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Well, after a few months of forced break I resumed working on the meniscus, who during polishing suffered from a conspicuous astigmatism, ( moreover underestimated by me ) and that took quite a while’ of work to get there ( quasi ) a capo.
But let's go in order and allow us to introduce you to the Ronchi-Telefono
essential accessory to perform tests in any place and condition, of the kind… you are at a wedding and you feel the urgent need to test a mirror ? 
no problem! with the Ronchi-Telephone you can easily do it e, finished the test, continue taking photos and videos of the newlyweds 
Returning to the mirror and his astigmatism, this is what it looked like during polishing.
The video highlights the arrangements of the two radii of curvature that determined the asymmetry of the figure, (with “R+” the shortest bend radius). The mirror was therefore placed in the position of maximum deformation , with the spokes at 45 ° with respect to the slit, because with the ray “R+” or “R-” parallel to the lattice-slit the resulting figure at Ronchi became regular in both cases.
Beyond astigmatism, different local irregularities that the step of grinding with tool al 50% she had evidently failed to standardize.
Now the mirror is on the mend, still convalescent with a small residue of peripheral astigmatism but which should conform without great difficulty. Is’ The technique described by Jean Marc Le Cleire in the extract excellently translated into Italian by Giulio was applied, which you can find some posts above. Some variations to this methodology were necessary to adapt it to the specific case, which in some respects was different from that treated by Le Cleire.
The reticle is hand held, therefore you will forgive the lack of fluidity and stability of the movement ( the ronchi-telephone is still experimental
) but it is important to see the action approaching the fire to verify that there are no changes in the inclination of the bands, at least in the central area, always in the position of maximum deformation.It can be seen in fact that at the edge of the radius R +, there is still an area with a smaller radius of curvature ( which expands and focuses first ) than the longest radius.
After this video, two more sessions were done with simple W strokes with a 21 cm, given that at this point it is not advisable to intervene on the entire quadrant, as the deformation is contained on a limited portion of the surface and not along the entire radius, as you can see well in the next video when only two to three bands are visible.
The defect is getting smaller, even if there is a consequent and foreseeable deepening of the center which must therefore be standardized.I believe that within a few sessions it will be possible to reach a uniform and regular sphere and begin parabolization.
To test and optimize you intend to make use of the right W strokes?
I was referring to the adaptation of the patina, a couple of minutes with good pressure or weight on the tool at each session before starting the runs.
Hi Carlo and welcome !
A 300 I think F5 is one of the best choices to try your hand at self-building a mirror, because it is quite complex, enough to experiment with hand most of the constructive aspects of a reflection optics, without ever jeopardizing the success of the project, which remains within the reach of even those approaching these creations for the first time.
On the machines, on the other hand, I don't really agree, my opinion is that the machines for the construction of optics are a point of arrival and not a starting point..
I mean, there are so many variables to keep under control during processing that, without adequate manual experience ( with fewer variables) , it would not be possible to determine if any manufacturing errors are due to the technique, from the strategy conceived, or from the machine not working properly.
But I repeat, it's just my opinion and in the field of application of mirror-machines both Giulio and Mirco could be much more precise than me!at this point I would aim straight for the sphere, with mirror below, Corsica 1/3 COC and slight right-left overflow ( three-four cm ) until regularization, always with the foresight to verify and optimize the adaptation to each session.
For the Ronchi try to decrease the intensity of the LED with a narrower slot, or to act on the intensity adjustments, exposure and contrast of the camera / webcam you are using.Hi Alberto and welcome again !
I moved your message by creating a new dedicated thread, I entered this title because it seems to me to be the synthesis of the speech,( but you can change it if that's not what you had in mind) and it is certainly a topic that interests most of those who are preparing to start “career” from Grattavetro.Having said that it is difficult to give a summary answer to all your questions, on each of the points you have highlighted a treatise could be written
My opinion is that it is sconsigliatissimo start a machining on those diameters, not to mention the short focal lengths, the risk , on the contrary, the greatest probability is that you will stop shortly after and abandon the project, not because you don't have the skills or wouldn't be able to do it, but because there is no training and experience that allows you to understand the simplest things and tools to gradually apply them to more complex things. With small diameters everything becomes easier and faster, you learn the same concepts and the same techniques in a much shorter time and any error is easily and quickly solved. Once done a little “school mirror” you are able to decide independently what the next step to take is, having experienced the difficulties firsthand , problems and times that, as the diameter increases, increase exponentially.
But, more than mirrors, the human soul is much more complex which often leads us to initiate “mission impossible” for the simple taste of challenges to the limit of logic e, when we get something in our head, all the tips and advice of caution are of little use.
