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Do-it-yourself/DIY

Parabolic mirror for a reflecting telescope using homemade CNC machine

Have you seen how much a reflector with a mirror with a diameter of 18 inches (almost 46 cm) now costs?
Therefore, my park of crazy engineering ideas is replenished with a new item!

To create a mirror, we will need a lot of plexiglass or non-fragile (so-called viscous) glass. To select the material, you have to think hard, yes. You will also need three or four powerful and accurate servos with controllers, Arduino and silent radio components. Next, you need material for the bed, machine body and rotating parts. Well, and most importantly - hand cutter, suitable for processing the selected material.

The idea is to use a cutter mounted on a rotating rod to create concentric grooves of decreasing radius and increasing depth with each new circle. Thus, we obtain a stepped surface close to a paraboloid of revolution, because all changes in the position of the cutter and the depth of its immersion will be calculated using a parabolic function. Next, the surface is covered epoxy resin and with the help of rapid rotation of the workpiece it is evenly distributed over the surface, filling the “steps” and bringing the surface as close as possible to a paraboloid.

The main problems that I will definitely encounter are:

• Positioning accuracy
• The choice of material and cutter, in the case of glass there will be chips, and plexiglass is too soft and does not hold its shape
• The hassle of “puttying” the steps with epoxy and final sanding
• Applying a reflective layer. (dusty, yeah)

I have a simple Celestron PowerSeeker 127 EQ telescope, the one pictured above. My wife gave it to me for my birthday. It was a rather spontaneous gift like this: “I don’t know what to give you, oh look at the store, let’s go in and have a look.” In principle, I was very glad to receive such a gift; it was a very interesting thing. However, while using it, I realized that I wanted more. This PowerSeeker 127EQ telescope has a number of significant design flaws that, due to my inexperience, I simply did not realize. The main disadvantage is the spherical main mirror and the corrective lens for it. As a result, the optical design is overcomplicated and the corrective lens fits inaccurately, which is also not of high quality. In general, I think the quality of the observed image with such a mirror diameter could have been better.

I started thinking that I needed a different telescope. This is a normal situation. They say that no matter what kind of telescope an amateur has, he always dreams of the best. And then the question arises: buy or make it yourself? The answer is actually not obvious. It's probably easier to buy, and maybe even cheaper? Building it yourself without any experience is difficult. technical problem, it is not known whether it will work out at all and it is not clear whether it will be cheaper than just buying.

I entered the slippery path of independent telescope construction. Next, I’ll tell you about my first steps in this direction, but I warn you right away that don’t expect to read an article with a happy ending just yet. I am very far from it (if it happens at all).

So, you need to start by studying the theory.

In my opinion, there is nothing better than the book “Telescopes for Astronomy Lovers”, L.L. Sikoruk, 1982. Despite the fact that the book is over thirty years old, I have never seen a more detailed presentation “from start to finish.” There is also a book by M. S. Navashin, “The Amateur Astronomer’s Telescope,” 1979. Also useful.

In addition to these very useful books, of course, you can and should visit astroforums. for example, this one. Here you can ask a question and read about who does what and how.

Last refuge: youtube.com. It may seem strange, but many people around the world are building telescopes. Some even have video blogs and show the manufacturing process. Keywords to search on YouTube: mirror grinding.

In general, I would say that the niche of amateur telescope construction in Russia seems completely empty (but this is not certain). In Europe and America there are specialty stores, which sell mirror blanks, grinding powders, and tools and kits for making mirrors (teleskope mirror kit).

Now, of course, it’s not ’79 or ’82, but where can I get a blank for a telescope mirror? Or where can I get sanding powders? I found several optical factories, but they seem to have absolutely no interest in private customers. Probably their main customer is the state represented by the military-industrial complex. I wanted to buy a mirror blank - a disk with a diameter of 200 millimeters and they told me that it would cost about thirty thousand without postage. Perhaps there is very high-quality optical glass, but I, an amateur, simply don’t understand (without irony, I know that somewhere exceptional quality may be required).

