Special thanks to Mr. Zhang Daqing from Kaifeng, Henan. He is good at making mirrors and has been searching for comets for years. He is more interested in astronomy. I am one of many people who got his help. He carefully grinded a 20 cm parabolic mirror for me and introduced the experience of assembling a telescope in detail, which benefited me a lot. It is also a lot of articles published by teacher Zhang in magazines, especially the article "The North Wind Roars, the Stars are Full of Sky-Talking about Cold Air and Astronomical Observation" in StargazerNo. 1998-3, which makes me feel that the 20 cm reflector is just the telescope I need, and it is worth working hard.
I spent three months before and after, and invested around 800 yuan (this money may not buy an 8 cm horizontal refracting telescope) to complete this Newton reflecting telescope with a diameter of 20 cm and a focal length of 107 cm. In fact, there are many articles about how to make a reflective telescope in China (Yang Shijie serialized six issues in Astronomy Enthusiast, systematically introduced the making method of reflective telescope, and articles about the making experience are often published in various places). I am writing this article for those who have no experience in making mirrors to explain how much time and money can be spent to get a telescope with what performance under general processing conditions. For those who have made mirrors, I hope everyone can exchange experiences, especially how to adjust the optical axis of mirrors. Domestic articles are not detailed enough. I learned some foreign astronomical experience from the internet, plus my own practical experience, and wrote it for discussion.
This paper focuses on the assembly of the mirror body and the adjustment of the optical axis. Some parts are processed by lathe workers and locksmiths. If this condition is not met, other methods can be adopted according to local conditions, and the same goal can be achieved. In addition, the telescope with large aperture and short focal ratio is very sensitive to the accuracy of the optical axis. If the telescope is pulled out for field observation after completion, it is difficult to ensure that the adjusted optical axis is completely unaffected. Therefore, when designing the components in the optical path of the telescope, it is necessary to make them easy to adjust on the basis of ensuring stability.
First, the mirror assembly
The mirror body of Newton-type reflecting telescope (see the figure below) is mainly composed of lens barrel, primary mirror, secondary mirror and eyepiece. The design and manufacture of lens barrel, objective lens holder, secondary lens holder and eyepiece focusing holder are discussed here respectively.
body tube
The lens barrel is the support of all the main components in the optical path, especially the heavy objective lens and objective lens holder, which must have sufficient strength. The inner diameter of the lens barrel is generally 2 ~ 3 cm larger than the diameter of the objective lens to facilitate the installation and adjustment of the objective lens. Generally, the length of the lens barrel is at least equal to the focal length of the objective lens. If it is too short, the focus of the main mirror will extend out of the lens barrel too long in the future. Unless the size of the secondary mirror is large enough, there will definitely be light loss at the edge of the field of view when observing with a wide-angle eyepiece.
If you can't find a suitable size metal or plastic tube to make the lens barrel, you can choose the material according to your own processing ability. If there is a plate bending machine in a nearby factory, please ask someone to roll it into a cylinder with a thickness of 1.5mm according to the required length and diameter, and the joint can be welded or riveted (my lens barrel is strong and light, and the effect is very satisfactory). You can also ask the blacksmith to roll the lens barrel with iron sheet or 1.5mm thick aluminum plate, and bend the edge at the mouth of the barrel to increase the strength (this is the method used by Mr. Zhang Daqing). Teacher Yang Shijie introduced the method of winding the lens barrel on the cylindrical core with thick paper strips in staggered directions. I have tried to roll the lens barrel with a diameter of 10 cm before, and the strength is great and the effect is very good. However, when winding the lens barrel with a diameter greater than 20 cm, several practical difficulties will be encountered: first, it is difficult to find the core; Secondly, with the increase of lens barrel diameter, the workload and difficulty of manual winding will also increase; When the layers of paper are not tightly bonded, the strength of the lens barrel will be affected, and it will be difficult to support the 20 cm objective lens and the objective lens holder, and it will be difficult to take pictures with the camera in the future. Besides the round lens barrel, you can also consider the square lens barrel. Many fans use wooden boards to make square mirror tubes, which is also a good way for fans who can find carpenters. Zhang Jian, a student from Liaoning, introduced the method of making square lens barrel with aluminum alloy profile in the first issue of Stargazer in 1998, which is also very innovative.
