TELESCOPE COLLIMATION
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Collimation of a Newton system telescope using a pinhole collimator.



  Telescope collimation is aimed at correct alignment of: telescope mirrors, telescope structure, focuser.


  It is better to learn collimation on a large telescope, because access is easier and everything is easier to see.


  The tutorial has been divided into 3 independent parts, click on the name to be transferred to the appropriate department.


1. SIMPLIFIED COLIMATION - FOR COMPLETE BEGINNERS

2. EXTENDED COLIMATION - FOR EXPERIENCED

3. ULTRA COLIMATION - FOR ADVANCED





TELESCOPIC COLLIMATION - simplified - for complete beginners

It will be a simplified collimation of the telescope, its aim is to obtain a decent level, which is enough at the beginning.



Technical elements - for beginners


        -- Collimator --

For the collimation we will use a very simple and free collimator, which you can make on your own.

Model I (old version) - We need a box for a photo film, if we do not have one, you can go to a photographer in the area and ask for an empty and unnecessary one. There are several types of boxes, but not all of them fit the 1.25" eyepiece extractor, we need a version with a smaller diameter and smooth walls. Cut out the bottom of the box and make a small hole with a diameter of 1-3 mm in its lid, exactly in the middle. On the inside of the lid, glue a reflective ring in the form of a flat technical washer used for bolts and nuts.

  1-1


Unfortunately, a sign of the times, clich├ęs are a thing of the past, it is more and more difficult to obtain their boxes, so ...

Model II (new version) - We need the right juice in a bottle, we are interested in its cap. Fortunately, many models of caps have an inner ring that fits perfectly with the 1.25" astronomical eyepiece sleeves, so it is enough to provide such a cap with a hole and a reflective ring, and then press it on the acquired sleeve and we have a ready-made collimator.
Many products have a cap very close to the size we need, but they are a bit too tight, and that's enough to disqualify them from our applications. This correct cap size will fit all 1.25" sleeves, even the better and worse products, and as you know, the sleeves can differ in diameter, despite the standardization of their size.

We can leave the cap in its original form, but having a bit of enthusiasm, we can cut off the outer ring and transform it into the object presented below.

  1-2


By the way, these screw caps are cool plugs for astronomical glasses with a diameter of 1.25".


You can also make a collimator from an old astronomical eyepiece, print it on a 3D printer, buy it, I leave this issue to your resourcefulness.


        -- Reflective ring --

    Regardless of the version of the collimator, we stick a reflective ring on the underside of the lid, shiny but porous, because this type of surface acts as a reflector used on clothes, as it catches light from different sides. When the used reflective ring is smooth as a mirror, it only reflects the directed light, it only reflects when it is perfectly in tune with our eyes and the light source, which makes it 99% dark for us, so it is not useful. It's something like a watch glass reflecting the sun's rays, when it flashes in our eyes only when it is perfectly aimed at our eyes, at other angles, nothing flashes for us.
    The size of the used reflective ring matters, which you can read more HERE below.   (If necessary, you will come back here with the BACK button in your browser)

We use this type of collimator installed in an eyepiece extractor just like an astronomical eyepiece.




        -- Main mirror center marker --

    To collimate the telescope, we will need a marker in the center of the main mirror. We use two types of simple markers, a sticker in the shape of a circle - , or a sticker in the shape of a ring - O . If you do not have any marker in the form of a sticker on the mirror, you should stick it yourself. Unfortunately, for this purpose, we have to pull the main mirror out of the telescope tube.
    I make markers from black stickers I bought at a stationery store. Place the marker as precisely as possible in the center of the main mirror, while taking care not to touch, soil or damage its aluminum coating.
    We place the marker once and leave it there permanently, because it will be needed for the next collimations. It does not adversely affect the image obtained by the telescope, because the center of the main mirror itself does not participate in generating the image, because it is obscured by the secondary mirror.
    For collimation with a pinhole collimator, a circular marker is enough, for collimation with a laser collimator, a ring marker is necessary due to the need to reflect the laser beam in the ring hole.

  Before you take any action, please read the description below.

  2-1    circle marker


  2-2    ring marker



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------------ Reflective ring in the collimator and marker on the main mirror ----------------


I will describe to you a little more about the dependencies of these two elements. The presented graphics will be on a large scale to make it easier for you to see what I am writing about.

The size of the reflective ring in the collimator determines the size of the marker on the primary mirror and vice versa, the size of the marker on the primary mirror determines the size of the reflective ring in the collimator. If we do not have a marker on the main mirror yet and are just installing it, we can afford to adjust these two elements according to our preferences. If we already have a marker on the main mirror, we have to choose the appropriate reflective ring in the collimator.

    The components are described below.

  2-3


1 - Reflective ring.
2 - Hole in the collimator (through which we look).
3 - Circular marker.
4 - Ring marker.

Analysis of the issue.

We have two sizes of the reflective ring in the collimator and two types of markers on the main mirror to consider, one marker is circular and the other marker is ring.

