Stages of mirror making
Making a quality mirror is a complex and delicate process that cannot be achieved only with machine work. There is need for highly skilled manual work from experts in this field. The methodology and the way to making optical mirrors have not changed significantly over the decades. The only real improvements have been in the methods for measuring the optical surface; such as laser interferometry.
We decided to share with you the stages we go through to craft our mirrors, as well as our post production tests and quality evaluation. The mirror we are showing is the one used for an order we received for a Dobson telescope with an aperture of 355mm and a f/5 focal ratio. The model is our lightweight, collapsible and mobile “Gemini” construction. The first step is to select the appropriate material for the job – in this case, a special mirror with a specific width and diameter. For the chosen diameter - 355 mm, the glass must be borosilicate with a low coefficient of thermal expansion. Because the optic surface is so precisely made, even the slightest meteorological and mechanical fluctuations can have an impact on the visual quality. If the glass is of insufficient thickness or placed inappropriately it can get deformed due to its own weight. We found that the optimal thickness, price and type of glass to use is a specially selected borosilicate glass BOROFLOAT®33 from the German company Scott with a thickness of 25mm. In order to eliminate any existing surface tensions which often cause astigmatism, the rear side of the mirror is ground to a flat surface with an abrasive to the number 320.
The first thing that needs to be done is to roughly grind out the front of the mirror until the desires radius of curvature of the spherical surface is achieved. This can be done in several different ways, one of which is done by hand, would be to use a metal disk or tube and abrasives. This way the risk of making a mistake or having an uneven surface is smaller. In addition, you reduce unnecessary thinning of the glass piece. Our mirror was grinded out to our target radius with curvature R=3500 mm using abrasives with numbers 40 and 70 which takes about 8 hours. The accuracy of the achieved spherical surface is measured and regulated by spherometer indicator.
After the rough grinding is done, the mirror was finely grinded using a special machine using different sizes of abrasive. Grinding from number 70 to number 1000 took about 6 hours on the machine. When grinding with the finer abrasives, the way we control smoothness is using the so called sharpie test (a grid is drawn on the surface of the mirror using a marker). Doing this ensures that during the grinding process of the spherical surface remains even on all sides in any profile. In order to avoid astigmatism and uneven surface it is very important to rotate the mirror frequently.
After fine grinding surface of the mirror, it must now be polished. Polishing was performed using cerium oxide and a specially designed tool which has a surface made of an optical pitch. The polishing is also done using a machine and polishing the whole mirror took us about 10 hours.
The quality of the polish is checked visually and by using a laser pointer. Quality and correctness of the spherical surface is checked and monitored with a device using the Foucault method of shadows and using Ronchi's method of optical testing. What should be observed by Ronchi's method is straight parallel lines with upright edges along the entire surface of the mirror. To avoid astigmatism, again, the mirror must be rotated frequently. Polishing ends only when the tests give excellent results.
After the polishing is done the figuring is what following. This is done to remove any final imperfections and to modify the surface curvature to achieve the shape required for a given application. This is the most complex and responsible stage of the mirror making. Producing a quality mirror with such a large diameter requires experience and knowledge, and a lot of patience and perseverance. Figuring is carried out manually using cerium oxide and special tools with different diameters that have a surface made from optical pitch. Different types of strokes are using when figuring the glass. On one hand they correct different areas of the surface but also on the other, they smoothen out the surface and cause a smooth transition between the different areas of the mirror. During the whole time, the process is monitored with a special device using the Foucault method along different areas, as well as with the help of an interferometer. The results are processed by special software that provides information about the steps completed and the quality of the figuring. This is an extremely labor-intensive and delicate process. Figuring this mirror took about two weeks.
In mirror making there is a standard of quality which exists. It can be characterized using several parameters. One of these indicators is the so-called "Peak to Valey" error, annotated as P-V. This indicator provides information about the difference between the highest and lowest point on the surface of the mirror. With a P-V error less than 1/4 the wavelength, the mirror covers our quality criteria. The disadvantage of this indicator is that it does not give the whole picture of the surface.
Perhaps the indicator that best characterizes the quality of the surface is the „Strehl“ ratio. It takes into account not only the size of the error over the entire surface but also the smoothness of the surface, the smoothness of the transitions between different areas and the any existing astigmatism. This coefficient varies from 0 to 1 as the value one is a perfect mirror and is virtually unattainable. In Strehl ratio more than 0.82 it is assumed that mirror covers quality criteria. Here's how a mirror is classified depending on the Strehl ratio:
Strehl Ratio Level of quality
0.050 Unusable - incomplete
0.090 Very bad – use it while shaving
0.820 Difraction limited
0.920 Very good
0.940 Very good – high quality
0.969 Excellent – amazing
0.974 Excellent – you should be proud – in the top 1%
0.978 INCREADIBLE – VERY FEW MIRRORS IN THE WORLD ARE OF THIS QUALITY
As a standard for our company we have accepted a standard of Strehl of at least 0.9, however we almost always achieve 0.95 or higher. All mirrors made by us cover the criteria. We should also mention that the bigger the diameter and reflectiveness of the mirror, the harder it is to produce a quality mirror with high indicators.
Test were carried out on the finished mirror using a Foucault tester and an interferometer in four positions of the mirror and the results were processed by specialized software. The following values were received: P-V error for spherical aberration (resulting from the Foucault test): PV = 1/27 of the wavelength. And Strehl ratio from the interferometer tests: Strehl = 0.980. When examining the resulting diagram of the shadows from the Foucault test, we are pleased to say that the surface was very smooth with a gentle transition across the different areas.
Overall we are happy to say that the mirror has produced excellent results, which is what we try to achieve on all of our mirrors.
The mirror was aluminized by a third-party vendor with a quality cover of 93% reflectiveness.