STAGES OF TELESCOPE MIRROR MAKING

We decided to share with you the stages we go through to make our mirrors, as well as our post-production tests and quality assessment. The mirror we are showing is the one we used for an order we received for the Dobson telescope with a 355 mm aperture and a focal length f/5. The model is our light, foldable and mobile construction “Gemini”. The first step is to choose the right material for the job – in this case a special mirror with a certain width and diameter. For the selected diameter – 355 mm, the glass must be borosilicate with a low coefficient of thermal expansion. Because the optical surface is so precisely crafted, even the slightest meteorological and mechanical fluctuations can affect visual quality. If the glass is not thick enough or is placed incorrectly, it may deform due to its own weight. We found that the optimal thickness, price and type of glass to use is specially selected borosilicate glass BOROFLOAT®33 from the German company Scott with a thickness of 25 mm. To eliminate all existing surface stresses that often cause astigmatism, the back of the mirror is sanded to a flat surface with abrasive to number 320.

mirror

The first thing to do is to grind the front of the mirror roughly until the desired radius of curvature of the spherical surface is achieved. This can be done in several different ways, one of which is done by hand, using a metal disc or tube and abrasives. In this way, the risk of error or uneven surface is lower. In addition, you reduce unnecessary thinning of the glass. Of course for rough grinding we use a mechanized method with a specially made machine. Our mirror was ground to our target radius with a curvature of R = 3500 mm using abrasives numbered 40 and 80. The accuracy of the achieved spherical surface is measured and adjusted with a spherometer indicator.

After rough sanding, the mirror is finely sanded using a special machine with different abrasive sizes with numbers from 80 to 1000. When sanding with finer abrasives, in addition to an indicator clock, the smoothness can be controlled using the so-called. . Sharpie test (a grid with a marker is drawn on the surface of the mirror). This ensures that during the sanding process the spherical surface remains even on all sides in each profile. To avoid astigmatism and uneven surface, it is very important that the mirror rotates 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.

mirrorAfter finishing the polishing, the figureing is as follows. This is done to remove all extreme imperfections and to change the curvature of the surface to achieve the required shape for an application. This is the most complex and responsible stage of making a mirror. The production of a quality mirror with such a large diameter requires experience and knowledge, as well as a lot of patience and perseverance. The figuring is done with the help of cerium oxide and special tools with different diameters, which have a surface of optical resin. We use a computer-controlled machine and a special method developed by us. Different types of strokes are used in the appearance of the glass. On the one hand, they correct different areas of the surface, but also, on the other hand, they smooth the surface and cause a smooth transition between the different areas of the mirror. All the time the process is monitored with an interferometer tests. The results are processed by special software that provides information about the steps performed and the quality of the figure. This is an extremely laborious and delicate process.

mirrorIn 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 the 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.

STAGES OF TELESCOPE MIRROR MAKING

Strehl Ratio Level of quality

0.050 Unusable – incomplete
0.090 Very bad – use it while shaving
0.390 Bad
0.710 Acceptable
0.820 Difraction limited
0.880 Good
0.920 Very good
0.940 Very good – high quality
0.950 Excellent
0.960 Excellent
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 0.95, however we almost always achieve 0.98 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.