As can be seen below, the combination of reflective ring A and marker 1 (A + 1) is not a happy combination, due to the fact that in the collimation process, with small inaccuracies, it is difficult to assess their fit. As you can see, a much better combination is the combination of reflective ring B and marker 1 (B + 1), because you can clearly see their mutual shifts and correct fit (B + 1 center).
Another unfortunate combination is the reflective ring A and marker 2 (A + 2), it is difficult to assess the correctness of collimation, the objects cover each other, in this case it is worth using a larger reflective ring, i.e. B (B + 2). We can clearly see how reflective ring B and marker 2 form an easy to observe duo (B + 2 centers).

I hope it is already clear what to consider when choosing the type and size of the marker on the main mirror and the collimator's reflective pad.

  2-4






        -- Lighting --

    During collimation, the telescope can be pointed at the bright daytime sky or a light bulb in a room under the ceiling, but the best results will be obtained by shining a bright fluorescent lamp into the telescope tube, in the place indicated in the graphic below. The fluorescent lamp strongly illuminates the main mirror, and by the way, the collimator from the inside, which gives us full control over what is happening in the telescope's collimation process.

    ATTENTION - I use an energy-saving spiral fluorescent lamp (in a shield, as below), because it gives the most favorable lighting, but you should know that breaking it (especially when switched on) is very dangerous, due to the mercury vapor that will then be released into the environment. The problem concerns all fluorescent lamps, including those under your ceiling and in lamps. An additional disadvantage of such a solution is the need for mains power supply from a socket, which in the case of field work is a serious obstacle.

  3-1






        -- Attaching the main telescope mirror - The main telescope mirror --


  The construction of the telescope requires that it is possible to tilt the main mirror (to a small extent) in different directions, for this purpose, the main mirror mounting consists of two independent elements. The first part of the mount is stationary and is permanently attached to the telescope tube. The second part of the mount is movable and the main mirror is attached to it. Both elements connect three points with an adjustable distance, changing their length makes the movable element of the mount (and with it the main mirror of the telescope) freely tiltable in relation to the fixed element.
To make it easier, let's imagine a stool on three legs, where the seat will be a mirror and each leg as a support point, by adjusting the length of the legs, we can tilt the stool in different directions.
The main mirror mountings have different designs, they are solid and openwork circles, there are three-arm or triangular frames, but they all have some common features, namely, the way of handling the three adjustable support points mentioned earlier. So that you know how to use these devices, I have prepared the following basic version descriptions.


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    Version 1. Main mirror mounting adjustable by means of three bolts, on which are deflecting springs.

  This type of mounting the main mirror is the most pleasant for collimation, but it does not guarantee stiffness, therefore, especially with larger mirrors, it will not work well in astrophotography. The movable part of the main mirror mounting, under the influence of the mirror's weight, can slide downwards.
Collimation is easy, because it consists in turning only three screws, and screwing them in or out tilts the main mirror mounts together with the mirror in the desired direction. Strong springs strung on the bolts constantly erase the clearances, they expand when unscrewing the adjustment bolts, and shrink when the adjustment bolts are tightened.

  4-1



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    Version 2. The fixing of the main mirror is adjustable with three pairs of screws, each pair has a pull screw and a push back screw, the fixing has six screws in total.

  This type of mounting the main mirror is difficult to collimate, but it guarantees considerable stiffness, therefore, especially with larger mirrors, it will work well in astrophotography.
The fixture of the main mirror has three pairs of bolts in each pair, one bolt attracts the moving part to the fixed part and the other bolt pushes the moving part away from the fixed part, thus eliminating the resulting play.
Collimating this type of mounting the main mirror is difficult because it consists in skilfully turning six screws, and screwing them in or out tilts the movable part with the mirror in the desired direction. Unfortunately, in order to move the main mirror further away, you first need to loosen the attracting bolt, and the resulting play to be removed by tightening the push-off bolt. In contrast, to attract the primary mirror, you first need to loosen the repulsion bolt, and only then can the movable portion of the mounting of the primary mirror be attracted by the pull bolt.
When we attract the moving part of the main mirror mounting with the attracting bolt without loosening the repulsive bolt, we will break the thread, because the moving part of the main mirror mounting resting on the repulsive bolt has nowhere to move.


The graphic below shows two common configurations for this type of attachment.

  4-2


  4-3




The animation below shows the operation of a pair of bolts. The pull screw is at the top. The push back screw is at the bottom.
  4-4



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    Version 3. Main mirror mounting adjustable by means of three bolts, with repulsion springs, but additionally stiffened with push-off bolts.

The fixing of the main mirror has three attracting bolts with springs (every 120°), and alternating between them, three push-off bolts (every 120°), which together create support points every 60°. This is a poor version of the main mirror mounting.
This type of main mirror mounting is difficult to collimate, but gives an average stiffness value, so, especially with larger mirrors, it will somehow work in astrophotography, but the collimation of this type of main mirror mounting is a nightmare.

  4-5


  The collimation of this type of main mirror mounting is difficult and consists in initially withdrawing the repulsion bolts (see the graphic below), then adjusting with three bolts with springs. Unfortunately, with larger and heavier mirrors, the springs do not manage to push out the moving part of the main mirror mounting, which sometimes, when removing the screws, ends with the necessity of pushing the movable part of the main mirror mounting with your thumbs. Yes, you read that right, sometimes you have to help yourself with your thumbs. Once we manage to somehow set the main mirror properly, it's time to wedge it with repulsive screws. Unfortunately then, due to the non-rigid fixation of the main mirror, and the fact that the push bolts are far from the pull bolts (staggered at 60 degrees), when we start to tighten the push bolts, what we have achieved so far is lost due to deformation of the hopeless mount main mirror.