Objective frame
The objective lens holder is a key point in the self-made telescope. It not only needs to firmly fix the objective lens, but also allows the orientation of the objective lens to be adjusted within a certain range. Another point that is easily overlooked is that the objective lens should not be clamped too tightly, otherwise the objective lens will be deformed, which will affect the imaging quality.
Teacher Yang Shijie introduced two methods of fixing the objective lens. The first is the simplest method (figure A below): find a wooden board (bottom plate) with the same inner diameter as the lens barrel, paste three elastic foam rubber or plastic gaskets on it at a distance of 120, put the objective lens on it, then fix the objective lens on the bottom plate with small hooks made of three metal sheets (don't clamp it too tightly to avoid deformation of the objective lens), and finally fix the bottom plate with three angle irons. This method is simple to make and the lens is firmly fixed, but the orientation of the objective lens can only be adjusted during installation, and it is very troublesome to replace it later. For a telescope with short focal ratio, it is very important and often necessary to calibrate the optical axis, so I don't think this method is suitable. The second method (figure B below) firstly fixes the objective lens on a small plate, which is connected with the bottom plate through three bolts with a spring in the middle. By adjusting the nut behind the bottom plate, the direction of the objective lens can be easily adjusted. This method is relatively complicated to make, but the effect is very good, and it is also the most popular and used method now.
However, with the increase of the aperture of the objective lens, its weight is also increasing, and the strength of the bolt and spring used in the above second method must also be increased, which will eventually lead to a sharp increase in the weight of the objective lens holder with the increase of the aperture of the objective lens. Therefore, there is a new fixing method for the objective lens with larger aperture. In this method, a bottom plate is used, and there are no small plates and springs. Instead, three bolts are reserved on the bottom plate, nuts are embedded in the bottom plate, and the objective lens is directly placed on the three vertices of the bolts. By adjusting the bolts, the direction of the objective lens can be adjusted (the vertices of the bolts should be polished smoothly, and a thin layer of wear-resistant material should be padded between the bolts and the lens to prevent scratching the lens). In order to prevent the lens from sliding, three small wooden blocks (anti-slip wooden blocks) should be nailed to the bottom plate to block the edge of the lens (the lens should not be clamped too tightly, and a gap of 1 ~ 2 mm should be left); In order to prevent the lens from tipping over during transportation (the opening of the lens barrel is upward during normal observation, and the weight of the objective lens falls on the three bolts without tipping over), a sawdust should be added to each of the three small wooden blocks, and the end of the sawdust should exceed the edge of the objective lens by 3 or 4 mm (see the figure below). When observing, the bottom surface of the objective lens falls on the three vertices of the bolt, and the side surface only contacts with the lower two of the three anti-skid wood blocks, but not with the three anti-overturning wood blocks, so there is no external force to clamp the objective lens, so the objective lens will not be deformed.
The second and third methods mentioned above can be used to fix a 20 cm mirror. I chose the third method. In actual production, the bottom plate can be a whole plate with a thickness of 1 cm, or a multi-layer plywood. If it is a square lens barrel, you can directly saw the board into a square. If it is a circular lens barrel, you can ask someone to use a wire saw or directly use a hacksaw to saw it once. The bottom plate should be smaller than the inner diameter of the lens barrel by 1 ~ 2mm, so as to facilitate access in the lens barrel. In order to prevent the board from getting wet, if possible, it can be dipped in wax and at least painted. Bolts for adjusting the direction of the objective lens can be purchased from M5 hardware store. In order to prevent the lens from being scratched, I put three layers of transparent tape on the back of the lens where it contacts the screw. In order to prevent the screw from loosening, I didn't use a nut, but directly drilled a hole with a diameter slightly smaller than that of the screw on the bottom plate, screwed the screw in, tightened it with the elasticity and tension of wood, and at the same time, it can be easily adjusted with the help of a screwdriver. The angle iron connecting the lens barrel and the bottom plate must be firm. I chose an angle iron with a thickness of 2.5 mm and a width of 15 mm, and connected it to the bottom plate with two bolts (the strength is much greater than that with wood screws directly), and also connected it to the lens barrel with bolts. The holes drilled in the lens barrel and the angle iron should be aligned, and the aperture just passes through the fixing screw, so as to ensure that the relative position of the objective lens and the lens barrel will not change every time the objective lens holder is installed in the future, and lay a good foundation for adjusting the optical axis in the future. According to the actual situation, different methods can be used to make anti-skid wood blocks and anti-overturning wood chips. Attention should be paid to ensure the safety of the objective lens, and at the same time, the objective lens should have a certain free space.
