Mirror grinding

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As a mirror grinding by hand or machine - - making arched precise will mirror designated as especially in astronomy for reflecting telescopes are used. The cut of smaller mirrors is a popular hobby of some amateur astronomers .

In addition to glass , glass ceramic - above all Zerodur from Schott - is used as mirror material today . Telescope mirrors made of metal were ground until around 1900 . This was done early on because large glass blanks could not be cast homogeneously enough. Modern types of glass, and later types of glass ceramic, have a very low coefficient of expansion. After casting, the mirror blanks must be cooled down very slowly (for up to several months in the case of large specimens) in order to achieve homogeneity and freedom from mechanical stresses in the glass.

Mirror shapes and material

The surface shape of astronomical mirrors is either spherical or parabolic and must be at least 10% of the light wavelength, i.e. H. accurate to at least 0.05 micrometers. That sounds difficult, but for a spherical mirror 20 cm in diameter it can be done in 1 to 2 days. A higher image quality is achieved with parabolic mirrors , which requires about twice the working time and special test equipment.

The working technique is basically simple, but requires harmonious movements and high concentration at the beginning. You start with two round, flat cast glass panes of the desired mirror size, of which the thinner ( blank ) becomes the mirror and the thicker one serves as a counterpart (grinding tool or "tool"). The counterpart can also be made of granite or even cut round from a pavement slab .

Coarse and fine sanding

First you have to "bevel" the blank of the mirror with a grindstone , otherwise the edges will break off during grinding; 1 to 2 mm bevel is sufficient. Then the necessary recess is made in the blank with coarse silicon carbide ( carborundum or "Karbo", best grain size 80). The sanding itself is done with swaying long strokes with a large side overhang. Extensive circular or oval movements, in which the blank always "keeps moving", are even more effective. The height of the arrow (recess opposite the edge), which results from the desired curvature or focal length of the mirror, is checked again and again .

manual grinding of a 30 cm mirror

Further processing takes place with increasingly finer carbo-water pulp. As a result of this grinding material introduced between the disks and the circular-rotating rubbing, the blank gradually assumes the spherical shape and becomes a (concave) concave mirror , the counterpart to a convexly curved disk.

In the course of time finer and finer grinding powder is used, while the last work steps , the actual polishing, take place with polishing red or cerium oxide and the shape of the cut can be checked by simply examining the reflected image.

Shape checking and parabolization

The spherical shape is usually checked with the Foucault knife edge test , which checks the quality of the reflection in the center of the sphere. Parabolization can then begin using typical test images or a measuring probe and PC program.

With a parabolic mirror , the curvature at the edge of the mirror is weaker than with a spherical mirror, so that parallel rays of light meet exactly at the focal point. The shaping takes place as above, but under constant visual control.

There is increasing literature on test methods. In addition to the knife-edge method, the mirror quality is also often checked with the Ronchi grid or the Lyot test .

Grinding of large lenses

The manufacture of optical lenses is carried out in a similar manner, but requires a great deal of experience. Amateurs can only grind objective lenses that are large enough for processing. The manufacture of eyepieces is reserved for optical workshops.

The necessary grinding accuracy is less than with mirrors, because the resulting errors in the optical path length with refraction are smaller than those with reflection (at least for n <3 ).

Refractive
index n 		Optical path difference
Absolutely about the
mechanical error 		Relative to the
error resulting from reflection
1.4 0−40% −1/5
1.5 0−50% −1/4
1.67 0−67% −1/3
2 −100% −1/2
3 −200% −1/1
mirror + 200% +1

For this purpose, the processing of lenses and their polishing must be carried out so far that the surfaces become completely transparent.

See also

Literature and web links

Web links for mirror testing

Individual evidence

  1. ↑ Test methods, polishing and correction of mirror grinding  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , www.artastro.de@1@ 2Template: Dead Link / www.artastro.de