How to deal with this cell?

As I wrote, first we remove the repulsion bolts (as below), then with the screws with springs we collimate the main mirror to the middle hole of the secondary mirror mounting (of course, after unscrewing the secondary mirror with the housing from the mounting).

  4-6



And when we get the perfect position of the main mirror, slowly screw the repulsion screws until they gently touch their tips to the moving part of the main mirror mounting, as shown in the graphic below.

  4-7


  Next, we tighten each repulsion bolt half a turn, look through the secondary mirror's central mounting hole, how much we've broken the primary mirror collimation, and then, sensitively, only tighten the repulsion bolts that give us the desired marker displacement to recover what we have broke down. Due to the fact that at this stage (in this type of mounting) the reactions of the main mirror are absolutely unpredictable, it is best to do it with two people. One operator looks through the hole and commands the other operator (That screw! Another screw! Twist a little more! Stop! Undo what you just did!). It's much faster and easier that way. In this type of attachment, it is better that the primary mirror, once collimated, is not moved at all during the use of the telescope, unless you want to ... disassemble the secondary mirror again and repeat the whole procedure again.


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  Okay, you will say, we tilt this mirror in all directions, but for what purpose?
The main mirror of the telescope has an optical axis (invisible to us) extending from its center. Our task in the collimation process is to direct this optical axis to the collimator opening located in the eyepiece extractor.


  5-1




We aim the optical axis of the main mirror at the target like a laser, and as you know, one animation is worth more than a thousand words, so see below. By adjusting the main mirror's cell, we cause it to tilt to different sides, our task is to direct the optical axis of the main mirror (purple line coming from the center of the mirror) to the right place of the secondary mirror, to finally hit the collimator hole.

  5-2






        -- Mount the secondary mirror of the telescope - Secondary mirror of the telescope --


  In the telescope, the main mirror projects the image straight ahead, i.e. towards the inlet of the telescope's tube. But we do not want the image there, we need it on the side, in the eyepiece extractor, and to achieve this, the image must be directed there by means of a second, additional flat mirror. The secondary mirror, because this is the name of the additional flat mirror in the Newtonian telescope, accomplishes this task. The secondary mirror is mounted at the inlet of the tube, but its mounting is quite specific, as it must provide adequate support and at the same time cover as little as possible the inlet of the tube. The most common are the three-leg and four-leg mounts. Moreover, this mount, as well as the primary mirror mount, must also be able to adjust the secondary mirror in different positions, and for this purpose, typically has four screws. The middle screw attracts the secondary mirror body to the mount and the three outer ones push it away. Depending on the desired secondary mirror movement, loosen and tighten specific screws.

The animation below shows the basic movements of the secondary mirror.

  6-1



By collimating the secondary mirror, we cause (among other things) its tilting to different sides. Our task is to position the secondary mirror so that the optical axis of the main mirror (the purple line coming from the center of the main mirror) points to the collimator opening.

One animation replaces a thousand words ...
  6-2








---------- Telescope collimation - simplified - for beginners - DESCRIPTION ----------


    At the moment, due to the fact that your experience does not allow for more, we will focus on simplified collimation. The time will come for more advanced activities.


- Collimation of the main mirror is performed at the telescope's tube placed vertically.

- Collimation, disassembly and assembly of the secondary mirror is done with the telescope tube positioned horizontally, thanks to which, in the event of a moment of inattention, the secondary mirror will not fall directly onto the main mirror.

- We keep the tools firmly and we take our time.



        | THING IS IMPORTANT AT THE BEGINNING |

    The collimation described below should be made with the collimator hole located at the point of focusing the telescope, otherwise it may give inaccurate results.

    How to determine where the focus is in the telescope, see HERE.

    However, if meeting this condition proves to be too difficult for you, set the eyepiece extractor to the middle of the working range and then collimate the telescope.




    We put the collimator with the hole into the eyepiece extractor (just like we put the eyepiece), set the eyepiece extractor according to the remark above and look through the aperture.


  It is possible that our telescope is well collimated, then our eyes will see the view as shown in the graphic below.

  7-1




  The graphic below explains what exactly we are looking at.

  7-2
  1 - Secondary mirror holder.

  2 - The inner wall of the telescope tube.

  3 - Secondary mirror.

  4 - Reflection of the primary mirror in the secondary mirror.

  5 - Reflection of the secondary mirror and its mountings in the primary mirror, again reflected in the secondary mirror.

  6 - The inner wall of the spectacle extractor sleeve.

  7 - Collimator cover seen from the inside.

  8 - Collimator reflective ring.

  9 - Circle marker in the center of the primary mirror.

    But in this inconspicuous graphic there are several key relationships, in order to correctly collimate the telescope, you must be aware of their existence.
  Namely ...


  The distances between the edge of the secondary mirror and the inner wall of the eyepiece should be the same.

  7-3




  The distances between the edge of the secondary mirror and the outline of the primary mirror reflected in it should be the same.