Eyepiece focusing base
The position of the eyepiece focusing base is determined by the diameter of the main lens barrel, the focal length of the main lens and the distance from the focal plane of the main lens to the main lens barrel. You can draw a map to scale and then measure the specific position from the map.
The eyepiece focusing base is required to support the eyepiece stably within a certain range (2 ~ 3 cm) and facilitate focusing. Its axis (that is, the axis of the eyepiece) is required to be as vertical and intersecting as possible with the axis of the main lens barrel. If you plan to take pictures with a camera in the future, it must also have enough strength.
If it is difficult to fix the eyepiece focusing base on the circular lens barrel, it can be divided into two parts: first make a plane, and then fix the eyepiece focusing barrel on the basis of this plane.
How to make a plane? It is a simple and effective method to find a short section of aluminum profile in the decoration shop and fix it on the outer wall of the main lens barrel with bolts (as shown below). Note that it is best to find aluminum profiles with a thickness not less than 1 mm, so that the strength can be guaranteed.
There is another way to make a plane: fix a support plate on the inner wall of the main lens barrel (see the picture below). Generally, there is a gap of 1 ~ 2 cm between the main mirror and the lens barrel, so there is no need to worry that the support plate and the eyepiece focusing base will block the light of the main mirror.
This is my method. The supporting plate is made of 120mm× 100mm× 2mm steel plate (see the figure below). Both sides are bent, and four mounting holes are punched on each side. Then make corresponding holes in the lens barrel, and fix the support plate on the inner wall of the lens barrel with bolts. Considering that the camera will be connected in the future, the tray will be more stressed, so there are more mounting holes and thicker materials. If the axis of the eyepiece focusing cylinder is found to be a little crooked, the thickness of the gasket used for each bolt can be changed. (There is a long "mounting hole for the secondary mirror bracket" in the picture, which is to prepare for the next installation of the secondary mirror. )
With the plane, the eyepiece focusing base is easy to fix. You can make a flange with an aluminum pipe truck and then fix it on the plane with bolts. As for focusing, you can use pull focusing, and fix it with top wire after adjustment, and the actual use effect is also good.
Secondary mirror bracket
There are two basic requirements for installing the secondary mirror. First, its direction and position can be adjusted within a certain range, which is to prepare for the adjustment of the optical axis in the future; Second, it is firmly fixed, avoiding the trouble of frequent repositioning in the future, so that we can have more energy to enjoy the beautiful starry sky brought by the telescope.
Here is a design, which is based on the support plate mentioned above, paying attention to the adjustability of the secondary mirror in all directions and giving consideration to reliability. Refer to the figure below for details.
The sketches of the four parts used are as follows:
The assembly method is as follows: insert one side of the T-shaped body into the groove of the cylinder, and connect the T-shaped body and the cylinder with M3 bolts. The cylinder and the auxiliary mirror support rod are connected by a connecting piece, and a knocking end of the auxiliary mirror support rod is fixed in the auxiliary mirror support rod mounting hole of the support plate by two nuts.
The mounting hole of the secondary mirror support rod is actually not a hole but a groove, and the secondary mirror support rod can move left and right; The connecte piece can slide up and down along that supporting rod of the secondary mirror; The cylinder can move back and forth in the hole of the connector and rotate left and right; The secondary mirror can rotate around the bolt of the cylinder to adjust the elevation angle. The convenient adjustment of the direction of the secondary mirror lays the foundation for the accurate adjustment of the optical axis in the future.
The above design requires high processing conditions, while Mr. Zhang Daqing's design is relatively simple.
