OVERVIEW OF ALL WORK SESSIONS
The mirror processing was carried out with a mirror-o-matic type machine, characterized by a rotating table and an oscillating arm on which the tool is fixed. The latter can be positioned at different points along the arm, allowing you to vary the offset with respect to the center of the mirror and therefore modulate the polishing action. To improve the stability of the support, a disk of soft material was applied to the rotating table, useful for absorbing any irregularities in the support surface and reducing the probability of generating astigmatism during the processing phases. To this measure, always to limit the onset of astigmatism, random rotation of the mirror every 10–15 minutes was added, a simple but effective practice to uniformly distribute pressures and prevent unwanted deformations of the optical surface.
The rotary table rotates at approx 8 rpm and the arm swings approx 22 times per minute.
Below is the graph with the trend of aberrations as work sessions vary.
In particular, various macro-phases can be distinguished, which will be described in detail in subsequent chapters:
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- At first, where a very small tool was used (30%) with which some techniques were tested.
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- A second one, with tool al 75% the diameter of the mirror, in which I began to standardize the optical surface and identify the "neutral" position of the tool, Able to work evenly and generate the ball naturally.
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- A third, a little experimental, which allowed me to eliminate the main aberrations (astigmatism and trefoil).
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- And the last one, always with a tool 75%, for the last phase of surface uniformity and elimination of spherical residues.
Figure 6. Graph of the trend of aberrations during the work sessions
WORK SESSION #3
In the work session 3 a targeted technique was adopted for the reduction of primary spherical aberration through the use of a sub-diameter tool equal to 30% the diameter of the mirror. The approach was based on controlled tangential runs, designed to preferentially insist on the “hump” of spherical aberration, maximizing the abrasive action in the region where the deviation from the ideal profile was most evident. In reality, in this specific work session, the main objective was to understand if I was able to predict the outcome of a work session, note the machine settings. One process or another would have been indifferent given the objective of the session, but while I was at it I decided to set the camera movements in such a way as to at least reduce the amount of spherical aberration.
Figure 7. Work session described (O:30%)
The machining zone was defined between the radius 75 mm and the radius 195 mm, with maximum excavation expected around 135 mm. Since a tool usually has a real effectiveness limited to 65% of its operational radius, it was necessary to carefully calibrate the position of the oscillations. The area corresponding to 60-65% of the tool radius was therefore made to coincide with i 75 mm from the center of the mirror in the innermost position, while at the maximum excursion of the working arm this useful region must have been at approximately 195 mm.
Figure 8. Machine parameter settings
Below is the image of the error profile with respect to the sphere present in the session 2, with superimposed lines indicating the set work area. The dashed green lines indicate the edges of the tool, the orange lines the area al 65% of the tool radius and the red line, the expected area of greatest work.
This setting allowed the volume of material to be removed along the radial profile to be precisely defined, getting a dig that, as shown in the map below, it developed exactly in the expected range. The image below represents the difference between the session measurement 3 and the measurement made in session 2, that is, it exactly represents the volume of glass removed during the work session..
Figure 11 images of the glass removed during the work session #3.
In addition to confirming the consistency between prediction and result obtained, the experiment also provided a valid estimate of the tool removal capacity, quantifying the removal in terms of nm/min (8 nm/min with 2kg weight on the tool), fundamental data for planning subsequent processes.
This particular machine setting is not ideal for continuing processing as it tends to dig in a very specific way around the area 135mm from the mirror axis, and not to work ideally on the entire surface. Moreover, the small size of the tool (30%) did not manage to improve the astigmatism in the slightest, nor the clover, nor the other aberrations. To improve the entire surface and continue towards a good sphere I decided to continue with a larger tool 75%.
WORK SESSION WITH TOOL 75%
Some considerations on the work sessions carried out with the tool of dimensions equal to 75% the diameter of the mirror.
In all these sessions I worked in an almost "fixed post" manner, that is, with the tool practically stationary in a given position and only left free to rotate.. I mean as if fixed after, because in reality I always leave the oscillating arm of the machine a stroke of around 25/30mm, this is to prevent the tool edge from always passing over the same area of the mirror, creating grooves. I also varied the radial offset of the tool with respect to the mirror center every time 10/15 min approximately in a range between 0 e 20 mm also to avoid zonal errors.