  7-4




  TThe distances between the marker on the main mirror and the inner wall of the focuser should be the same.

  7-5




  Only the distances marked below with red arrows do not have to be equal, the visible asymmetry results from the intended asymmetry of the secondary mirror mounting, but for the moment, let's not go into this aspect, you will gain more experience, then you will explore the subject.

  7-6



If, however, you would like to see now what asymmetric secondary mirror offset is, CLICK   If necessary, you will be back here with the BACK button in your browser.


If, unfortunately, you did not see the above view in the collimator, it's time to get down to business, i.e. collimation.





        -- Secondary mirror collimation --


    We will start the collimation from the secondary mirror. The first drawings do not contain the reflections that can be seen in the secondary mirror, because it is not important at this point in the collimation.
The following inaccuracies in the position of the secondary mirror usually occur simultaneously, but for the sake of clarity, we will break them down into individual cases.

    The first common situation is shown in the graphic below. As you can see, the secondary mirror is shifted too high in relation to the walls of the eyepiece extractor, sometimes it is shifted too low, and should be placed centrally. The black ring in the graphic is the inner walls of our spectacle lens. The distance marked with red arrows should be the same, to achieve this, in this case, slightly unscrew the green screw and screw in the blue one. Your bolt arrangement may be different, and this case only illustrates the problem.
    You should also remember to loosen some screws first and then tighten others, nothing by force, everything with feeling.

  8-1  See what you are striving for.


    Another common view is shown in the graphic below. As you can see, the secondary mirror is shifted to the left, it is placed too shallow in the tube (because the depth of the telescope tube is on the right and the outlet is on the left), by analogy, when the secondary mirror is shifted to the right, it is too deep in the telescope tube. The secondary mirror should be centrally located, the distance marked with red arrows should be the same, to achieve this, in this case shown in the graphic, remove the red center screw and screw in the three outer screws, green, yellow and blue. As a result of our actions, the mirror will move to the right (deeper), in the case when the mirror is too deep, the actions should be performed in the opposite way, analogous to the situation.

  8-2  See what you are striving for.



  8-3   A correctly positioned secondary mirror in relation to the walls of the eyepiece extractor looks as below.




    Now you can pay attention to what you can see in the reflection of our secondary mirror, and the view below is often presented. The secondary mirror is twisted in its axis and to correct this, it is best to loosen the central (red) screw very slightly, but this should be done so as not to lose what we have previously set, then, firmly grasp the body to which the secondary mirror is attached, and gently turn the whole thing in the direction you want. In the case presented in the graphic below, in the direction indicated by the arrow, otherwise, analogous to the situation.

  8-4  See what you are striving for.




    This time, the secondary mirror with the mounting is tilted towards us, to correct it, you should (by turning the collimation screws appropriately) tilt it away from us.

  8-5  See what you are striving for.                                                                                                                                             Below is a top view.




    This time the secondary mirror with the mounting is tilted away from us, to correct it, you have to tilt it towards us (by turning the collimation screws accordingly).

  8-6  See what you are striving for.                                                                                                                                             Below is a top view.




    If we got the view below, it means that we have done everything correctly and our activities at the secondary mirror are already finished.

The graphics did not include what is reflected in the primary mirror reflected in the secondary mirror, as it was irrelevant at this point in the collimation.

  8-7






        -- Primary mirror collimation --


    Now we can take a look at what can be seen in the reflection of the primary mirror reflected in the secondary mirror (graphic below).
We will see a bright background there, sky, ceiling, or something else, depending on what we aim the telescope at. Against this background, you can see the mounting of the secondary mirror, and the secondary mirror itself, most likely shifted relative to the outline of the primary mirror, and the central marker on the primary mirror (black dot). We also see, from the inside, the lid of our collimator and a reflective ring glued to it, and in the center of the reflective ring, a hole, the one through which we look.

  9-1    See what you are striving for.


  As a reminder, a collimator.


  To correct the inaccuracies disclosed above, we will rotate the primary mirror cell screws this time, bearing in mind the handling shown and described at the beginning of this tutorial. We make small turns with the screws, constantly checking the view in the collimator to know what the effect of our actions is. If the reflexive ring of the collimator and the marker on the main mirror come closer to each other as a result of our rotation of the screws, we continue our activities until the marker of the main mirror is perfectly in the center of the ring in the collimator. If, however, as a result of our actions, the marker and the ring move away from each other, then we have to turn the screws in the opposite direction. Some time we will wander, and the reflexive ring of the collimator will not always want to get closer to the marker, but after getting into practice, setting them up will only take a moment. If the telescope is so large that we cannot turn the screws and watch what is happening at the same time, it is worth using the help of another person, we are in command, and the helper rotates the screws until a satisfactory effect is obtained.


A correctly collimated telescope in the collimator looks like this.

  9-2



But wait ... you call ... it is asymmetrical there, the outline of the secondary mirror is shifted to the right, as shown by the red arrows in the graphic below.
You are right, but it has to be this way, everything is correct, this outline will be shifted slightly towards the main mirror, this is normal, this asymmetry is due to the intended asymmetry of the secondary mirror mounting. CLICK   If necessary, you will be back here with the BACK button in your browser.