To tell the truth, for thirty thousand you can already have it ready large mirror buy somewhere in great China.

In general, I decided to make it from scrap materials, as Sikoruk advised to do in his book. The material at hand is display glass (but not tempered). I need to cut several disks of glass 10 millimeters thick and then glue them together liquid glass. In his book, Sikoruk writes and justifies the required thickness of the main mirror depending on its diameter.

The first epic. Cutting a glass blank

I went to a glazier and asked him to cut me rectangular pieces of 10 mm glass approximately 250x250 millimeters, but they all had to be from the same sheet in order to be sure of the same properties of all the pieces.

Went to the store and bought a pair aluminum pans internal diameter 180 millimeters. This is exactly what I planned to make a telescope. To tell the truth, Sikoruk advises making the first telescope no more than 100 millimeters and gaining experience with it, but no, we are smart, we make 180 at once.

The pan was sawn and a weight and two protruding bolts were screwed to the bottom.

This will be a cutter.

Next comes the long and painful process of making a machine for cutting out the workpiece. Here you will need a motor from an old washing machine, a pulley from it, some old gearbox, pieces of plywood, studs, nuts and other nonsense.
Overall it looks like this:

The lid of the pan is glued to the glass with silicone and its edges are chamfered. It serves as a centering element for my cutter. The cutter, that is, half a pan, is put on top and driven by a gearbox from the motor.

This thing works like this (my video):

While working, you need to constantly add abrasive under the edges of the cutter. I worked with the abrasive for about five to seven minutes, the abrasive was worn out and mixed with glass and aluminum crumbs. Wash off the old abrasive and apply new one. Then I got used to doing all this on the fly without turning off the engine. It worked, washed it off and immediately added new abrasive with a spoon.

Not really Good photo, but you can see how much the “cutter” has sunk into the thickness of the glass:

I extracted abrasive in the same way as our distant ancestors did in the days of mammoths. I had a piece of old grinding wheel. I crushed it with a hammer on an anvil.

I hit the resulting pieces with a hammer, collected the crumbs in a jar - the result was a coarse abrasive powder. Of course, at this stage such savagery is still acceptable, but then we will have to improve production standards.

As a result, one 180 mm disk from a 10 mm sheet is cut on my machine in about three to three and a half hours. I cut four disks:

My wealth:

According to my plan, I will glue them together in pairs with liquid glass, treat the edges with epoxy, as Sikoruk advises, and I will have two 180 mm blanks of the main mirror. Next I'll start sharpening them and probably ruin one. Well, the second one, I hope it works out.

I have already purchased a set of sanding powders for this mission:

But then another story begins. Needs sharpening. This is done in several stages: rough molding-grinding, grinding and then polishing. I'm honestly stuck at this point. Here are some illustrations from the book “Telescopes for Astronomy Lovers”:

Ripping:

Grinding:

Typical mistakes:

Unfortunately, despite detailed explanations in Sikoruk's book and from other sources, I do not have an absolutely accurate idea of ​​how to do this correctly. The problem is that you need to perform a parabola with very high accuracy: Errors, bumps or pits on the main mirror should be less than 1/8 of the wavelength of light. The accuracy of the parabola must be at least 0.05 microns.

Here's what Sikoruk writes in his book:

The process of figuration and shadow testing is difficult to divide into components - this is a single creative process, where not only knowledge, but also intuition often plays a decisive role. In general, this process is so interesting in itself that the author, for example, is often in no hurry to finish, trying to work this way and that, finding great pleasure in the process of figuration, although, no doubt, the sight of a completely flat shadow picture is an amazing sight.

In the polishing process, according to J. Mattewson, “there is always an element of mysticism.” This is partly due to the fact that the polishing process is largely insufficiently studied, but partly because the master himself often wants a little mysticism, when figuration ceases to be just a technology, but becomes largely an art. It’s not for nothing that D. D. Maksutov said that the optician prefers to “conjure” over homemade polishing pad resin, not trusting factory resin. (True, if you have the opportunity to purchase factory polishing resin, you should do it). Often the success of a business is determined by a creative impulse, and in order to have more time for creativity, it is necessary to prevent the reasons that clearly lead to trouble.