Find a long piece of iron and connect the lens barrel with both ends bent at 90 degrees; Find a piece of wood, cut a slit in the center of one end, clamp the long iron piece, and cut the other end into a 45-degree inclined plane; The fixture of the secondary mirror is oval, which is equivalent to the size of the secondary mirror. Four claws extend from all sides, and after bending 90 degrees, you can catch the secondary mirror. The secondary mirror clip is cut with thin iron sheet and connected with the wood block with two wood screws.
This design can be completed with very common tools, and the shielding of the main mirror is very small; As long as the machining is accurate, leave some allowance when punching, and it is not a problem to adjust the optical axis later.
At this point, the design and manufacture of the lens barrel is completed. Before use, it is best to remove the primary and secondary mirrors, and spray a layer of black matte paint (available in decorative materials stores and canned, with a price of about 16 yuan) on the inner wall of the lens barrel. The effect is acceptable.
Second, the production of picture frames
For the 20 cm reflective telescope, if there are not enough equatorial telescopes, then we should not hesitate to choose a horizontal support called Dobson structure.
Is this building the John of America? Dobson invented it in 1970s, which is simple, light, stable and practical, and has long been popular all over the world.
The following is an exploded view of Dobson structure. It mainly has three parts:
Ear (upper left)
The Ear is the axis that the telescope rotates in the vertical direction. Round plastic or aluminum blocks with a diameter not less than 10 cm can be symmetrically fixed on both sides of the center of gravity of the lens barrel and directly fixed on the main lens barrel, or a wooden frame can be placed on the lens barrel and the ears can be fixed on the wooden frame (this can adjust the position of the ears and is more conducive to the balance of the main lens).
Box (above)
Made of wood, there are two V-shaped grooves in the upper part, which just fit the ears, and the center of the bottom is perforated.
Bottom plate (upper right)
Made of wood, three protrusions (which can be made of plastic) are evenly distributed with a shaft in the middle.
When in use, the box is placed on the bottom plate and supported by three plastic blocks, and the shaft on the bottom plate passes through the central hole at the bottom of the box, so that the box can flexibly and stably rotate 360 degrees horizontally around the shaft of the bottom plate; Put the ear of the lens barrel on the V-shaped groove of the box, and the lens barrel can rotate vertically within 90 degrees. In this way, the Dobson stent is completed.
As long as the three plastic blocks on the bottom plate are separated and the friction coefficient of each contact surface is appropriate, the Dobson device is very convenient to use and the telescope can rotate flexibly when looking for the target. Once the target is found, the telescope will not bounce or shake once it is released. The practical application shows that the target is still very stable in the eyepiece field of view even in the case of high magnification.
Not being able to track automatically is a disadvantage. Many foreign fans add motors to their two shafts, and the motor speed is controlled by computer to realize automatic tracking, and the effect is good. Those who are interested may wish to have a try.
Third, the adjustment of the optical axis.
After the telescope was built, when we put into observation hopefully, we found that the image quality was flat and even the stars could not converge into one point. Don't doubt that there is something wrong with the mirror at this time. The problem is probably only with the lens assembly. After readjusting the optical axis, the telescope may present a completely different scene.
The imaging of parabolic mirror has a characteristic, which is perfect on the optical axis and has no aberration, but there will be obvious coma when it leaves the optical axis (the star point has a small tail). On the optical axis, using the eyepiece of the general field of view, the star point in the center of the field of view is very sharp, but in fact the aberration at the edge of the field of view is not easy to detect. If it is outside the optical axis, the stars in the whole field of view may be unreal, and the farther away from the optical axis, the more serious this situation is.
How to adjust the optical axis?
When the two optical axes in the mirror optical system: the optical axis of the primary mirror (objective lens) and the optical axis of the eyepiece all pass through the same point on the secondary mirror and completely overlap after being reflected by the secondary mirror, that is, become one optical axis, the optical axis is adjusted.