  9-3



  Finally, remember that:
- not all secondary mirrors set at an angle of 45° form a perfect circle.
- the secondary mirrors are not always glued at the factory with a perfect offset.
- tubes, even of expensive telescopes, are sometimes ovoid and wavy.
- mounting the eyepiece extractor on the telescope tube, perfectly correct, is a real feat.

The real world is not perfect, only the demo graphics are pixel-accurate, so not every telescope can be perfectly collimated. When you get a decent result, be proud of yourself and don't be agonized by small inaccuracies.
Such a small digression :)



  9-4  As a reminder, description of the items.
  1 - Secondary mirror holder.

  2 - The inner wall of the telescope tube.

  3 - Secondary mirror.

  4 - Reflection of the primary mirror in the secondary mirror.

  5 - Reflection of the secondary mirror and its mountings in the primary mirror, again reflected in the secondary mirror.

  6 - The inner wall of the spectacle extractor sleeve.

  7 - Collimator cover seen from the inside.

  8 - Collimator reflective ring.

  9 - Circle marker in the center of the primary mirror.







TELESCOPE COLIMATION - extended - for experienced


    The simplified collimation description presented above is correct, it allows a beginner astronomy adept to take courage in adjusting the optics of a newly acquired telescope, however, to make a more precise collimation, you need to rummage in our equipment even deeper, unfortunately, this requires more knowledge, courage, time and self-denial.



Technical elements - for advanced users



        -- !!! RIGOR !!! --

    Telescope collimation performed with a pinhole collimator, in the scope of the test:

    - the circumference of the secondary mirror in the inner circumference of the eyepiece sleeve

    - the contour of the main mirror reflected in the secondary mirror

        and

    - collimation of the telescope with a camera

    It MUST be done in the FOCUS of the telescope.   

    In the case of a collimator, a hole located at the focus of the telescope. In the case of a webcam, a lens placed at the focus of the telescope.

    How to determine where the focus is in the telescope? Read HERE. Why do you have to meet this condition? Read HERE.




        -- Secondary mirror Offset --

What is secondary mirror shift? I will explain it to you like this ...

    The light from space objects traverses the cosmos, when it reaches us, it enters the telescope as a parallel beam of light rays, say a cylinder.

  10-1



    Such a cylinder of light strikes the main mirror, which is parabolic or elliptical and therefore focuses the light in one place, just like a lens focuses the sun's rays. This point where the rays meet is called the focal point. The cylinder of light reflected from the main mirror turns into a cone of light.

  10-2


    Unfortunately, as you can see in the picture above, the focus is in front of the telescope. It is impossible to see the sky through a telescope by placing your head with the eyepiece at the mouth of the tube, therefore a second, small, flat mirror is added, a secondary mirror that directs the light to the side to the eyepiece extractor.

  10-3


The thing is, the light we want to direct to the side is already a cone, not a cylinder.

    To reflect a cylinder of light at an angle of 90° you need a centrally located elliptical mirror (graphic below - top), but to reflect a cone of light, at an angle of 90° (graphic below - bottom), the elliptical mirror will no longer be mounted symmetrically, it will have to be slightly shifted.
    For easier understanding of the problem, draw any straight cone and divide it with a line at an angle of 45°, one part of the dividing line will always be longer (right yellow), than the other (left red). In a divided cylinder, the right side (yellow) and the left side (red) will be of the same length. So as you can see, in order to cover the entire cone of light with the secondary mirror, you have to move it slightly. By what value? It depends on the size of the main mirror and its focal length.

  10-4



    Below I present two cases, the first, a telescope with an offset secondary mirror, and the second, a telescope with a secondary mirror mounted symmetrically / centrally (no offset).

  A telescope with a secondary mirror having an asymmetric shift.
  10-5



  A telescope with a secondary mirror that does not have an asymmetric shift.
  10-6



    As you can see, the lack of shift of the secondary mirror results in the light escaping from the top (orange circle), which should be directed towards the eyepiece lens, and at the bottom (yellow circle), an idle secondary mirror protrudes.

  A telescope with a secondary mirror that does not have an asymmetrical shift (zoom).
  10-7










        -- Collimating diaphragm at the end of the eyepiece sleeve --


Sometimes during the collimation, looking through the hole of our collimator, it is difficult to tell whether or not the object in the distance is in the center, and then something that would narrow the field of view would be useful. For this purpose, we make a circular diaphragm out of cardboard. To make it, you can use a special compass for cutting out circles, such compasses can be purchased in stationery stores. We cut out a circle from the cardboard with a diameter matching the sleeve of our eyepiece extractor, more precisely, the sleeve inlet from the side of the telescope tube (graphic below - red). In the cut out circle, very precisely, in the very center, we cut a round hole with a diameter of 5-10 mm. This diaphragm, in combination with our pinhole collimator, will provide us with high accuracy of optics alignment, as you will see later. There are collimators in the form of a long tube with a cross made of wires at the end, unfortunately, it is enough to mount them in the eyepiece extractor slightly crookedly so that they do not fulfill their task properly. The diaphragm I am offering you, in a duet with a collimator, absolutely centers the sleeve of the eyepiece extractor.


For the sake of description, I will call it the "red" diaphragm, I hope it's obvious that it can be any color.

