THE BLANK AT THE MENISCUS
The blank was made from a calcium-sodium glass disc from 610 mm in diameter and 20 mm thick that has been placed in an oven, brought to softening temperature , collapsed by gravity into a pre-built mold with the design spherical curvature and cooled in a controlled manner to avoid internal tensions.
The meniscus was then ready to be worked thanks to the use of the rotating table specially built to support the convex shape of the back.
An initial check of the shape revealed that the meniscus was far from having a spherical surface, the geometric shape which presented evident astigmatisms and an average error compared to the spherical figure of 1 mm circa.
Using a glass utensil 400 mm diametro, with abrasives and strokes concentrated on the midline area, it allowed the two surfaces in contact to conform to the spherical one.
During this process, the "lower" areas emerged compared to the sphere that was being generated, which remained shiny and not affected by the tool/abrasive action, until they are completely "absorbed"
THE TOOLS
The surface of the blank has been uniformed and the sequence of abrasives up to the grain has been completed 800, it was possible to proceed with the polishing phase , creating sub-diameter tools for polishing
The pitch patina was created in "reverse" mode, i.e. by directly pouring the molten mixture onto the mirror in which aluminum sheets had been positioned to avoid the meniscus from sticking to the glass. Once cooled, the patina was glued to the tool using a new layer of molten pitch.
This procedure only to avoid holding the mirror horizontally above the molten mixture to give the shape to the tool.
However, completion of polishing revealed residual astigmatism which needed to be corrected before moving on to parabolization.
THE UNEXPECTED PROBLEMS
Corrected the astigmatism and reached the sphere ( Final ROC 3560 mm , effective diameter 604 mm, F 2.95 ), The manual parabolization phase has begun on a rotary table , which immediately highlighted processing anomalies with in-depth techniques starting from the center ( w-run or center-to-center )
The in-depth analysis indeed, after a few initial sessions, it was not generated with the regularity of a conical surface , but it showed a more significant "digging" action in the median area compared to the center and edge.
This prevented us from continuing to generate depth in the center on a regular basis, as the continued application of "normal" techniques brought the figure back towards the sphere/ellipse, distancing it from the parabolic shape and with more marked defects at the edge.
The cause of these anomalies is to be found in the "cell" support which does not allow continuous support of the glass but only in eighteen points. In fact, this type of support is specifically designed to distribute the weight of the mirror in an optimal way and limit deformations due to the weight to a minimum..
But this feature shows its benefits in mirror operating conditions ( mounted on his telescope ), It is different in conditions of processing on a moving support and with the exertion of pressure on the glass by the tools.
The pressure applied during the tool strokes, even if you try to apply it as consistently as possible, it does not receive a homogeneous reaction from the entire surface of the mirror but will be greater in the support points and less in the center and at the edge where the support does not exert any direct support action.
THE SOLUTIONS AND TECHNIQUES
Therefore it was necessary to change the approach to the construction of the parable.
In practice the mirror was worked as if an error had to be constantly corrected , in this case excessive excavation in the median area.
I therefore chose to change the construction of the parabola with the al setting 70% the diameter, in order to have two "macro-zones" to work separately, limiting the action on the median-peripheral area to a minimum , where the defect was generated.
Two more sub-diameter tools were then built , one 20% to work the edge and one to the 35% for the remaining central area, using the techniques summarized in this diagram:
- Strokes from the small extension on the mirror edge with localized pressure on the tool edge, to delve deeper towards the edge
- “Epicyclical” races on the peripheral area. The tool center describes a circular trajectory as the mirror rotates, with light pressure applied to the center, “stretch” the curvature rays of the peripheral area.
- W-shaped strokes with light pressure in the center to "dig" the central part without going beyond the 70% the diameter.
The W-shaped runs were used as connecting runs ,only for short sessions and without putting any pressure on the tool, only push and pull at the edges.
The ratio between the number of strokes of the various types determined the overall action on the figure. In general, an equal number of strokes in the center combined with the same number of strokes at the edge were used.
For example in a session , every 10 complete turns of the mirror with type runs 3 were carried out 5 complete laps with type racing 1 e 5 laps with type racing 2.
This type of processing does not lead to a stable geometric figure of the mirror , it is easy to "go beyond" the parable and arrive at a hyperbole. However, it was possible to intervene and correct the figure simply by changing the relationship between the runs in the center and those at the edge depending on whether you want to "go back" or continue.
TEST FOR THE FINAL PARABLE
The Ronchi Test was used throughout the entire process, while the Caustic test, tests we talked about in detail in this article was performed in the final part, especially during the final touches to achieve and verify the final parabolic shape. These are the final reports:
A sequence of final images at the Ronchi Test ( in higer place ), from intra to extra-focal starting from the left, compare with those ( in lower place) generated with the software.
