Collimation of a RC telescope using the Takahashi Collimation Microscope.
1. Figure 1 and key shows a perfectly collimated telescope using the Takahashi collimation scope.
2. It is important to understand that EVERYTHING you see through the Takahashi collimation scope is WITHIN the bounds of the secondary mirror. It is all a reflection of the system as seen in the secondary mirror. YOU CANNOT SEE OUT THE FRONT OF THE SCOPE. Please refer to the following key:
· A = Centre dot of the Takahashi collimation scope.
· B = the centre dot / circle mark on the secondary mirror.
· C = the white centre of the Takahashi collimation scope.
· D = the inner black end of the Takahashi collimation scope and the grey circle is the very end of the Takahashi collimation scope. How well you see this and what you see can depend on how well the inside of the Takahashi scope is illuminated.
· E = the primary mirror baffle tube.
· F = the secondary mirror baffle / light shield tube.
· G = Secondary spider support arm. What you are seeing is a reflection of the spider arm in the primary mirror that is being reflected back to your eye by the secondary mirror. (A reflection of a reflection).
· H = A further reflection of the spider arm. However, this time it is an image of the arm that is being reflected from the secondary to the primary, back to the secondary then finally to your eye (a reflection of a reflection of a reflection).
· I = the light annulus.
3. The ultimate aim is to achieve perfect concentricity of all the elements shown. The collimation is then confirmed and, if necessary, finely adjusted with a star test.
4. The process is iterative and as you adjust one element, then you may need to go back to refine the adjustment of another.
Understanding the elements, adjustments and image information.
5. Before you start there are some basic principles that need to be understood that will help you during the collimation process.
6. Every telescope has one mechanical optical axis. This is a function of the mechanical assembly of the telescope and all components that lie in the optical path. Examples are:
· Adaptor or extension tubes,
· The telescope tube,
· The telescope end rings,
· Spider assembly,
· Rear cell back plate,
· Camera / eyepiece.
· Mirror assemblies and baffles.
There may be others elements affecting the optical axis or variations in its position based on the type / make of scope or your set up. It is important to remember that the use of all this equipment can produce flex or sag in the mechanical system during the collimation process and when the scope is in use and is very difficult to avoid in the real world. This can of course change the optical axis to some degree. The aim would be to perform the collimation with a perfectly stiff system. However, you should take all precautions that you can to reduce any flexure that can take place so that the use of different accessories has the minimum impact on the position of the optical axis. It is highly recommended that you use robust, screw threaded adaptors and extensions to mount focusers, cameras or eyepieces etc. Focuser stiffness, flexure and quality are extremely important (back lash is not important in terms of collimation).
7. The alignment of the Tak scope centre dot (A) and the secondary mirror centre dot / circle (B) is achieved by tilting and tipping the secondary mirror using the secondary collimation screws (figures 2 and 3). This aligns the secondary optical axis with the telescope mechanical optical axis. It can also affect the alignment of the spider reflections (G) and (H) to some degree and will effect the alignment of the primary mirror as seen through the concentricity of the secondary baffle (F), primary baffle (E) and light annulus (I).
Figure 2 Figure 3
Figure 4 Figure 5
Figure 6 Figure 7
The collimation process.
9. Bearing in mind the discussion at paragraph 6 start by making sure that you have your normal image train in place. This includes your focuser and any extension tubes that are required to reach focus either visually or with any camera. It is not essential to try to collimate with every variation but you should have the majority of elements in place, particularly those that lead to the longest length hanging off the back of the telescope. The Tak collimation scope has a generous focus adjustment but the appearance of the collimation image, particularly the light annulus (I), is affected by the back focus position of the Tak scope. Indeed, if you are to far forward, then it may not be visible at all.
10. Insert the Tak collimation scope into the focuser. And try to make sure it is as central as possible. Make reference to its position to make sure you can replace it as close as possible to the original position if you have cause to remove it during the collimation process.
11. Take a few minutes to familiarise yourself with the image through the Tak scope. Move the Tak scope focuser in and out to view each of the collimation image elements in turn. Can you see, focus and identify all the elements clearly? If not then you need to adjust the position of the Tak collimation scope by using more or less extension adaptors in your image train. Try racking the telescope focuser in and out.
12. Try to assess the current collimation of the system. Of particular interest is the alignment of the spider reflections (G) and (H). If these are not aligned but the other elements look close to correct collimation then you will need to adjust the spider or back plate to mechanically centre the secondary assembly onto the telescope optical axis, which will throw all the other elements out of adjustment. However bear in mind that any adjustment can affect the overall alignment.
13. This is the most important step and should be carried out before and after every other adjustment except for the very last alignment of the primary mirror. The aim of this step is to achieve alignment and concentricity of the secondary mirror dot / circle (B) with the centre dot of the Tak scope (A). You should also aim to achieve concentricity of (B) with the white area of the Tak scope (C), however, this should follow naturally with alignment of (A) and (B). If you have already assessed that you need to align the secondary spider assembly to the telescope optical axis as discussed in paragraph 12 then your first adjustment of the dots does not need to be perfect, as it will need to be repeated later.
14. Once you have achieved the alignment of the secondary (B) and Tak (A) dots take a look at the concentricity of the secondary baffle (F), primary baffle (E) and light annulus (I). If they are not concentric then make an adjustment to the primary mirror tilt / tip. This does not need to be perfect at this stage as the main aim of the adjustment at this stage is to assess the alignment of the spider supports (G) and (H). Notice that as you adjust the primary mirror tilt / tip, the position of the reflections (H) changes. If the spider reflections are out of alignment proceed to paragraph 15. If they are aligned then you need to refine the collimation of the secondary and primary by repeating paragraphs 13 and 14 in the correct order to achieve the best concentricity you can.
15. If your spider reflections (G) and (H) are not aligned make adjustments to the secondary spider assembly mechanical centring and / or back plate to bring them closer to alignment. Do not try to achieve this all in one step as making these adjustments upsets the alignment of the other elements and they in turn upset the alignment of the spider reflections. One point to bear in mind is that you do not want to displace the spider so far that you produce any significant “bend” in any of the spider arms as shown in figure 8. deformation of this type will show up as double diffraction spikes on bright star images. Obviously the amount you can adjust without reaching this condition depends on the adjustments available at you spider assembly. Some scopes allow lateral movement of the spider as well as push pull, which will allow you to keep the spider arms straight. Also be careful not to induce twist into the spider arms. Once you have made this initial adjustment you need to repeat paragraphs 13 and 14 in the correct order until all the elements are aligned to the best of your ability.
16. Once you have achieved alignment of all the elements using the Tak collimation scope then you need to carry out a star check to confirm the collimation. You may not need to make any further adjustments once you have confirmed the collimation on a star. However, if you do need to make any further adjustments on the star test then these should be carried out on the primary mirror tilt / tip adjustment only. Changing the collimation of the secondary will take the scope out of collimation very quickly. This article does not include the detailed discussion necessary to communicate all the information required about a star test but a very good discussion on the topic can be found on Thierry Legault’s website.