  11-1



"Red" diaphragm embedded in the sleeve of the eyepiece extractor.

  11-2







---------- Telescope collimation - extended - for experienced - DESCRIPTION ----------


    The main mirror is collimated with the tube standing vertically. The collimation, disassembly and assembly of the secondary mirror is done with the tube in a horizontal position, so that in the event of a moment of inattention, the mirror does not glide down towards the main mirror. We hold the tools firmly and we take our time.





        -- Centering the secondary mirror mount --

    Secondary mirror attachment, i.e. a place for attaching the secondary mirror located at the mouth of the tube (three-arm or four-arm). Unfortunately, it is often not placed centrally in the factory, in the center of the round tube of the telescope, and it should be, and our task, at the beginning, will be to position this element properly.

    We start by carefully unscrewing the secondary mirror with the base to which it is glued. Then, with an ordinary school compass, we measure the distance from the central mounting hole of the secondary mirror to the walls of the telescope tube (light green arrows in the graphic) to see if the element is centered. If there are differences in distances, they are corrected using the length adjustment screws of the arms of the secondary mirror mounting.

  12-1






        -- Correct setting of the eyepiece extractor --

NOTE - the following method is applicable to telescopes with a full tube tube, while in the case of telescopes with lattice and folding tubes, however, the only way to truly set the eyepiece extractor is THIS method.


    Very often, at the factory, the eyepiece extractor was not installed correctly, our task will be to correctly position this element. But what does that mean correct, and for what purpose are we doing it? In order to collimate the telescope properly, the eyepiece lens must be set so that its optical axis intersects with the optical axis of the main mirror, what's more, it should cross perfectly at right angles. For this, we will try to achieve it with the help of the steps below.

  13-1


   To begin with, we need a reference point to which we can set the eyepiece extractor. For this purpose, in the inside of the tube, exactly on the opposite side of the eyepiece extract, we will set a point at which we aim the optical axis of the eyepiece extractor.

We will determine the location of the marker in two stages:

1. Measure the inner circumference of the telescope tube to find the point where arrows x1 and x2 meet, on the opposite side of the eyepiece extractor. Do as indicated in the graphic below, see the white line with arrows. Using a strip of paper, on the inside of the telescope tube, measure its inside circumference. We shorten such a strip of paper carefully, piece by piece, until we choose its length perfectly. After adjusting its length, take it out of the tube and use a ruler to divide it into two equal sections to mark the place marked with a blue arrow. We put the paper strip back into the tube and with its help we have a designated contact point for the arrows x1 and x2. By transferring the dimension from the paper strip to the telescope tube, we mark this position on the inner wall of the tube. When doing this, I suggest that you do not trace the black paint to the tube, it is better to stick a piece of painter's tape or something similar on this point, and then draw marks on the tape with a pencil.
  Theoretically, we should measure the circumference from the center of the extractor sleeve, but since this sleeve is round and therefore symmetrical, it does not matter that the measurement was made from its outer walls. This is simpler, because we do not have the problem of determining the center of the sleeve (yellow arrows touch, to the right, in the picture below).

  13-2



    Now we can start determining the dimension - y.

2. To do this, use a ruler to determine the identical distances y1 and y2. on both sides of the telescope tube.
First, from the edge of the tube, to the center of the eyepiece lens, we take the dimension y1, and then, on the opposite side of the tube, we measure the same distance, i.e. the dimension y2. (see graphic below)

  13-3



When on the circumference of the telescope tube x1 = x2, and the distance y1 = y2, it means that the cross for setting the eyepiece lens is exactly where it belongs.


  13-4


Having the designated marker, we can start checking whether the eyepiece extractor is set correctly. If it is, then it aims its axis exactly at our marker, if not, it aims somewhere nearby.
So we install our pinhole collimator (in the place where the observation glasses are mounted in the eyepiece extractor) and look into it, which can be seen.

  View in the collimator - marker on the opposite side of the telescope tube.
  13-5


What is what.
  13-6


But it's hard to tell if the marker is perfectly in the middle, we need something to help us make sure ... and for that ....

... we attach the "red" diaphragm as shown in the graphic below. Description of the diaphragm HERE.   If necessary, you will come back here with the BACK button in your browser.

  13-7



  13-8


    The diaphragm is mounted, so looking through the collimator opening, we can only see a small section of the inside of the tube (see the graphic below), the secondary mirror is temporarily absent. Our marker should be placed exactly in the center of this small section / pinhole (graphic below), if this is the case, we have properly attached eyepiece extract, and if it does not, we have incorrectly attached eyepiece extract and we need to correct its position. When we have a model with tilt adjustment, the task is easier, but when we do not have tilt adjustment, we have to act in a different way. This requires loosening our eyepiece and placing flat washers of the appropriate thickness under its corners, until the collimator, diaphragm hole and marker on the telescope tube are aligned, as shown in the graphic below.

  The view in the collimator we are aiming for.
  13-9


  What is what.
  13-10


    After obtaining the above, take out the "red" diaphragm and place the telescope tube VERTICALLY.

Now we will perform a test to confirm whether there is an intersection of the axis of the eyepiece extractor and the axis of the main mirror.

