already been seen and written, the notion-application transfer is not so immediate, theory and procedures must be adapted and customized to the specific case and a path full of possible variants is generated, that seem to promise to get you to your destination more comfortably and quickly, until you come across other anomalies that you didn't even assume existed.An example above all: I don't know if it depends on the shape of the meniscus, by the pushed focal ratio, from the thickness, from the media or myself, the fact is that the same techniques with sub-diameter have different effectiveness if done at the edge, in the center or in the middle area. In practice, the principle that has accompanied me so far fails… In the end just take this into account ( fortunately, it occurs constantly and progressively from center to edge ) and everything returns as before, but that's exactly what you don't expect to happen.
Fortunately, I noticed it at this stage, not knowing during the parabolization would have led to a long series of corrections with unexpected results.
So in order not to miss anything, I also started another mirror which I will write about in another dedicated discussion and a little’ “nostalgic”
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!
MicheleHi Michele, I go to order:
I reread almost the entire thread but I'm not sure - you have always worked from the beginning with the tool above (TOT – Tool On Top) and on a cell a 9 supports?
The cell is a 18 points ( there is also the photo ) and having used only sub-diameter tools since grinding, the mirror was always positioned on his cell, so yes…
You've treated the back of the meniscus in some way? Always hold the mirror in the same places?
The back has been sanded with grain 600 , just to avoid possible unwanted reflections during testing.
No, positioning is “random”, after moving the mirror I reposition it on the cell according to the consolidated method “at random” , Jokes aside, I don't think it has any use to keep the mirror in exactly the same position on the stand, rather… )Last question: you have always used a tile tool(?)- to a 50%?
If you are talking about tiles, I assume that you are referring to roughing / grinding… I have always used glass on glass, I have no direct experience with other materials for the tool, I get along well with glass both with full diameter and, as in this case, for a tool al 50%.
A little less than 7 cents mm ! This is what it will take to dig to parabolize the mirror
Anyone who has already scratched a parable knows very well that this “small” value represents hours and hours of work with pitch , cerium oxide and elbow grease ! 
I have already started the study, I will not follow the classic and very used processing at 70% ( deepening of the center and of the edge, leaving the area al 70% the diameter ), although this will involve more excavation ( in a smaller area though ).
Digging the edge frankly worries me, with these radii of curvature so pushed I saw that to find a nice riveted edge, ( then difficult and challenging to correct in a F2.2) even a single session in which the longitudinal overflow of the tool during the strokes is greater than the transverse one may suffice.
The excavation will be done all in the center, the edge will be treated as little as necessary.
I have not yet taken pictures of the small deformation achieved so far, therefore I publish the immediately preceding ones, when the sphere was now corrected by any astigmatism even if it still needed a better connection between the worked areas, as can be seen from the bands of the Ronchi slightly “rough” and not really straight. It must also be said that the smartphone is generally not the best for taking photographs of this kind, they succeed best with a very simple webcam which, however optically “low” and economical, it allows a better management of exposure and contrast with dedicated controls, much better than the automatisms of a mobile phone. See you soon with the first images of the mirror on the way to the parable. 
Heilaaaaaaa what a nice job that is coming out
Very Massimo
I am in complete agreement with the method of proceeding you have chosen for parabolization. On such a thin and fast mirror, I too would have chosen to work towards the center.
A big good luck, for the continuation.
I stay tuned
Bravo Massimo! I agree with the compliments and the excellent idea of parabolising starting from the center, despite the 700 thousand nanometers to be removed on a smaller area with a more peaceful processing.
Thanks Giulio
even if you scared me you missed an extra zero ! fortunately the nanometers to be removed are “solo” 70.000
However, a few hundred nanometers ( perhaps 1000
) I have certainly dug them and you are starting to see some timid signs of deepening, therefore there will remain yes and no others 69.000 
, nanometer more, nanometer less…
the beautiful ( so to speak ) is that the Romchi test can be used for a little longer, only in these early stages still close to the sphere. The focal ratio is so high that as soon as the conic constant begins to assume values lower than -0.3 ( or so ) the test itself will become almost unreadable.
Already, I'm sorry…I calculated the tenths which instead are hundredths!!
On Ronchi you catch me unprepared but curious:
I need you to explain to me the reason for Ronchi's next useful illegibility, as the parable deepens.
Tell me if and where am I wrong, but unprepared I would seem to guess that the lines with which Ronchi shows the sphere, gradually becoming the curves of the parabola being excavated, they might show up (maybe maybe) with local accentuations, for the correction of which it becomes difficult to estimate the quantities of work?
Thank you.
Anyway congratulations!
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