To give a very quick and not in-depth opinion to your questions;1A – may suffice with an adequate support cell for the mirror.
1B – the sphere for the grattavetro, the conic that is being designed for optical design software. In any case, the difference is a few microns and there is no need for such precision for the focal length, unless telescopes are to be mass-produced.
1- – and2 – I do not know, I have never tried and I have not read of studies done on glued glass for problems and behavior at an optical and mechanical level.
3 – I very much hope so, because that's exactly what I'm doing right now
Thanks for the appreciation to the Blog, We are very pleased to know that what we do helps to stimulate the curiosity and interest of enthusiasts.
the distance between the mirrors in a Newtonian is arbitrary, it is chosen by the designer according to the extraction of the focal plane from the optical tube to be obtained.
In the example above, i set the value for “Is” (Optical Axis focal plane distance ) of 200 mm from the optical axis, to have an extraction of the focal plane of 50 mm from a hypothetical tube of 300 mm in silence. ( maybe it's a bit’ little bit )
Keep in mind that the secondary axis must be decentralized with respect to the primary ( value “Dx” in the table) , the closer you get to the primary, the greater the offset of the secondary will be.hello Frank, this happens because Stefanosky doesn't trust Grattavetro
and then he goes to read the American sites / forums where they have an acronym for everything.
In this case it refers, eat your intention, to the position of the mirror and tool during machining, from which MOT =”Mirror On Top” or TOT =”Tool On Top”… but in my opinion, “mirror above” e “mirror under”, works just as well Atmos is a beautiful software created by Massimo Riccardi, one of the best Italian opticians ( designer of optical systems also for Officina Stellare ) and is available both in the Demo version ( free and more than enough for the needs of non-professional grattavetro ) than in the full version. The demo version is limited to the design and analysis of a system of up to three optical surfaces.
To obtain the arrow of a parabolic mirror, just simulate a newton using the preconfigured Atmos cards and obtain the value “Mirror Sagitta”
The most attentive and scrupulous will notice (some time ago Giulio noticed it immediately) that the value of the arrow differs by a few hundredths of a mm ( at fault ) than that calculated mathematically on the radius of the equivalent sphere.
This happens because Atmos calculates the value of the arrow referred not to the sphere but to the final Parabola, obtained by lowering the edges and leaving the center unchanged with respect to the sphere of origin.Ah here !
who knows why I was convinced that the tool was smaller… I'm starting to miss
However the sub-diameter will serve you, not immediately but you will need it all right !
so I should reduce the front overflow right?
If it were a tool a full diameter and, run to W “narrow”, means reducing overflow and increasing the frequency of segments “come and go”.
In the event of a smaller tool an offset must always be maintained along the entire edge, otherwise the center will work more.
Indicatively in this way:
I would also like to open a small parenthesis on the cerium oxide used in the work sessions.
Since a good part of the product ends up on the glass edge or even on the support where the glass rests due to the washout, it is counterproductive to recover it and use it again?On this I would not be able to answer you fully, Giulio or Mirco should be heard, who know life, death and miracles ( especially miracles ) on materials and their application in the optical field.
I can say that I have always avoided recycling cerium for various reasons, including possible contamination with impurities ( powders , various residues ) but also because I believe that Cerium loses its abrasive effectiveness quite quickly while it is being used, or at least that's my feeling…Bravo Luca, you are working well !
The figure is evolving exactly as it should. This gives us a first piece of information and that is that the patina is well adapted. It seems obvious but it is an important test, because when the technique ( applied correctly ) does not lead to the desired result “fault” it's just about the patina and its adaptation.Now your crafting will need to be a bit’ more “dynamics”, meaning that you will have to follow the narrowing of the central hole, consequently reducing the radius of the tool center trajectory.
-However, always stay slightly outside, do not go inside or even over the change of curvature with too much precision. At the same time, increases the extension of the stroke as necessary to keep the offset on the mirror edge constant, which must be the same as the previous runs with a wider trajectory radius.
Consequently, as you approach with the trajectory in the center, even the w strokes will have to decrease the amplitude. ( I wouldn't worry too much at the time of small roughness, they will go away slowly, always working with less pressure ) You can think that, when the hole is eliminated, the decentralized and w-shaped strokes coincide in center-to-center runs which will uniform the sphere properly.