Through the center hole of the secondary mirror mounting, into the telescope tube, using a thread, lower a small weight until it reaches the center of the primary mirror. Secure the weight from the bottom with felt, thanks to which you will be able to put it on the central marker of the main mirror without being afraid of scratching the mirror. The weight does not swing annoyingly on the threads when it is standing. Then pull the thread a little so that it is taut. Now the axis intersection test is ready. When looking through the collimator's opening, you should see a hanging thread, and on the opposite side of the telescope's tube, in the very center, there is a marker. The thread should hang in the middle of the marker.

  View in the collimator (telescope tube positioned vertically).
  13-11



  General view (telescope tube positioned vertically).
  13-12



If the thread is shifted (see the picture below), it means that the secondary mirror mounting is not correctly positioned in relation to the optical axis of the eyepiece extractor, the axes do not intersect. Theoretically, if the legs of the secondary mirror mounting are of the same length and the distance from its center to the edges of the tube is the same around the entire circumference, and the eyepiece extractor is mounted axially, these three elements should fit perfectly, but if they are not, the reason is most likely the deformation of the telescope tube, our errors, or other inaccuracies.

  View in the collimator (telescope tube positioned vertically). Axis intersection alignment error, thread is shifted to the side.
  13-13




    After the secondary mirror mount and the eyepiece extractor have been correctly positioned, it's time to set the primary mirror.





        -- Correct positioning of the main mirror --


  The main mirror is collimated with the telescope tube in a vertical position, because then the main mirror is evenly supported. While observing the main mirror through the center hole of the secondary mirror mounting (the secondary mirror is still missing), position the primary mirror so that the marker in its center is aligned with the hole through which you are looking. To better see the hole through which we are looking, it is worth illuminating the middle ring of the secondary mirror mounting from the inside with a small flashlight (see the graphic below).

  14-1






        -- Correct positioning of the secondary mirror --

This is where it gets tough, as the secondary mirror has so many axes of freedom that its correct positioning is a real challenge.



        -- !!! RIGOR !!! --

    Telescope collimation performed with a pinhole collimator, in the scope of the test:

    - the circumference of the secondary mirror in the inner circumference of the eyepiece sleeve

    - the contour of the main mirror reflected in the secondary mirror

        and

    - collimation of the telescope with a camera

    It MUST be done in the FOCUS of the telescope.   

    In the case of a collimator, a hole located at the focus of the telescope. In the case of a webcam, a lens placed at the focus of the telescope.

    How to determine where the focus is in the telescope? Read HERE. Why do you have to meet this condition? Read HERE.




We will start with the fact that there is some virtual (and real) place in the telescope tube, a point where the optical axis of the main mirror and the axis of the eyepiece slide intersect. Moreover, by assumption, both axes intersect at this point perfectly at right angles, and moreover, it is precisely to this point that the secondary mirror must be precisely placed, at an ideal angle. Our entire collimation now tends to do so. This place is marked with a blue point in the pictures below.

  15-1



The mentioned place is marked with a blue dot.

  15-2



    We proceed to the careful installation of the secondary mirror in the telescope. Now, looking through the collimator itself (without the "red" diaphragm in the extractor), adjust the secondary mirror so that it is in the center of the extractor sleeve and the main mirror marker coincides with the reflective ring in the collimator as precisely as possible. Only the marker is of interest to us at this point.
We put on the circular diaphragm again and only now the inaccuracy in the position of the secondary mirror or its correct positioning will be revealed. The most common is the wrong mounting depth, it should be corrected, in the meantime removing and putting on the "red" diaphragm, until you get the view as in the "Situation 1" below.





        -- Now there will be a study of different situations --



  Situation 1 - Everything is great, we are perfect.

Below, there is a view in a pinhole collimator with an additional diaphragm at the end of the extract..

  16-1 | Everything is equal and symmetrical to each other.
.



  16-2 | The situation in the telescope - perfect.
.



  16-3 | Closeup - perfect.
.FFF




Recommendation? Go HERE   If necessary, you will come back here with the BACK button in your browser.






  Situation 2 - It's not good, we are not perfect.

Below, there is a view in a pinhole collimator with an additional "red" diaphragm at the end of the eyepiece extractor sleeve.

  17-1 | The matched marker on the main mirror and the reflexive ring of the collimator are not centered in the hole, they are shifted towards the main mirror (right)
.



  17-2 | Situation in the telescope - the point of contact of the axis falls inside the secondary mirror.
.



  17-3 | Close-up - the secondary mirror is mounted too deep and the sight in the collimator is caused by the fact that ...
.



  17-4 | ... instead of doing the following (take the secondary mirror back a little) ...
.



  ... you started tilting them side to side until the markers were in line, but the effect is as below ...

  17-5 | ... the Q1 and Q2 angles are identical, the reflection angle = the incidence angle, the markers align, but not at the right depth
.



  17-6 | View reminder in the collimator.
.




Recommendation? Install the secondary mirror less deeply.






  Situation 3 - not good, we are not perfect.

Below, there is a view in a pinhole collimator with an additional "red" diaphragm at the end of the eyepiece extractor sleeve.

  18-1 | The matched marker on the main mirror and the reflective ring of the collimator are not centrally in the hole, they are shifted towards the tube inlet (left).
.



