The very mention of the word “collimation” causes much anxiety amongst amateur astronomers. A google websearch for “collimation” produces literally hundreds of detailed accounts of blood and sweat with many a frustrated astroimager brought to the brink of despair. This is no such tale. In fact, like many other things in life, when I finally decided to take the plunge as it were, I found things a lot different than what they were “reported” to be. I decided to write this entry as a series of questions and answers as though you the reader were right here interviewing me. I felt that would cover everything sufficiently for a single blog post. I hope anyone who owns one of these or is planning to acquire one will find it useful
What is collimation?: Collimation is simply the process by which you align the components in your optical system to bring the light from the object to it’s best focus. Typically you’re talking about a reflector telescope which employs 2 mirrors of different geometrical configurations that are spaced apart varying distances inside of the optical tube. These mirrors have to be aligned in such a way as to enable all of the light to arrive at a single point.
What is unique about the Tak 180 ED?:
The Tak 180ED employs a hyperbolic primary mirror, a flat secondary mirror and a doublet “corrector lens” to produce a wide flat field corrected for coma and spherical aberration. The focal length is extremely short, only 500mm (F 2.8) for a mirror 7 inches in diameter! This makes it more challenging to focus and collimate as the depth of field is extremely short. The secondary is offset from the primary center to enable more illumination of the field. It differs from the classic Newtonian telescope in that the primary of the newtonian is parabolic, not hyperbolic, rendering the field sizes smaller typically for similar aperture.
Why is there such a mystique behind collimating these telescopes?: I’m not sure about there being a “mystique” other than many “tales” that are out on the web regarding how difficult many people have found it. No question you do have to pay attention to details but that is no different than many other things we do in this hobby.
Was there anything different about your telescope that came into play during the collimating process?: Actually there was. I acquired this telescope from a friend who had the stock focuser removed and replaced with an extremely heavy duty focuser designed by Paul Van Slyke. Unfortunately these are no longer made but lucky for me I do have one on this scope and it is built like a tank. I was very concerned about even touching it for fear of causing some misalignment problem but clearly he demonstrated how his focuser did not alter the steps required for collimation. The corrector lens was on there pretty firmly and I did have to remove it carefully with a strap wrench but it was actually fairly easy to do.
Was there any special equipment, laser collimators or other tools you had to acquire?: That was actually the biggest surprise to me. Perhaps this was unique for the 180 but luckily my friend saved all of the collimation tools provided by Takahashi for this scope and I did not need anything other than an open 17mm wrench for the primary screws.
The items above were included and this diagram in the instruction manual shows the precise order of assembly. The “collimating tube” has a cross hair made of fine nylon threads at 90 degrees to each other that you tape down into slots in the tube. The eyepiece is a pinhole with a side port.
Did you get any assistance from anyone during this process?: That is really the most important thing when doing anything for the first time and with any uncertainty, ALWAYS get help from someone more experienced. I contacted Texas Nautical Takahashi or “land sea and sky” like everyone else who has a question about their Tak scope in the US. There is a fellow named Fred who is the expert in these matters and has been there for years, but unfortunately he was out of the office. I spoke to one of his colleagues who basically told me to read the manual and follow the directions! So that’s exactly what I did! I found the manual to be really well written and directions easy to follow.
So how did this go down once you were told to “read the directions”?: Actually a very interesting thing I discovered before that happened occurred when I tried to collimate it by only adjusting the the secondary with the CCD inspector software, assuming collimation was close. This is my “goto” method for collimation which I have done for a variety of scopes and been successful over the past several years. What I realized was that even after the software seemed to indicate good collimation it didn’t look right. The secondary was offset as expected but the area surrounding the secondary was not concentric. I knew then that it wasn’t right.
After that I realized that I had to start from the beginning, reading the directions. When I assembled the collimation tools and screwed that in to the focuser I could see right away that the collimation was off. There is a dot on the secondary and a circle on the primary to assist you in making your adjustments. I really don’t have anything else special to report about the specifics other than I followed the directions, starting with adjusting the secondary to the focuser which involved both tilting the secondary and rotating it gradually until the spot was lined up, and then adjusting the primary to the secondary.
Can you provide any tips or any other important things you learned besides “read the directions”?: Yes certainly. I don’t mean to oversimplify things but honestly it really boiled down to exactly that. Here are a few tips you might find useful:
- The directions: you may or may not have access to these especially if purchasing the scope used so please feel free to download the pdf file here
- Don’t try to reinvent the wheel as it were. Ask an expert in the field.
- Don’t be afraid to remove the corrector! It just screws on to the focuser!
- CCD inspector is a very valuable tool for collimation but I believe you need to be cautious relying on it exclusively for fast optical setups like this.
- I recommend using a flat panel or other light box set up for aligning the secondary with the telescope mounted and horizontal to the ground. It was fairly easy to make the collimation adjustments while looking in the eyepiece and adjusting the secondary. It is really just trial and error. Small adjustments either rotating the secondary or turning the “push” screws. A “push” screw is just a screw that pushes against the secondary or primary cell to tilt it. I did not have to adjust the axial position of the secondary front to back at all.
- The primary collimation screws are pretty coarse and difficult to separate and work on each one individually. I did not have the wrench provided by Tak so the one I used was perhaps fatter. You will need a 17mm open wrench and a screwdriver. There are 3 components. At the base is the locking nut and furthest back is the locking screw. The middle screw is what tilts the mirror. You have to loosen everything and then I found that turning the middle screw by hand was the most effective. Again you really have to trial and error it to see what works. Once it is lined up I tightened the locking nut by hand and a little more with the 17mm open wrench but then was VERY careful about tightening the back locking screw with the screwdriver because a couple of times I did that the middle screw was turned and threw the collimation off.
- Once the secondary is aligned, put the scope vertical pointing to the sky and do the primary collimation in that position.
Ok. So can we see any results of what you did?: Of course, we’re all waiting for that! I confirmed the visual appearance to be correct based on the manual diagram, then tested the alignment on the Omega Centauri cluster. I believe you will agree that collimation is very good now!
Any final comments about the process?: Listen to the experts. In this case they said read and follow the directions and use the tools provided, which obviously worked! Don’t be afraid to remove parts from your scope if they need to be. Most importantly, don’t believe the horror stories that are out there 🙂
Thanks for reading!