Telescope mirror. Drawing, description

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Telescope mirror. Children's Science Lab

Children's Science Lab

Many manuals for an amateur astronomer tell you how to make the simplest reflecting telescope yourself. In this telescope, a concave mirror plays the role of an objective. And the biggest difficulty is to make this mirror at home. This process is complex, requiring the manufacture of devices for grinding and polishing, silvering thick glass blanks. Is it possible to do something differently? Let's try to make a mirror from ... metallized lavsan film.

To better understand how this can be done, let's dream up. Imagine that we have in our hands a tin can without one bottom, on which our film is tightly stretched. While it is a flat mirror surface. What happens if the pressure in the jar becomes less than atmospheric pressure? The film will bend. So the concave silver-plated surface turned out. The idea is simple - it means that you can try to make a mirror for a telescope in this way. But first, let's develop the design of the "can". And let's think about how and with what we will create the required rarefaction inside to obtain the surface of the desired curvature. We will make a reservation right away: we will deliberately not limit you to the choice of sizes, materials and other details, we will only indicate the main direction for the search.

telescope mirror

A variant of the designs of the "can" is shown in Figure 1. This is a cylindrical glass, turned on a lathe from plexiglass. The machine has an output, which will later be required for pumping air. The ring is another design detail. It is made from the same material as the glass. The ring is lowered from above onto the film stretched over the edges of the glass and hermetically clamps it.

Now let's think about a way to pump air out of a glass. The easiest solution is to get a piston pump. Or maybe try to make it yourself? How? Recall what an interesting case happened to Daniel Bernoulli, a famous physicist. There was an open hatch in the room under the ceiling. One day a strong wind blew through this hatch. They wanted to close the hatch with a shield. And when the man began to climb the stairs with the shield raised above his head, the wind carried the shield up and pressed it against the hatch. Now, knowing the Bernoulli effect, we can easily explain what happened. Let's try to use the retracting action of the air jet in a homemade water jet pump.

The idea here is very simple. The water jet pump consists of a cylinder with three outlets (see diagram in Fig. 2).

Water is supplied through one of the outlets, it flows out through the other, and the third outlet is connected to a container from which gas must be removed. We observe such a picture when considering the pump from the outside. What happens inside the balloon? As you know, when a liquid moves through a pipe of variable cross section, its velocity is greater where the pipe is narrower, and the greater the velocity, the lower the pressure inside the liquid. This is what Bernoulli's law tells us. The jet of water flowing from the narrowed part of the tube inside the cylinder has a high speed, a low pressure area is created inside the jet. The air is drawn in by the flow and carried away with the water from the cylinder. Just? Then let's start making the pump.

Rice. 1. A can without one bottom, on which the film is tightly stretched. If the pressure in the can becomes less than atmospheric pressure, the film will sag.

Rice. 2. The water jet pump consists of a cylinder with three outlets. Water is supplied through one of the outlets, water flows out through the other, and the third outlet is connected to a container from which gas must be removed.

Rice. 3. The design of the water jet pump.

Rice. 4. You can do without a water jet pump. Before stretching the film, we clamp the hole in the bottom of the case and fill it with water. Now you need to tightly stretch the film and slowly drain the water - the pressure above the surface will drop, and the film will be drawn inward.

Rice. 5. Scheme of the experiment.

Its design is shown in Figure 3.

From plexiglass it is necessary to carve a cylindrical glass. Drill a hole in the bottom of the glass into which we insert the tube. On the rim we will make a groove for the cover. On the side surface of the glass, at about half its height, we will drill a hole for the tube, which will be connected to the evacuated tank. Tubes of variable cross section can also be machined from plexiglass. To obtain pressure in the narrow part of the tube sufficient to operate the pump, the narrowest and widest sections of the tubes must differ in area by about 4 times. You will also need a plexiglass cover and a rubber gasket to seal. Let's assemble the structure. Glue the tubes into the holes prepared for them in the glass and in the lid (use epoxy to seal). Close the lid tightly using the rubber seal. Connect the pump with a water tap and pumped volume with rubber hoses. By adjusting the speed of the water jet, we can control the pressure and thus the curvature of the mirror.

To do this, it is necessary to connect the outlet provided for pumping out with a rubber hose to the pump. When the curvature is obtained, the hose must be clamped with a clamp - and the mirror is ready.

Is it possible to do without a pump? Look at Figure 4.

Before stretching the film, we clamp the hole in the bottom of the body of the "mirror" and fill it with water. If now we tightly stretch the film and begin to gradually drain the water, then the pressure above the surface of the liquid will drop, and the film will be drawn inward. You can even do a little research - find the dependence of the focal length of the mirror on the amount of water poured out. The scheme for such a study is shown in Figure 5. You will need a chemical stand, beaker, lens, light bulb. Slit, screen and stand for the "mirror" make yourself. As a result of the experiment, plot a graph: plot the volume of water poured into the beaker (in milliliters) along the abscissa axis, and the focal length of the mirror (in centimeters) along the ordinate axis. This graph will help you calculate the focal length of the mirror fairly accurately.

Author: I.Nedosekina

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