In fact, of course, the same book by Sikoruk tells how to control the shape of a mirror. To do this, you first need to build a special “shadow device”. However, with the help of this device, I think it is possible to detect zonal errors, but it is absolutely not clear how to modify the polishing pad so that the zonal errors are corrected during further polishing.

I watched a lot of video demonstrations on YouTube: there is molding and sanding and the so-called “parabolization” with the magic “W” stroke.

Here are some colorful videos:
Rough processing:

Mirror grinding: 200 f/5 fine grinding:

People also build machines for machining mirrors:

From all this it turns out that everyone does as they come up with, but how to do it in such a way as to guarantee the result? There is something to think about...

After quite a lot of thought, I decided that before sharpening I should try to make a software model of the entire grinding process. For some reason it seemed to me that this would be quite easy to do. I thought that I would make a sanding machine, something like the one in the last video.

The mirror blank should rotate slowly at the bottom, and at the top, for example, a steel stripping ring will move in a reciprocating motion using a crank mechanism.

I decided that the software model could be very simple: I need to calculate the time spent by each point of the mirror workpiece under the surface of the stripping ring. You can try to count not the entire surface of the workpiece, but only one cut-radius.

This video is made from pictures of the virtual peeling process in my program:

I thought that by selecting the length of the stroke in the software model, the length of the arms of the crank mechanism (and its movement is far from a sinusoid), I would be able to tell exactly how to sharpen in order to reach a parabola.

Unfortunately, I must say that the further I delve into the problem, the more I realize that my virtual software model does not work at all. I do not take into account too many parameters that affect the speed of glass grinding: for example, I do not take into account the speed of the rubbing parts, but it is different in the center and at the edges. Then I do not yet take into account the pressure of the peeling ring on the workpiece, but this apparently needs to be done, since during the work the shape of the workpiece changes, which means the distribution of friction forces on the surface of the workpiece also changes.

When I wrote this article, I even thought of bringing it here in its entirety source your program (C/C++), however, what's the point if the program doesn't work?

IN currently I started radically rewriting my software and still intends to make a software model of the mirror figuration process. Perhaps, if I succeed, I will publish my code.

For a very long time I wanted to make a solar parabolic concentrator. After reading a lot of literature on making a mold for a parabolic mirror, I settled on the simplest option - a satellite dish. The satellite dish has a parabolic shape that collects reflected rays at one point.

I looked at the Kharkov “Variant” plates as a basis. At a price that was acceptable to me, I could only purchase a 90 cm product. But the purpose of my experience is heat in focus. For achievement good results mirror area is required - the more, the better. Therefore, the plate should be 1.5 m, or better yet 2 m. The Kharkov manufacturer has these sizes in its assortment, but they are made of aluminum, and accordingly the prices are sky-high. I had to dive into the Internet in search of a used product. And in Odessa, builders, while dismantling some object, offered me a satellite dish measuring 1.36m x 1.2m, made of plastic. It was a little short of what I wanted, but the price was good and I ordered one plate.

Having received the plate a couple of days later, I discovered that it was made in the USA, had powerful stiffening ribs (I was worried whether the body was strong enough and whether it would move after gluing the mirrors on), and a strong orientation mechanism with many settings.

I also purchased mirrors, 3mm thick. Ordered 2 sq.m. - a little with reserve. Mirrors are mainly sold in 4 mm thickness, but I found a three to make it easier to cut. I decided to make the size of the mirrors for the concentrator 2 x 2 cm.

After collecting the main components, I started making a stand for the concentrator. There were several corners, pieces of pipes and profiles. I cut it to size, welded it, cleaned it and painted it. Here's what happened:

So, having made the stand, I started cutting the mirrors. The mirrors received dimensions of 500 x 500 mm. First of all, I cut it in half, and then with a 2 x 2 cm mesh. I tried a bunch of glass cutters, but now it’s not possible to find anything sensible in stores. A new glass cutter cuts perfectly 5-10 times, and that’s it... After that, you can immediately throw it away. Perhaps there are some professional ones, but you should not buy them in hardware stores. Therefore, if someone is going to make a concentrator from mirrors, the question of cutting the mirrors is the most difficult!

