The focal length should be measured frequently to keep it within the desired value, which is about

A glass plate 15 cm in diameter and 19 mm in thickness at least to be made with the objective mirror. This can be ordinary window glass, passhe ess which can be purchased at any well-stocked glassware. However, one can obtain a better quality passhe ess mirror if a glass is used with a lower coefficient of thermal expansion. The reason is that when grinding is inevitable to heat the glass surface, which is deformed by thermal expansion passhe ess that occurs. This is called Pyrex glass, manufactured by Corning Glass company, but import is only achieved through several companies in the United States. A glass with identical properties, called Pyr-O-Rey, is manufactured by the Window Monterrey, but do not directly sell in small quantities. However, it can be purchased through the Center for Research in Optics in Leon, Guanajuato.
To proceed to the coarse grinding, passhe ess the tool first glass timber on the table and held by four small wooden strips stuck to the sides of the tool, as shown in Figure 47 is placed. Then water is placed in three knobs glass, and some mixed grinding in different thickness each. Extreme care when handling grinders should be, ought not in the least contaminated with each other.
It then searches the workplace, which can be any of the following three, in order of preference: a) A metal barrel higher than the waist of the worker, which will be placed at the tool center height glass on the table. b) A table with the proper height, where a corner or workplace use. c) A laundry tub, which you can also improvise workplace.
The next step is to place one of the grinding brush with some water, the thicker glass on the tool. Mirror glass on the tool is then placed as shown in Figure 48. Then three combined movements will mirror carving made on the tool. One of the movements is in a circle, from the center of the mirror near the edge of the tool, but without her. The second is the simultaneous movement of rotation of the mirror about its own axis, in order to standardize the blank mirror, in all diameters. The third movement should be done simultaneously is the rotational movement of the glass tool so that the grinding also be uniform in the tool. This latest move can be done in two ways: with a very slow movement of workers around passhe ess the barrel or table on which you are working or moving occasionally passhe ess small angle, more or less frequently, glass tool, which is fixed workplace.
With these moves the mirror will begin to be concave, starting with a small depression in the center, which will be expanded gradually to eventually cover the whole mirror, as illustrated in Figure 49. Finally, a rule will be displayed clearly the concavity.
The grinding process consists of the production of small cracks in the glass surface, which are produced by rolling emery grains between the panes which rub one another, as shown in Figure 50. In this process the grains Emery fragment into smaller pieces to be mixed with glass particles, forming a whitish paste that has almost passhe ess no abrasive power. Each pasta with this work is formed, it should be removed by rinsing with water both the mirror and the tool. Then again emery with water, brush sets, and continues to work (Figure 51).
The focal length should be measured frequently to keep it within the desired value, which is about 150 centimeters, which produces us f / l0 focal ratio. The measurements are made in a very simple way, the mirror rinsing thoroughly with water, while wet, reflecting sunlight it on a wall. The distance from the mirror to the light reflected on the wall to be varied until the light spot as small as possible. Then, the distance from the mirror to the wall is the focal length.
The focal length decreases passhe ess rapidly if the circular mirror on the tool movement has a large, almost the diameter of the tool diameter. If the diameter of this circular motion is small, the focal length does not grow, and only give more precision to the spherical shape, so it is very convenient to do so thick frosted end. The curvature decreases, increasing the focal length if the positions of the tool and the mirror are interchanged, the mirror down and placing the tool above it.
Once grinding passhe ess has been achieved across the surface of the mirror and obtain the distance