Primary mirror meniscus 600 F2.2

[Massimo Marconi]

Right observation Giulio
However, the asymmetry that can be seen does not depend on the surface of the glass, indeed, however, turning the mirror does not change the figure.
The causes are to be found in the “approximation” part tester, in the lattice “long” and by the pushed focal ratio.

1- the grid is not perfectly centered and the lines have a width “media” of 0,125 mm (lattice made with office laser printer on tracing paper). A test showing hundreds of nanometers will also be affected by small lattice asymmetries, since the opposite bands to the mirror center are not guaranteed to be in a perfectly symmetrical position.

2- the camera in this setup, it was on the side of the source and not above it as is usually the case. The distance was a couple of centimeters, therefore the camera was one centimeter from the optical axis. In an F2.2 this distance is enough to create an asymmetry with the Ronchi.

as we have always maintained, the Ronchi and its setup are not adequate for nanometric checks on the surface, other tools are needed.
In this case, the overview is sufficient to verify the good general progress of the processing, while for the conic control limited to Ronchi, we can focus only on a band in the middle position and compare it with the software simulation, but it will always remain an evaluation linked to the observer's impression.

[Giulio TiberinI]

An average grid line width of 0.125mm creates a lattice with four lines per millimeter, and it is already a good result for a DIY lattice.

Two years ago I ordered some from 20 lines per inch at the US publishing house Willmann Bell, but they returned the money with the communication that they no longer sent it, causes frequent loss by postal services….A stupid excuse like the lady who owns the company. But today I understand that the cause could well have been the one that made her close the business.

Even I used a double frequency grating, I have no idea it wouldn't make your job appreciably better

[Massimo Marconi]

Little time to devote to scratching , and the work proceeds in very small steps. However, we are at the point of necessarily having to start taking measurements. We are just beyond the conic constant of K=-0.7, therefore quite close to the parable. In the Ronchi images you can see how the peripheral part remains high compared to the center, therefore, measures are needed to return the extent of this anomaly to the parabolic shape before it becomes difficult to correct.

I had already noted the difficulty of processing the edge / periphery, which seems to "resist" the excavation much better than the center, I highlighted it a few posts ago ... what's new is that the tool gives 21 cm used so far, began to be hardly usable by the conic constant of K-0.5 on.

From that moment in fact, the in-depth races no longer gave the desired result, anomalies on the parabolic shape began to be more and more present. The difference between the radii of curvature between the edge and the center began to be too high even for an al tool 30% the diameter. Up to that point he had done very well, but then I had to replace it with an even smaller sub-diameter. At the moment I am working with tool from 125 mm, which seems to "hold up" the adaptation both in the center and in the periphery, but everything suggests that the final touches will be done with even less tools or in any case, with different tools, each of which is suitable for a specific area.

[skround]

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?

[Massimo Marconi]

Hi Michele thanks ! ,
strokes with sub-diameter are always the same, ring roads along a circular sector for the local modification of the ROC , to W for the maintenance / restoration of the parabolic shape, to Y for the deepening of the center and other unlikely “customized techniques” works , sometimes ( fortunately few ) we invent ourselves driven by the desire to "experiment" new solutions, however, in most cases, they do more damage than the problem, therefore I will avoid describing them

For the time spent I have no idea, I was unable to work the mirror more than a few hours a week during this period and I dedicated a part to the problem of the tool, trying to understand what are the limits of adaptation for a given sub-diameter as a function of the different radii of curvature of the mirror and the techniques used, in practice, how much the patina can be deformed to maintain constant contact with the glass in the different areas.

So I think the question can be answered with another reasoning:
-With a good half hour session with cerium oxide, if done right, you can at least dig 200 glass nanometers in the mirror center ( not on the whole mirror obviously ).
– For a 600 / F2.2 the switch from -0,35 until -0,7 in the value of K it foresees that about 2.5 hundredths of a mm of glass in the center, that is 25000 nanometers high..
-then, in theory 62.5 hour ( 125 sessions ) should be enough , therefore I can understand that in the last two months I have dedicated at least 7.8 hour ( minute more minute less ) per week to scratch.

[skround]

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!

[Massimo Marconi]

It could be done, maybe not quite with footage of the sessions that, Sunglasses, I think they are uninteresting, perhaps better with the execution of some tests. in fact, some time ago Mirco and Giulio had started with the publication of videos also on our Youtube channel.

Manual processing is the only one that allows you to manage such high focal ratios, I don't see how it is possible to machine an F2 unless robotic technologies are used that are beyond the reach of mere mortals, which in any case reproduce manual processing and the intervention of a real-time programmer who knows how to intervene in the right way at the right time is always necessary.
the deformations and therefore the variation of the local ROC of a parabola / hyperbola in these focal lengths assume values ​​that are too high compared to the required tolerance of 65 nanometers high., beyond the possibilities of any machine understood in the artisan sense of the term, which however remain very useful in the roughing phase, polishing and sphere.