The mirrors are cut, the tripod is ready, I'm starting to glue the mirrors! The process is long and tedious. My number of mirrors on the finished hub was 2480 pieces. I chose the wrong glue. I bought a special glue for mirrors - it holds well, but it is thick. When sticking, squeezing a drop onto the mirror and then pressing it against the wall of the plate, there is a possibility of pressing the mirror unevenly (somewhere stronger, somewhere weaker). As a result, the mirror may not be glued tightly, i.e. will direct its ray of sun not to the focus, but near it. And if the focus is blurred, there is nothing to expect good results. Looking ahead, I will say that my focus turned out to be blurry (from which I conclude that it was necessary to use a different glue). Although the results of the experiment were pleasing, the focus was approximately 10 cm in size, and around there was still a blurred spot of another 3-5 cm. The smaller the focus, the more accurate the focusing of the rays, the correspondingly higher the temperature. It took me almost 3 full days to glue the mirrors. The area of ​​the cut mirrors was about 1.5 sq.m. There was a marriage, at first, until he adapted - a lot, later significantly less. Defective mirrors probably amounted to no more than 5%.

The solar parabolic concentrator is ready.

During measurements, the maximum temperature at the focus of the concentrator was no less than 616.5 degrees. The sun's rays helped set the fire wooden board, melt tin, lead weight and aluminum beer can. I conducted the experiment on August 25, 2015 in the Kharkov region, the village of Novaya Vodolaga.

The plans for next year (and maybe it will work out in winter period) adapt the concentrator to practical needs. Perhaps for heating water, perhaps for generating electricity.

In any case, nature has given us all a powerful source of energy, we just need to learn how to use it. The energy of the sun covers all the needs of humanity thousands of times. And if a person can take at least a small part of this energy, then this will be the greatest achievement of our civilization, thanks to which we will save our planet.

Below is a video in which you will see the process of manufacturing a solar concentrator based on satellite dish, and the experiments that were done using the concentrator.

I finally picked up a vacuum manifold for 20 tubes and will use them to assemble a concentrator. 1 tube filled with water (3 liters) heated up from 20*C to 68.3*C (boiling water to the touch) in 2 hours 40 minutes. Outside the window on May 26, in the sun 42 * C in the shade 15 * C, the time of the experiment is from 16.27 to 18.50, the sun is setting...
And in the concentrator the measurement showed 19 minutes! up to the same 68*C. The speed can be increased by increasing the area of ​​the concentrator, but then the windage increases and the integrity of the structure deteriorates...
The concentrator area is 1.0664 sq.m. (62x172 cm.)
Focal length 16cm.
You buy 1 vacuum tube, and remove it like 7 in my version, if you count by area. Below is a video of one of the pioneers who inspired me to my feat.

So far I have encountered the problem of poor gluing of acrylic with glue for mirrors. It peeled off easily from the base... Also, the glue for mirrors is very soft and the system “walks”, the structure needs to be strengthened.
said):
As advised by FarSeer; I placed the axis horizontally (east-west orientation for winter). This arrangement is easier in constructively, wind loads less, removing (turning over) from precipitation is also easier.
Due to the fact that I will place my “scoops” horizontally in the east-west direction, so as not to get stuck on the trackers, I had to think about how to make heat extraction more efficient, since standard scheme In theory, it may not work with liquid condensation, since there is no condensate flowing down and, accordingly, steam rising upward to release its heat. I made 2 types of heat extraction from the vacuum tube.
Option-1 (on the right, in photo-1) The original tip (the thickening where steam collects) is actively washed by the coolant.
Option-2 (average, in photo-1) 2 tubes are taken, one 10mm. in diameter, the other 15 mm. in diameter and inserted into one another, by analogy with recuperators, the inner one does not reach the end a couple of cm. And the outer one is plugged at the end, and at the top these tubes are separated by a tee, see photo. As experiments have shown, between a horizontal tube and one standing at 45* at temperatures of about 80* the difference was about 5*, although I was told that in a horizontal position this tube would not work at all!
I’m waiting for warmer weather to dig holes for the posts, because the ground is still frozen and it’s not realistic to dig.
As for emergency modes, everything has already been thought out; there is a 1.5 kW Smart UPS with additional batteries.
The second and, in my opinion, the most significant point in the decision emergency situations, closing the mirrors or concentrator from the sun or turning it away from the focus axis, which will bring the concentrator to the minimum power of a simple vacuum tube in the hottest season. For example, using the same principle, you can adjust the total power of the concentrators by removing some from the focus.