Figure 12. Machine parameter settings with al. tool 75%
The tool protruded from the edge of the mirror for circ 18/20% Returning to the intriguing rotating table and the fixed post that would make it a useful machine.
This position (also suggested by the PolSim program and verified experimentally) allows you to work the entire surface uniformly.
Figure 13. Polishing simulation using PolSim software
Below is the graph of the progress of the main aberrations during the work sessions. As can be seen, the extent of astigmatism did not vary during the work sessions, while the clover showed a very slight but constant improvement. The spherical also shows a trend of improvement, except in those sessions highlighted by the circle, during which I varied the edge of the tool to identify the "neutral" position, that is, that position of the tool, which uniformly hollows out the mirror, directing it naturally towards the sphere. During all these sessions a constant improvement of tertiary spherical is also noted, coma and secondary astigmatism.
REDUCTION OF ASTIGMATISM AND CLOVER
Despite the use of a tool equal to 75% the diameter of the mirror, mounted in fixed post with a decentralization designed to obtain neutral machining, the astigmatism present on the surface showed no signs of improving, while the clover showed only minimal evolution. After numerous attempts, I decided to exploit a usually unwanted phenomenon to my advantage: the spontaneous onset of astigmatism when the mirror does not rest uniformly on the rotating surface of the machine. So I thought, to artificially introduce non-uniform supports (with shims) to force the appearance of an astigmatism opposite to the existing one.
I performed many tests using them as shims, rubbers (large and small) plastic splints (long, wide, ends, court), but I obtained the best result with crescent-shaped rubber shims, positioned near the edge of the mirror and in correspondence with the peaks of astigmatism.
This configuration induces a slight deformation of the mirror, raising precisely the "high" areas of astigmatism, forcing the tool to work intensively on those areas. Below is an example of how the two crescent-shaped thicknesses were arranged before a work session and the consequent excavation on the mirror after the session. It is clear that the mainly corrected aberration was astigmatism.
Figure 15. On the left the rubber pads used for the Astig. On the right, image of the most excavated areas.
For the clover I took a similar approach, but instead of two thicknesses, I used three smaller rubbers, placing them in the points corresponding to the "high" areas of aberration. In case astigmatism and trefoil were present simultaneously, the three supports were arranged following the peaks of the combined aberrations.
Below is an example of how the three thicknesses were arranged before a work session and the consequent excavation on the mirror after the session. In this case mainly trefoil and astigmatism were corrected. Astigmatism was also corrected and not just exclusively clover, because the supports were not arranged on an equilateral triangle, but deliberately along an elongated triangle aligned precisely along the direction of the astigmatism. In this way I managed to reduce both aberrations.
Figure 16. On the left the rubber pads used for the Trif. On the right, image of the most excavated areas.
The processing was always carried out with a tool 75% of the diameter in fixed post with neutral decentralization, so as to maintain predictable excavation action.
Iteration after iteration, this method allowed me to progressively reduce both astigmatism and trefoil, while in parallel I eliminated spherical aberration, approaching in a stable and continuous way the desired spherical shape.
Under the trend of aberrations as the work sessions continue. It is evident as from the moment I started placing the shims under the mirror (from session #13 to the session #25), astigmatism and trefoil aberrations began to decrease significantly to almost nothing.
LAST SESSIONS AND REACHING THE SPHERE
The last processing sessions were dedicated to the uniformity of the optical surface and the complete elimination of spherical residues.
During these more delicate phases, the mirror was periodically rotated on its support surface to keep the main aberrations under control: astigmatism, coma and the characteristic cloverleaf deformation that can emerge when the pressures are not perfectly symmetrical.
Below is an image of the Ronchi a test 4 l/mm: the presence of straight and regular lines testifies to the achievement of a well-corrected sphere, uniform to the edge.
At the same time, numerous interferograms were acquired, one of which shown below. Also in this case the linearity of the fringes is a clear indicator of the good quality of the optical figure.
Dozens of interferograms were acquired, also rotating the mirror in order to eliminate the astigmatisms of the measurement set-up.
The average of many interferograms acquired, shown below, provides a reliable and stable evaluation of the final form achieved.
Now that a good sphere has been reached, it is possible to move on to the parabolization phase.