PARABOLIZATION
After bringing the mirror to the sphere, the moment of parabolization has arrived, which for many is perhaps the most delicate phase of the entire process.
In this specific case, a machine configuration was adopted designed to preserve the edge of the mirror as much as possible, which in the initial spherical surface was already clean and free of significant defects. For this reason we chose to proceed with a method that favors gradualness, even at the cost of longer processing times.
The new machine setting:
To start, I mounted the tool on the 50% the diameter of the mirror.
The races are center to center, e The tool edge was set to protrude by an average of 18% the tool diameter, beyond the edge of the mirror. I say "average" because the overflow is not identical on both sides: on one side it is a little larger than the other. This small asymmetry serves to prevent the machine from always repeating the exact same path, and helps to better blend the excavation in the area near the edge of the mirror.
Figure 22. Machine settings for the parabolization phase
Every 10–15 minutes of work I did two things:
– I rotated the mirror slightly with respect to the rotating plane.
– I moved the tool attachment along the arm of the machine, varying the distance between 0 e 50 mm. A 0 mm the tool worked longer, while in 50 mm the work session time was shorter (example 10 min a 0mm, 3 min a 50mm). Practically, it is a way to blur the action of the tool edge in the central area, avoiding excavations that are too clear.
Compared to the classic technique which keeps the curvature constant a 0,707 D, here more material is removed in the central part and more patience is required. However, the advantage is to better control the progress of the work and above all the risk of ruining the edge is reduced to a minimum.
And anyone who has already had to deal with a clinched or raised edge knows how difficult it is to then bring everything back into tolerance.
All the tools used during the parabolization phase had a "star" shape, that is, I had a sort of serrated edge imprinted on them, of extension equal to 15% of the tool diameter to have an even more nuanced effect on the machined area and minimize the grooves left by the tool edge. To create the "star" shape I simply pressed the pitch tool onto some nice thick cardboard, showing the outline of the areas to be removed. Below is an example
Figure 23. Sequence of operations to impress the star shape on the pitch tool
PROGRESSION OF WORK SESSIONS
The progression of the parabolization shows a regular evolution towards the optimal figure. Session after session, the analysis of the error maps highlights a clear reduction in spherical aberration, which represents the main objective of this processing phase. The convergence curve appears stable, a sign that the processing parameters have been set correctly.
Other aberrations such as coma, astigmatism and local irregularities, remain constantly close to zero, a behavior that demonstrates excellent adaptation of the pitch and uniform contact during the work session. Placing the mirror on the rotating surface proves to be effective, avoiding microflexions that could have introduced unwanted defects.
Figure 24. Graph of the trend of aberrations during the parabolization sessions
The evaluation of the private error profiles of the spherical component allows the occurrence of secondary defects to be observed with greater sensitivity, often too faint to emerge clearly in full maps, but still potentially dangerous if allowed to accumulate. The trend highlights an excavation that proceeds regularly from the edge towards the center, confirming the good direction of the work. However, the central area tended to remain slightly flat, generating a small hill that, session after session, became more pronounced.
Figure 25. Private error profiles of the spherical component
Another recurring element was the appearance of a raised ring at approximately 0.5D, of modest entity but nevertheless carefully monitored. These two defects, although limited, they require targeted interventions to prevent them from amplifying over time.
To keep the surface under control, every 5–6 sessions it was necessary to intervene on the central hill using a tool equal to 30% of diameter and short strokes CoC, for a few minutes, generally no more than 8–10. This periodic correction prevented the central zone from rising too much and kept the progression of the excavation consistent with the desired parabolic shape.
The raised ring, instead, it was occasionally corrected with tangential runs localized at the hump. However, its size did not show a tendency to grow, remaining always well contained.
Overall, the analysis of the error profiles confirmed that the processing proceeded in a controlled and predictable manner, with easily manageable minor defects and no significant drift from the expected optical geometry.
FINISHING TOUCHES AND FINAL MEASUREMENTS
The final phase of parabolizing the mirror required more controlled work, with shorter work sessions and more frequent measurements. Even the last sessions were carried out entirely with al tools 50% and ran CoC, until a conic constant close to the ideal value is reached. The latest measurements have in fact indicated a constant of −0.993, practically coinciding with the theoretical parabola.
Achieved this goal, an in-depth measurement campaign began to verify the real quality of the surface. Ronchi tests and a series of interferometric measurements were performed, acquiring around twenty interferograms for each of the four orientations of the mirror (0°, 90°, 180° e 270°). This strategy allowed us to average the results and minimize systematic errors.
Below are a couple of images of the Ronchi test in intrafocal with a grating 4 l/mm.
Figure 26. Ronchi intrafocal test of the parabolic surface
Below are two examples of interferograms, one taken with a mirror at 0° and one with a mirror rotated 90°.
Figure 27. Interferogrammi (0° e 90°) of the parabolic surface
The resulting analysis highlighted a mirror of excellent quality, with 21 nm RMS of SFE and a Strehl ratio equal to 0.93, therefore well beyond the diffraction limit. Below is the map of the final measurement.
Figure 28. Aberration map of the parabolic surface obtained
And below is also the graph of’ MTF, PSF and the star test simulation.
Figure 29. MTF, PSF and star test simulation of the parabolic surface obtained
To validate the result, the measurement was then repeated using Nanomefos 1000, that is, a professional interferometric machine, which confirmed the validity of the measurements. Below is the comparison of the maps obtained with the Bath interferometer and the Nanomefos 1000. The similarity of the two maps and the quantitative estimate of the total aberration are surprising, 21-22 nm with the Bath and 21.75nm with the Nanomefos 1000 (truly incredible!!!)
Figure 30. Comparison between results obtained with the Bath interferometer and Nanomefos 1000
Further improvement would be theoretically possible, but neither justified nor necessary, especially considering the intended visual use. Furthermore, the central hill is completely obscured by the secondary mirror. For this reason I can consider the work completed with full satisfaction, and the mirror is ready for the final phase: the aluminization, which will make it operational and ready for the sky.