Regarding the works of Tom O , with all respect and admiration for anyone working on these exciting projects, I find it more interested in the constructive and mechanical part of the project than in the optical part, which is instead the one that interests me personally most and often, not to say always, I don't agree with the choices “US” to finalize these fast mirrors with the Ronchi or ultra-short focal lengths with Foucault, I believe that it will be difficult to have good results, but maybe I'm wrong and I'd like someone to prove that the proper and well-known limits of these tests are actually “circumventable” with some particular strategy unknown to scholars and researchers of applied optics all over the world.

Then it happens that the telescope is beautiful, built with ingenious and innovative construction solutions, it is carried in a suitcase and assembled in 5 minutes, holds collimation for years, the alumination is perfect and the optics go up to temperature in 30 seconds… pity that, however, you see 'na ciofeca ! At that point how to blame the supporters of the “these mirrors cannot be done” ?

[skround]

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?

[Massimo Marconi]

Hi Michele, any measure is valid if, as you know better than I., the limits and tolerances of the measuring instrument used are clear, therefore any test is "good" when evaluated for what it can actually measure. Each test has its strengths and weaknesses , knowing them and applying them correctly is in my opinion a good strategy, comparing them with the results of other types of tests is even better ...

All that said, , it is not in Grattavetro's philosophy to judge the work of people worthy of esteem and admiration like Tom, in the absence of a cross-examination.
We certainly have different opinions, as is normal in life, personally I have already expressed my doubts in using the Ronchi on short focal lengths and more generally for the finalization of a reflection lens therefore, judging with the risk of exploiting a publication of someone else's work to give value to my thesis seems to me quite incorrect, especially if the environmental and experimental conditions of the test itself are not known.

We at Grattavetro have already "clashed" on this issue with the overseas grattavetro on the pages of Cloudy_Nights ( represented by the good Lockwood ) while for us there was ( Luckily) Giulio, which thanks to his preparation and command of the language, worthily represented the thoughts of all of us who write on this blog ...
We have been called into question, after the publication of some of our articles which are therefore also read by American gurus, we certainly didn't go looking for them and a very heated and interesting debate ensued… and even if, after a fair "battle" it ended in "tarallucci and wine", each remained almost on their own positions.

So sorry Michele but… how is Tom's mirror going, you should ask him, not to me ... if one day Tom will happen on these pages and is interested ( for some strange reason ) to my opinion on his mirror, I'll be happy to give it to you.

[Massimo Marconi]

After some time of forced pause, finally I was able to resume processing, it was time !
The mirror is now parabolic, that is, the conic constant K = -1 has been reached. At this point the road that separates from reaching the project hyperbola is really minimal ( so to speak )

It would also seem like a good parable and it's almost a shame to have to “mess up”. Clearly any qualitative judgment cannot be made with certainty with the Ronchi che, as we know, provides an overview of the surface condition also in relation to defects such as roughness, astigmatism and zonal errors but, nothing tells us about the size of any defect found, as nothing can tell us about the actual accuracy of the figure.

Moreover, with the achievement of the parable, the deformation with respect to the starting sphere has reached and exceeded the value of 6 hundredths of a millimeter in the center of the mirror. Dig this amount of glass with cerium oxide and manually, I do not wish it to anyone :-), but apart from physical fatigue, other problems have appeared, precisely in relation to the use of the Ronchi with such a high focal length:

– the caustic of reflection for a 600 Parabolic f2.2 assumes values ​​of several mm on the focal plane. This means that at certain distances, in which the central areas will be in extra-focal while the peripheral ones still in intra-focal, the size of the light spot is greater than most of the photographic lenses of the webcams and mobile phones with which I had so far taken the images of Ronchi's tests. In particular, it is not possible with these devices to see the mirror area fully illuminated by the source.

In “visual”, that is, looking through the lattice, you can run the test normally, the mirror is uniformly illuminated from intra to extra-focal of all areas, but with my usual webcam I can only visualize correctly only near the Roc in the central area.

So I tried with a compact camera, with which the situation has improved significantly, but I think you will need a reflex to perform the test normally as in visual.

In the video you can see what is described, ie the race “helpful” offset with fully illuminated mirror. I also had to “improvise” a tester assembling all the junk available in the tavern to have a functional pseudo-tester with a compact camera

in the following image, the best you can do with this camera: we are near the fire in the central area, you cannot go beyond without starting to see the areas “blind” not illuminated on the surface, just like at the beginning of the video.
Is’ however sufficient to make a comparison with the image generated by the software, enough to understand where we are with the processing.

See you soon with the attempt “Reflex” to navigate more effectively between the centers of curvature of the various areas.
Clearly from now on it will be mandatory to make direct measurements on caustic, I therefore hope to be able to publish the first quantitative reports on the mirror soon.

[Giulio TiberinI]

Beautiful figure, and this already seems to me an exceptional visualization! Congratulations Massimo!

[Bartolomei Mirco]

Wow Massimo, beautiful picture…The surface looks nice smooth and even complimentoniiiii, you're doing a great job

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