Without inspection means, we can judge whether the optical axis is adjusted by actual observation. Find a sunny night with good atmospheric tranquility, and look at a star with the highest magnification of the telescope (the diameter of the main mirror is expressed in millimeters) (if there is no equatorial telescope, you can look at the North Star). Place the star point in the center of the eyepiece field of view (to reduce the aberration caused by the eyepiece), and carefully adjust the focal length, from out-of-focus adjustment to focusing, and then to focusing. If there is no problem with the adjustment of the optical axis, you can see a series of images from left to right as shown below (the circular ring in the figure is caused by the diffraction of light, and the shadow of the secondary mirror and its bracket will actually be seen after defocusing, which is not shown in the figure).
Whether the star image is really condensed, thin and sharp at the focus and whether the diffraction ring is concentric after defocusing all reflect the imaging quality of the telescope. If you can see several diffraction rings after defocusing, but they are not as perfect as the above picture, and there are some "burrs" around them evenly, which means that the mirror accuracy is slightly poor, but the optical axis is well adjusted. If the star point becomes a small fan after defocusing, the star image moves in the field of view of the eyepiece, and the divergence direction of the fan remains unchanged, indicating that the optical axis of the telescope needs to be adjusted.
Optical axis adjustment steps and auxiliary tools
Optical axis adjustment can be performed as follows:
1. Adjust the eyepiece focusing barrel to make it perpendicular to the axis of the main lens barrel.
2. Adjust the secondary mirror so that it is located on the axis of the main lens barrel.
3. Adjust the secondary mirror directly below the eyepiece focusing tube.
4. Adjust the direction of the secondary mirror so that the optical axis of the eyepiece points to the center of the primary mirror after being reflected by the secondary mirror.
5. Adjust the direction of the primary mirror so that its optical axis coincides with the optical axis of the eyepiece.
The above is just a general way to adjust the optical axis. There will be some problems in the specific operation process, and sometimes it is difficult to control the accuracy. Here are a few assistive tools:
1. Peephole with double crosshairs:
The outer diameter of the tube is the same as that of the eyepiece interface. Cover one end of the tube, dig a round hole with a diameter of 3~4mm in the center of the cover, and draw the other end of the tube symmetrically with a white cotton thread, and the distance between the two lines is 3 ~ 4 mm. The length of the lens barrel is determined as follows: put the peephole (outside the peephole) into the eyepiece focusing barrel, with one end of the peephole flush with the external port of the eyepiece focusing barrel, and one end of the double crosshair is away from the secondary mirror 20.
There is no limit to the material of the peeping tube (if you use the 3 1.7mm eyepiece interface, you can consider using the black box of Kodak film to make the peeping tube). The key is to be stable after inserting the eyepiece focusing tube, and not to shake too much. To straighten the double reticle, the connecting line between the small square at the intersection and the peephole should be the axis of the eyepiece focusing tube.
2. The primary mirror center positioning point
Cut a piece of black paper with a diameter of 5mm and stick it right in the center of the objective lens with double-sided tape. (Because the central area of the primary mirror does not participate in imaging, this black spot will not have a negative impact. )
3. Crosshair at the opening of the main lens barrel
Draw a crosshair at the opening of the main lens barrel with thick lines, requiring that the two lines are perpendicular to each other and the intersection point passes through the axis of the main lens barrel. (Pulling the reticle at the opening of the primary mirror may affect the work of the secondary mirror, so it is best to mark the positions of the four intersections between the reticle and the lens barrel. If you feel that the crosshair is in the way, you can remove it first and then pull it if necessary. )
These three tools are not complicated to make, but you will soon find them very useful. With their help, now we can start to adjust the optical axis of the telescope step by step.
0. Preset the direction of the primary mirror.
Remove the secondary mirror and adjust the bolt behind the primary mirror until the image formed by the intersection of crosshairs, the black dot in the center of the objective lens and the intersection of crosshairs is in a straight line when viewed from the front of the lens barrel opening, indicating that the primary mirror is basically pointing correctly. There is a special step below to adjust the main mirror. Joining this step in advance can make the later operation easier. )
1. Adjust the eyepiece focusing barrel to make it perpendicular to the main lens barrel.
Put the peephole into the eyepiece focusing barrel, and observe from the peephole, you can see that the line from the peephole to the double crosshair (actually the axis of the eyepiece focusing barrel) is extended again, and it will intersect with the wall of the main lens barrel at a certain point. Mark this point, measure its position with a ruler, and then refer to the position of the eyepiece focusing barrel in the main lens barrel to judge whether the eyepiece focusing barrel is perpendicular to the main lens barrel.
