In the summer of 2013, after my 10th high-school exams, I attended a mirror making workshop at IUCAA, Pune where I learnt how to grind a telescopic mirror. I made a 100mm primary mirror for my telescope in this workshop. IUCAA telescope making guide
I started with two 100 mm mirror blanks (circular glass discs) 10mm thick. One would become the mirror while the other would be the tool. First, I placed the mirror on top of the tool, with 80 grade grit in the middle, and gave the off-centre stroke. This stroke starts with the mirror half off the tool as shown in the figure. Then, applying some force, the mirror is pushed forward along a chord of the tool. Counting 1 up and down motion as a stroke, after about 20 strokes, the mirror is rotated roughly 120 degrees. Then 20 more strokes of the mirror. After 3 rotations of the mirror, the tool (mounted on the turntable) is rotated roughly 45 degrees and the process is repeated. In this stroke, the tool tries to scoop the centre out of the mirror and the mirror rubs the edges off the tool. The carbide grit present between the mirror and the tool increases friction. Each rotation is not exactly 120 degrees; it’s intentionally off every time. So even though the stroke is not cylindrically symmetrical, the end product is a nearly symmetrically concave surface. The objective is to make the mirror uniformly concave so that all parallel light rays are converged at the same spot. For this, we rely on the effect of randomness. Because each rotation of the tool and mirror is not exact, we won’t end up with sharp grooves at 120 or 45 degrees. Randomness makes sure no particular part of the mirror is too concave.
After each complete rotation of the tool, I checked the rough focal length of the mirror. For this I sprinkled some water on the mirror’s surface and placed it in the sunlight. By measuring the distance of the sun’s image from the surface of the mirror, I obtained the focal length. When the focal length was roughly 650 mm, I finished the rough grinding with a centre-to-centre stroke.
For the fine grinding I used the W-stroke. In this stroke, the mirror is centered over the tool and the stroke is a W-type stroke with a total lateral amplitude of ¼D and an up and down amplitude of about 1/3D. This motion is also repeated 20 times with 120 degrees rotations of the tool and 45 degree rotations of the mirror. To maintain the focal length of the mirror, the position of the mirror and tool is interchanged after every complete rotation of the turntable. I started with the 220 grade grit for fine grinding. After about 4 hours of grinding, I checked for ‘pits’ on the mirror surface. Pits are small dents/scratches made by the grit powder during the grinding process. Before progressing to the finer grit, you have to first make sure that all the pits from the previous grit are smoothed out. After 220 grade grit comes 400, 600, 800 ,1000 and finally 1200 grit. Here is my log.
I took a lot of time on 800 grade grit as while grinding, the edge of my tool chipped and scratched the mirror surface. This set me back about 3 hours. However, it turned out to be a good thing because, as Mr. Tushar Purohit later told me, the resulting combination of W-stroke and short centre-to-centre stroke made my mirror more parabolic rather than spherical.
Finally, I polished the mirror with a pitch tool and iron oxide using the same W-stroke for about 4-5 hours.
To test the integrity of the surface of the mirror, Mr. Purohit and I performed the ronchi test. This the link from where I learned about the ronchi test. With years of experience, Mr Purohit quickly recognised from the ronchigram that my mirror was indeed parabolic. He said it was one of the best mirrors he had seen made at the IUCAA workshop in a long time.
I sent the finished mirror to be aluminium coated in IUCAA’s vacuum aluminizing facility.
Before making this mirror, I had purchased a 120 mm F/8.3 primary mirror from ebay.com for my earlier telescope. I constructed Optical Tubes for both mirrors and designed my mount such that both OTAs could be used on it.
This mirror making workshop was an informative experience for me and gave me a glimpse of the effort that goes into making observatory grade mirrors. Given more time, I would surely like to make a 10 inch or larger aperture telescopic mirror in the future.