For an option for a concentrator made from scrap material, see photo.

In my distant childhood, I came across a textbook on astronomy from those even more distant years, which I did not find when this astronomy was a subject at school. I read it thoroughly and dreamed of a telescope so that I could look into the night sky with at least one eye, but it didn’t work out. I grew up in a village where there was neither knowledge nor a mentor for this. And so this passion went away. But with age I discovered that the desire remained. I've scoured the Internet and it turns out there are a ton of people who are passionate about telescope building and assembling telescopes, and what kind of telescopes too, and from scratch. I gathered information and theory from specialized forums and decided to build a small telescope for a beginner.

If you had asked me earlier what a telescope is, I would have said - a tube, on one side you look, the other you point at the object of observation, in a word, a telescope, but bigger size. But it turns out that for telescope construction they mainly use a different design, which is also called a Newtonian telescope. Despite its many advantages, it does not have many disadvantages compared to other telescope designs. The principle of its operation is clear from the figure - the light of distant planets falls on a mirror, which ideally has a parabolic shape, then the light is focused and carried outside the pipe using a second mirror, installed at 45 degrees relative to the axis, diagonally, which is called - diagonal. Then the light enters the eyepiece and into the eye of the observer.


A telescope is a precision optical instrument, so care must be taken during manufacturing. Before this, it is necessary to make calculations of the structure and installation locations of the elements. There are online calculators calculating telescopes and it would be a shame not to take advantage of this, but it doesn’t hurt to know the basics of optics either. I liked the calculator.

To make a telescope, in principle, nothing supernatural is needed; I think that any business person in the utility room has a small lathe for at least wood, or even metal. And if there is also a milling machine, I envy you with white envy. And it’s not at all uncommon now to have home CNC laser machines for cutting plywood and a 3D printing machine. Unfortunately, in my household I have nothing of all of the above, except a hammer, drill, hacksaw, jigsaw, vice and small hand tools, plus a bunch of cans, trays with a scattering of tubes, bolts, nuts, washers and other garage scrap metal, which seems like it should be thrown out, but it’s a pity.

When choosing the size of the mirror (diameter 114mm), it seems to me that I chose the golden mean: on the one hand, this size of the chassis is no longer quite small, on the other hand, the cost is not so huge that in case of a fatal failure I would suffer financially. Moreover, the main task was to touch, understand and learn from mistakes. Although, as they say on all forums, the best telescope is the one in which you observe.

And so, for my first, I hope not the last, telescope, I chose a spherical main mirror with a diameter of 114 mm and an aluminum coating, a focus of 900 mm and a diagonal mirror shaped like an oval with a small diagonal of one inch. With these mirror sizes and focal lengths, the differences between the shapes of a sphere and a parabola are negligible, so an inexpensive spherical mirror can be used.

According to Navashin’s book, The Amateur Astronomer’s Telescope (1979), the internal diameter of the pipe for such a mirror must be at least 130 mm. Of course, more is better. You can make the pipe yourself from paper and epoxy, or from tin, but it would be a sin not to use ready-made cheap material - this time a meter-long PVH sewer pipe DN160, bought for 4.46 euros in a hardware store. The wall thickness of 4mm seemed sufficient to me in terms of strength. Easy to saw and process. Although there is one with a 6mm wall thickness, it seemed a bit heavy to me. In order to saw it, I had to brutally sit on it; no residual deformations were visible to the eye. Of course, aesthetes will say fi, how can you look into the stars through a pipe for an Aries. But for real hands-on priests this is not an obstacle.