  18-2 | The situation in the telescope - the point of contact of the axis is in front of the secondary mirror.
.



  18-3 | Zoom - the secondary mirror is mounted too shallow, and the sight in the collimator is caused by the fact that ...
.



  18-4 | ... instead of doing the following (let the secondary mirror sink a little deeper) ...
.



... you started tilting them side to side until the markers were in line, but the effect is as below ...

  18-5 | ... the Q1 and Q2 angles are identical, the reflection angle = the incidence angle, the markers align, but not at the right depth
.



  18-6 | View reminder in the collimator.
.




Recommendation? Install the secondary mirror a little deeper.






  Situation 4 - not good, we are not perfect.

Below, there is a view in a pinhole collimator with an additional "red" diaphragm at the end of the eyepiece extractor sleeve.

  19-1 | The matched marker on the main mirror and the reflective ring of the collimator are not centered in the hole, they are shifted upwards.
.



  19-2 | SThe situation in the telescope - the axis of the eyepiece lens does not cross the axis of the main mirror, but goes under it.
.



Possible causes?

1 - The secondary mirror mount is not perfectly centered in the telescope tube.
2 - You have not set the eyepiece lens as accurately as you think.
3 - The axis of the main mirror does not exit exactly through the center hole of the secondary mirror mounting.






  Situation 5 - not good, we are not perfect.

Below, there is a view in a pinhole collimator with an additional "red" diaphragm at the end of the eyepiece extractor sleeve.

  20-1 | The matched marker on the main mirror and the reflective ring of the collimator are not centered in the hole, they are shifted downwards.
.



  20-2 | The situation in the telescope - the axis of the eyepiece lens does not cross the axis of the main mirror, but passes over it.
.



Possible causes?

1 - The secondary mirror mount is not perfectly centered in the telescope tube.
2 - You have not set the eyepiece lens as accurately as you think.
3 - The axis of the main mirror does not exit exactly through the center hole of the secondary mirror mounting.






    Now we can look at what can be seen in the secondary mirror in Situation 1, but without the "red" diaphragm restricting our field of vision.

however first ...


        - ! RIGOR ! -

    The following examination must be ABSOLUTELY performed with the collimator hole located in the FOCUS of the telescope.

    How to determine where the focus is in the telescope? Read HERE.

    Why do you have to meet this condition? Read HERE.




------------ Now without the "red" diaphragm we examine outlines and contours ----------------


  Situation 6 Continuation of Situation 1 - everything is set perfectly, not only that everything is coordinated as in situation 1, but also, after removing the "red" diaphragm, it turned out that the contour of the main mirror in the secondary mirror is perfect, and the perfect in the contour of the eyepiece tube, it means that we have perfect asymmetry of the secondary mirror.


  21-1 | The view in the red diaphragm collimator - perfect.
.



  21-2 | View in the collimator without the "red" diaphragm - perfect | Remember about the above-described RIGOR | |
.



  21-3 | The situation in the telescope - everything is correct.
.



  21-4 |View in the collimator - Red arrows indicate the visible asymmetry which results from the intended asymmetry of the secondary mirror mounting. Everything is OK.
| Remember about the above-described rigor |
.







  Situation 7 Continuation of Situation 1 - Seemingly it is perfect, everything is as in situation 1, unfortunately, after removing the "red" diaphragm, it turned out that the contour of the main mirror in the secondary mirror is not perfect, which means that we probably do not have a perfect one asymmetry of the secondary mirror mounting.


  22-1 | The view in the red diaphragm collimator - perfect.
.



| Remember about the above-described RIGIR |
  22-2 | The view in the collimator without the "red" diaphragm - less perfect, as you can see, the secondary mirror does not evenly cover the main mirror.
.



  22-3 | The view in the collimator - as we can see, collimating, everything is as it should be ... | Remember about the above-described RIGOR |
.



  22-4 | The situation in the telescope - ... but the secondary mirror is not glued properly.
.




What can you do about it? You have to detach the secondary mirror from the mount and stick it correctly, which is not a simple procedure. But ... this is not a serious ailment, struggling with it is a whim, your telescope will give correct images, unless the above inaccuracy is really large, or you are professionally involved in astrophotography, only then it is worth considering interference.

------------------------

But ... one more thing ... nothing is perfect in the real world, many of the activities described above have not been performed with the accuracy of the thickness of a human hair, any inaccuracies, including the one described above, may have a different cause than you think. So, tormenting oneself with thoughts now as to whether it is worth to rush to correct the placement of the secondary mirror, or not to hijack, it is better to turn into enthusiasm for re-collimations. If, despite the experience gained, the same flaws persist, then it is worth considering their precise elimination.

------------------------

    And finally. The telescope is now collimated correctly, but in larger telescopes, the main mirror, depending on the quality of the mounting, may move slightly depending on the angle of the telescope tube tilt, so if we photograph objects at the zenith and just above the horizon, we should always check the correctness of collimation. This time we only collimate the main mirror, because it is the cause of the errors. I omit the situation when, in poor telescopes, the secondary mirror also moves or the entire telescope tube bends, but the exact collimation of such specimens misses the point.








TELESCOPE COLIMATION - ULTRA - for advanced users


Well ...

                   



Empirical study of the path of the light cone.

  25-1





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