Collimation is the adjustment of the alignment of the various optical elements of an instrument relative to each other. To obtain the sharpest and sharpest image possible, it is essential to properly collimate the telescope's optics. Regardless of the quality of the instrument's optics, if it is not well collimated, the images will be of poor quality.
The collimation can be disturbed by the vibrations due to the transport of the instrument. For this reason, it is advisable to regularly check the collimation of your instrument. Here are two collimation procedures for a Schmidt-Cassegrain telescope:
- The standard collimation method
- Collimation with the Hartmann disk
1- The standard collimation method
For visual observation, collimate with your elbow. For planetary imaging, it should not be used. The reason is that most of the time the elbow is not perfectly centered with the optical plane of the telescope. Thus, the collimation will be a little different in the two ways of observation.
To check the collimation, point with an eyepiece of approximately 26 mm, a star of medium brightness (magnitude of 2 to 3 approximately) near the zenith. Allow the telescope to come to room temperature before starting this test. A temperature difference between it and the surrounding air causes distortion of the image. Center the image of the star in the field of view using the eyepiece, defocus the image until it appears as a ring of light surrounding a small dark disc such as present in the image of figure 1:
The only possible and necessary adjustments are made by means of three small screws located on the outside of the support of the secondary mirror (figure 2) which is on the front surface of the telescope:
- Look at the defocused image of the star to see in which direction the dark disc is offset from the center of the ring of light (1, fig. 1): notice which part of the ring is thinnest. Place your index finger on one of the collimation screws. You will see the shadow of your finger on the ring of light. Move your finger across the secondary mirror holder until its shadow is on the thinnest part of the ring. Look then where your finger is, if it points either on a screw or between 2 screws, this is the place that will have to be adjusted.
- Using the telescope joystick control keys, at the slowest speed, move the light ring to the edge of the eyepiece field of view, in the direction where the dark disc is off-center in the eye. light ring (2, fig. 1).
- Turn the adjustment screws while looking through the eyepiece. You will notice that the image of the star moves across the field. If the image goes out of view, you are turning the screw in the wrong direction. Rotate there in the opposite direction to bring the image of the ring towards the center of the field of view.
- If the screw you are turning becomes too easy to turn, tighten the other two, both equally tight. If the screw you are turning becomes too hard, loosen the other two, both equally strong.
- When the image is again in the center of the field of view (3, Fig. 1), carefully examine the concentricity of the rings. If you think the dark disc is still off-center in the same direction, continue adjustment by turning the same screw. If it's now on the opposite side, you've gone too far, and you need to turn the screw in the opposite direction. Always check at the same time the centering of the image in the field and the concentricity of the rings.
- It is possible that after a first adjustment, the dark disc has shifted in another direction. In this case, repeat steps 1 to 5.
- Repeat the test at higher magnification with a 9mm or smaller eyepiece. Any defect in concentricity at such magnification will only require very minor rework on the three screws. You will then have a well collimated telescope.
- In good atmospheric conditions with very low turbulence: to perfect the alignment, check the image of the star at the highest possible magnification, after having focused properly. It should appear as a small point (called the Airy disk) surrounded by diffraction rings. If necessary, fine-tune the collimation with very small retouching, until the Airy disk is centered in the diffraction rings. The distribution of light in the rings must be homogeneous. You now have the best possible alignment of the optics in your telescope.
The optimum setting will be obtained when the Airy disk (with the diffraction rings) can be seen properly aligned as in the example above. For photography of planets and other objects in the solar system, which requires perfect collimation, always check the telescope's collimation before taking an image, especially if the telescope has been moved.
2- Collimation with the Hartmann disk
The procedure described above is the standard eyepiece collimation method. Personally, I have adopted collimation using a three-hole Hartmann's disk in use with my CCD camera which is used for deep sky imaging or that used for photographing objects in the solar system. It should be noted that this method must be used with Schmidt-Cassegrain type telescopes.
The advantage of this technique is that it is used directly with the camera used to acquire the images, thus offering a collimation in accordance with the optical train of our imaging system. Another advantage is that it can be used in the presence of significant turbulence, therefore in all circumstances. Here is an example of Hartmann's record:
The disc is the diameter of the telescope. We install it in front. The central hole is used to access the collimation screws. The three holes at the periphery of the disc are aligned with the three collimation screws.
Checking the collimation
With the Hartmann disc, it is very easy and fast to check the collimation at all times. We aim at a bright star (magnitude 0 to 1) near the zenith, we install the disc in front of the tube and we focus on the star.
When the star is defocused, it appears as 3 separate stars on the screen. As we focus the instrument, these 3 stars converge to form only one when the focus is achieved. If we cannot converge the 3 stars into one, the telescope is decimated.
Hartmann disc manufacturing
It is very easy to make a Hartman record yourself. Here is the material necessary for its realization:
- Thick black cardboard larger than the diameter of its optical tube
- An angle protractor
- A compass
- A cutter
- A ruler
Cut the black cardboard to the diameter of the optical tube using a compass and cut out its circumference with scissors or a cutter. The cardboard should fit inside the tube. Three holes must be drilled at 120o on the periphery of the disc cut out. The diameter of the holes is about 1/5 to 1/6 that of the optical tube. Use the protractor, ruler, compass and cutter to make the holes. The holes will be drilled as far as possible from the center of the disc. Finally, drill a central hole to access the collimation screws. The position and cutting of the holes must be carefully done to maintain the accuracy of the collimation. See an example of the hartman disc at the beginning of this section.
Carrying out the collimation with the help of the Hartmann disk
Always match the three holes on the periphery of the Hartmann disc with the three collimation screws as shown in the following image:
1. Center a bright star near the zenith in the camera's field of view.
2. Put on the Hartmann disc and plug hole 1:
3. Focus until the 2 stars converge into one:
4. Remove the plug from hole # 1 and put it on hole # 2.
5. If you see 2 stars slightly apart, turn the # 1 collimation screw until you see a single star. If the screw becomes too easy to turn, tighten the other two, both equally strong. If the screw becomes too hard, loosen the other two, both of them.
6. Recenter the star in the camera's field of view.
7. Focus until the 2 stars converge into one.
8. Remove the plug from hole # 2 and put it on hole # 3.
9. If you see 2 stars slightly apart, turn the # 2 collimation screw until you see a single star.
10. Recenter the star in the camera's field of view.
11. Focus until the 2 stars converge into one.
12. Remove the plug from hole # 3 and put it on hole # 1
13. If you see 2 stars slightly apart, turn the # 3 collimation screw until you see a single star.
14. Do not plug any holes.
15. Focus until the 3 stars converge into one. If we do not manage to merge the 3 stars, repeat the process at point 1. This second iteration will align the 3 stars into one.
The Hartmann disc allows very good collimation at all times, even in the presence of significant turbulence. For the photography of planets and other objects of the solar system, which takes place in very low turbulence, it is recommended to check the setting on the Airy disk. Use your camera for planetary imaging with all the necessary equipment to photograph the object you want to take and point a bright star near the zenith. Position the star in the center of the screen. Focus precisely and examine the Airy disk on the screen. Use a short exposure time and adjust the contrasts to show the diffraction rings. If necessary, fine-tune the collimation with very small retouching, until the Airy disk is centered in the diffraction rings.
If you are using a monochrome camera to take the images, put your red filter in front. It will reduce air turbulence and thus facilitate very fine adjustments.
Sky Astro - CCD