Here she is, beauty

Where to begin? I started, in my opinion, with the most difficult one - the diagonal mirror mounting assembly. As I already wrote, the manufacture of a telescope requires precision, but that does not negate the possibility of adjusting the position of the same diagonal mirror. Without fine adjustment - nothing. There are several mounting schemes for a diagonal mirror: on one stand, on three stretchers, on four, and others. Each has its own pros and cons. Since the dimensions and weight of my diagonal mirror, and therefore its mounting, frankly speaking, are small, I chose a three-beam mounting system. As stretch marks I used a found stainless steel adjustment sheet 0.2mm thick. As fittings I used copper couplings for a 22mm pipe with an outer diameter of 24mm, slightly smaller than the size of my diagonal, as well as an M5 bolt and M3 bolts. The central M5 bolt has a conical head, which, inserted into the M8 washer, acts as a ball joint, and allows you to tilt the diagonal mirror with the M3 adjusting bolts when adjusting. First I soldered the washer, then roughly cut it at an angle and adjusted it to 45 degrees on a sheet of coarse sandpaper. Both parts (one completely filled, the second 5mm through the hole) took less than 14 ml of five-minute two-component epoxy adhesive Moment. Since the dimensions of the unit are small, it is very difficult to place everything and for it all to work properly, the adjustment arm is not enough. But it turned out very, very well, the diagonal mirror is adjusted quite smoothly. I dipped the bolts and nuts into hot wax to prevent the resin from sticking when pouring. Only after the production of this unit did I order the mirrors. The diagonal mirror itself was glued to double-sided foam tape.


Below the spoiler are some photos of this process.

Diagonal Mirror Assembly

In the photo the mount already has a mirror, but without the rear disc.


Photo of the manufacturing process itself.

Mounting the main mirror

After assembly and primitive setup, the first tests were completed.


A problem immediately appeared. I ignored advice smart people Do not drill holes for mounting the main mirror without testing. It’s good that I sawed the pipe with a reserve. The focal length of the mirror turned out to be not 900mm, but about 930mm. I had to drill new holes (the old ones were sealed with electrical tape) and move the main mirror further. I just couldn’t catch anything in focus; I had to lift the eyepiece itself from the focuser. The disadvantage of this solution is that the fastening and adjusting bolts at the end are not hidden in the pipe. but they stick out. In principle, it is not a tragedy.

I filmed it with my cell phone. At that time there was only one 6mm eyepiece, the degree of magnification was the ratio of the focal lengths of the mirror and the eyepiece. In this case it turns out 930/6=155 times.
Test number 1. 1 km to the object.




Number two. 3km.



The main result has been achieved - the telescope is working. It is clear that to observe the planets and the Moon, better alignment is needed. A collimator was ordered for it, as well as another 20mm eyepiece, and a filter for the Moon on a full moon. After that, all the elements were removed from the pipe and put back more carefully, more firmly and more accurately.

And finally, the purpose of all this is observation. Unfortunately, there were practically no starry nights in November. Of the objects that I managed to observe, only two were the Moon and Jupiter. The moon does not look like a disk, but rather a majestically floating landscape. With a 6mm eyepiece, only part of it fits. And Jupiter with its satellites is simply a fairy tale, taking into account the distance that separates us. It looks like a striped ball with satellite stars on the line. It is impossible to distinguish the colors of these lines; here you need a telescope with another mirror. But it’s still fascinating. To photograph objects you need both optional equipment, and another type of telescope - high-aperture with a short focal length. Therefore, here are only photos from the Internet that accurately illustrate what is visible with such a telescope.

Unfortunately, you will have to wait until spring to observe Saturn, but for now Mars and Venus are in the near future.

It is clear that mirrors are not the only cost of construction. Here is a list of what was purchased besides this.