This is actually with a full diameter tool, I our case, with smaller tool , the center-on-center races must always have some’ left / right overflow to compensate for the missing surface, in practice it ran to w with the width of the single pass very close to the next one, that is, very “narrow:.
so far the good news ( so to speak
), the bad one is that, in this way, we have further reduced the focal length of the mirror, not by much, barely noticeable, but we certainly did not feel the need, since it was already
quite complicated on its own hi Luca, Unfortunately, these are some of the problems for short focal lengths. The curvature is so pronounced that the tool struggles to maintain ( admitted that he got there) a good fit.
Consider that a contact difference of a few thousandths with pitch lasts the remde in those points ineffective.
In this case, contact is better on the central area.
During your work, The figure in the sequence of images has changed and a lot too.
The patina works more in the central area, the mirror above amplifies the excavation in the center to the point that now the center has a radius of curvature a few mm shorter than the edge.
This can be seen in the fact that while the edge in the Ronchi is still intrafocal, relatively far from the fire, the center has passed its roc and is in extra focus.solution:
since so far the center has been excavated, you have to dig the same amount from the periphery and make everything uniform:1-e’ the mirror below is preferable
2-decentralized races, along the circumference of 1/2 the mirror radius, or in any case with the tool center near the circumference that delimits the change of curvature on the mirror.
Runs with little pressure in the center and a few cm offset on the edge.
3- do not exceed the length of the strokes in the longitudinal direction ( penalty edge retorted ) to work the edge, if necessary, position the tool center near the edge and make very short strokes, three to five cm of extension, obviously with constant rotation around the mirror.
4- In this sentence, you will almost exclusively use the tool center, therefore it is not necessary to insist on adaptation, as the form will change from session to session.
5 – every two or three sessions do enough W runs “narrow” to even. then ronchi test to identify the median circumference that delimits the change in curvature, this is our center mirror trajectory for racing.
6- resume from the point 2 until uniformity is achieved.Advice: fa’ treasure of these issues because the essence of Grattavetro e ‘ this, understand mistakes and know how to correct them, but until you try them on your own skin you hardly learn anything. Who does not find himself dealing with these problems, he's not good he's just lucky.
Sure that, you too… something easier to get started no ?
There is an interesting article by Mirco about it, where the properties of various materials besides aluminum for mirrors are described and compared.
But imagine Michele, indeed thank you for your considerations regarding the project
As you say, the approach to this type of realization, which are unfortunately not supported by a defined literature and consolidated techniques, must be addressed in a way “experimental”, that is, by attempts, with all the consequent errors that anyway, they will be used to trace or exclude possible / sustainable paths. So any idea or even criticism is welcome, however, it will have the merit of providing additional elements of evaluation to arrive at the result.
In this sense, cell a 18 points was a choice dictated by the “somewhere you have to start too”, other interesting ideas had also been presented by Giulio, It would be nice to have an operational comparison between the different solutions regarding the support of the meniscus during the manufacturing phase.
This is a bit’ the “philosophy” about us Grattavetro, that we are certainly not among the most “good”, but only among the most “curious”, we like to put our nose where there is still uncertainty, aware of the industrial amount of errors that, in these cases, you risk committing. Hi Michele, I didn't write anything about the realization of the meniscus because I didn't do it. I commissioned the work for two thick glasses 20 mm with spherical meniscus ( one from 60 and the other from 80 cm in diameter ) to a company in Pisa, which creates curved glass with the mold softening technique and subsequent annealing, mainly for the nautical sector.
So I could not follow the various stages, but I could see that the finished glass is far from being spherical. ( not that I expected who knows what, even if a little’ I had hoped for it )
I mean that how “furnishing item” it looked spherical, but for the needs of us glass scratcher was so far out of the necessary tolerances to the point of requiring a complete reconstruction of the shape starting from the abrasive 80 until polishing.
The main problem was in astigmatism, the meniscus “freshly baked” it presented several radii of curvature in several sections with a difference in depth in some points of almost 0.5 mm, “Invisible” with the naked eye, but devastating for optical use.
With the sanding I managed to eliminate almost all astigmatism, currently after a partial polishing, only two perpendicular sections remain that have a different radius of curvature , residual astigmatism is what the tool gives 30 cm on the mirror from 60, together with the finer abrasive grain (800) however, he could not have corrected because the difference in depth on the surface is less than the dimensions of the granules themselves… with a full diameter tool the situation would have been resolved faster, also because the sub-diameter tool, in long runs, it adapts to changes of curvature on the surface with the result of digging even where it shouldn't.
So now, i am trying to correct this last residue of astigmatism that i estimated to generate an error around the lambda.
Thank you for your suggestion, I wasn't aware of Tom Otvos' work, I'm going to have a look