2. Adjust the secondary mirror so that it is located on the axis of the main lens barrel.
Remove the peephole, install the secondary mirror, and roughly adjust the direction of the secondary mirror, so that the eyes can see the image of the primary mirror reflected by the secondary mirror from the eyepiece focusing tube, and also see the image reflected by the secondary mirror and the reticle twice. From these images, we can see the relative position of the secondary mirror and the reticle. If the center of the secondary mirror coincides with the intersection of the reticle, it means that the secondary mirror is located on the axis of the main lens barrel, otherwise it needs to be adjusted accordingly.
3. Adjust the secondary mirror directly below the eyepiece focusing tube.
From the direction of the eyepiece focusing barrel, the secondary mirror is obviously located below the focusing barrel, but the accuracy after this viewing angle cannot be guaranteed. At this point, when installing the peephole, the outermost circle is the inner wall of the peephole (the double crosshair doesn't work now and can be ignored), and the secondary mirror is in the middle. If the contour of the outer circle of the secondary mirror and the contour of the inner wall of the peeping tube are concentric circles, the requirements are met, otherwise the secondary mirror should be adjusted in the axis direction of the primary mirror. (If the peephole is too small and the light is too dim to see clearly, you can stick a piece of white paper on the wall of the main lens barrel opposite the peephole. If the peephole is too thin, the length of the peephole can be lengthened or shortened. )
4. Adjust the direction of the secondary mirror so that the optical axis of the eyepiece points to the center of the primary mirror after being reflected by the secondary mirror.
On the basis of the previous step, while observing from the peephole with eyes, adjust the direction of the secondary mirror until the outline of the outer circle of the image formed by the primary mirror in the secondary mirror is concentric with the outline of the outer circle of the secondary mirror.
5. Adjust the direction of the primary mirror so that its optical axis coincides with the optical axis of the eyepiece.
Illuminate the double crosshairs of the peephole with a flashlight. When the eyes look through the peephole, they can see the image formed by the double reticle, and the image formed by the central point of the primary mirror and the double reticle after two reflections. Adjust the bolts on the back of the main mirror to make the above three concentric.
At this point, the optical axis of the mirror has been adjusted. The image that can be seen from the peephole is given below for reference.
In the above adjustment steps, according to the different design of the auxiliary mirror bracket, the next operation will have more or less influence on the result of the previous step, so you can go back to the previous operation if necessary, and it may take many iterations to finally get a satisfactory result. It will take some time to adjust the membership fee for the first time. Once adjusted, as long as the secondary mirror bracket is stable, the future work will be much easier. Even if the main mirror is reinstalled for transportation, it is generally only necessary to adjust the bolts behind the main mirror. With the help of the peephole, the telescope can be quickly adjusted to the best state.
Last but not least, it is generally considered that the intersection of the optical axis and the secondary mirror is at the center of the secondary mirror. In a telescope with long focal length, you can think so, but in a Newton-type reflective telescope with large aperture and short focal length, the size of the secondary mirror is also larger, and the distance from both ends of the long side of the secondary mirror to the eyepiece can no longer be approximately considered the same. Please refer to the following schematic diagram:
The optical axis intersects the point B of the secondary mirror, not the point A where the center of the secondary mirror is located. This is equivalent to the displacement of the secondary mirror from the central position to the primary mirror and away from the eyepiece. The displacements in these two directions can be calculated by the following formula:
Displacement = short side length of secondary mirror /(4* focal length ratio of primary mirror)
For example, if the short side of the secondary mirror of my telescope is 35mm and the focal length ratio of the primary mirror is 5, then the displacement in both directions is1.75mm..
If there is such a telescope with short focal length, this situation needs to be taken into account. Calculate the displacement. During the second adjustment, the secondary mirror should be slightly away from the eyepiece. The third step is adjustment. When we see that the contour of the outer circle of the secondary mirror and the contour of the inner wall of the peeping tube are concentric circles, in fact, the secondary mirror has shifted to the direction of the primary mirror, and no additional adjustment is needed.