[Lvas] collimation

Thu Mar 13 18:47:18 PDT 2008

Matt,

Here is an article I wrote a few years ago about collimation. Maybe that will help. I've since reviesed it for one of the later club newsletters, but this old version should still work for you. Sorry there are no drawings with it. If we meet at one of the upcoming star parties, I'd be glad to help in person.

Fred

COLLIMATION
By Fred Rayworth

Many technical issues affect Newtonian telescope performance, and the simplest one to cure is collimation. Have you ever aimed at a star, tried to focus it and found that the object flared to one side inside focus and flared to the other side outside focus? That is because the two mirrors that make up your optical system do not line up.
            Collimation is simple and one of the first steps in telescope setup. On a compound telescope such as the Schmidt-Cassegrain, collimation is a little more difficult and not normally done in the field. Because of the inherent mechanical design of a compound telescope, the optics stay in alignment much better than a Newtonian. For the purposes of this discussion, we will be discussing the Newtonian optical system.
            Light travels through the front of the tube and hits the main mirror at the back end. The bowl shaped mirror reflects the light up toward the front of the tube again. A forty-five degree flat mirror (diagonal) intercepts the light and reflects it out the side of the tube to the eyepiece. It is a simple yet effective system. Optics notwithstanding, the only things that can go wrong with this system are environmental factors such as water, dust, bad air, or when the mirrors do not line up properly.
            The mechanical nature of the mirror mount and diagonal spider assembly bring about changes in the alignment of the mirrors. We will discuss the diagonal assembly first.
            The diagonal suspends in the center of the tube by a single metal rod, two, three, or four vanes. These vanes (with the exception of the metal rod style) are very thin strips of metal designed to be rigid, yet produce minimal blockage in the optical path. The diagonal flat mirror mounts in the center of this assembly, called a spider. In the better spiders, the diagonal mount is adjustable with screws or nuts. In some homemade Dobsons, the diagonal glues to a mount that attaches to the spider and adjusts by bending.
            Spiders are flexible due to the relative flimsiness of the assembly but once aligned, they tend to stay in place with little drift. Alignment is usually required of the diagonal and not the spider, especially if it is removed or installed each time the telescope is used. This alignment usually consists of rotating the diagonal so that it aims through the focuser. If the diagonal assembly sees rough handling, it may on occasion require a tilt adjustment in addition to the rotation.
            The focuser is often neglected in the alignment procedure. The focuser is bolted or screwed to the side of the tube and is supposed to aim directly at the diagonal. If the tube sees rough handling or receives a few bumps, the focuser may shift slightly and partially cut off the cone of light from the mirrors. Though the mirrors align perfectly, the cone of light may come out of the focuser at an angle and the edge of the field cuts off. The affect may not be as noticeable as a mirror alignment issue (unless it is grossly out) and at worse, may result in a slight dimming of the image or a loss of contrast, usually at the edge of the field which is normally not a concern anyway. For the best view, do not neglect the focuser in the alignment process.
            The final component in the system is the mirror mount. The design of the mechanical assembly that holds the mirror allows for adjustment knowing that it is going to drift. Most mirror assemblies adjust at three points, these points accessible as bolts, wing nuts, or screws on the back of the mirror cell. Occasionally there is a mirror cell with four adjustment points but these are rare (I made one once in high school and did not realize the advantages of a three point system until years later).
            Even the best mirror cells may drift in alignment over time and rough handling. Another factor is that especially in the larger telescopes, removing and installing the mirror is part of the setup. Getting the mirror exactly in the same spot each time is prone to problems. I once made a 6” reflector with a square wood tube and the mirror was permanently mounted in the bottom. I did all the alignment with the diagonal. In this instance, whether out of skill or dumb luck, the system worked perfectly and despite dropping the tube numerous times, I only had to align it once! Go figure!
Now that we have a picture of each element, it is time to look at the actual alignment procedure. Before we begin the alignment, it is important to add a very simple but useful modification to the main (or primary) mirror. Many commercial mirrors already come with the center marked, but if it is not, you should to that first thing. This mark may be a small dot or a ring. I’ve used everything from a nail polish dot to self-sticking paper reinforcements. The diagonal blocks the center of the mirror, so a mark there will not affect the image. The purpose of this mark is to provide the exact center of the mirror and that will come into play later.
Once a telescope is initially set up, the diagonal spider will usually stay in alignment. As I said before, rough handling may affect this. If the hardware (screws or nuts) holding it to the tube work loose, or say your neighbor’s kid grabs it out of curiosity, it is a good idea to check it. The following procedure has two benefits, as it will initially align your spider and mirror at the same time. This procedure is usually not necessary in the field.
If it is possible, remove the diagonal and holder from the spider. In many designs, the diagonal holder screws into a center post on the spider. To remove it, remove any lock nuts and simply unscrew it from the spider. This procedure will leave a hole in the center of the spider that is just perfect for the next step. With the diagonal holder removed, look through the hole down the tube to the mirror. If everything is correct, the center dot on the mirror will be visible through the hole in the center of the spider and the spiders’ reflection will project on the center of the mirror. The dot on the mirror will also suspend over the center hole of the reflection of the spider. If not, adjust the mirror until it is. You should now look to see that the mirror reflects a round cone of light from outside the tube. By this, I mean that the sky you see reflecting off the mirror should be round and should not have part of the edge cut off by the end of the tube. If this is the case, it means the center of the diagonal spider is not exactly in the center of the tube and though it aligns with the mirror, it is causing the mirror to aim out the end of the tube at an angle, cutting off part of the light. In this very rare case, it may be necessary to center the spider in the tube. This is a rare problem except in a sloppily made tube assembly. In the case of a truss tube design, the problem is probably the rods or mounts that make up the tube assembly. I should mention here that another possibility is that in a solid tube design, the mirror cell and mirror are not centered in the bottom of the tube. If this is the case, the mirror and cell must be centered. Sometimes there are shims or adjustable clips that hold the mirror cell in place. You may have to adjust these to get the cell centered in the tube. Whatever the case, the end result should be that when everything is aligned, you should see a round reflection of the sky and no part of the tube cutting off the edge of the field.
In the case of a single rod diagonal holder, remove the rod from the tube assembly. Now make a mask with a center hole in it large enough to see the mirror (maybe ¼” to ½”) and place it on the end of the tube. Align the mirror so that it reflects the hole in the mask on the center mark. Re-install the diagonal rod and bend it until it aligns with the mirror.
Once you align the spider assembly (or single rod), it should stay there except under rough handling. Now install the diagonal (in the rod style it will already be attached) and align it with the focuser. To do this, rack the focuser out all the way and align the diagonal so it centers in the focuser. Now look at the reflection of the diagonal and spider in the mirror and adjust the diagonal so that the mirror center dot is in the center of the diagonal mirror as well as the spider arms. With everything centered, the mirror should reflect the spider, the diagonal, and a round image of the sky.
Now that I’ve gone over the initial alignment, we will talk about a normal alignment in the field. If the mirror and diagonal stay in the tube during transport, when you set the scope up, check the alignment and tweak the mirror until it centers in the diagonal and the focuser. Always rack the focuser out all the way and make sure the image of the diagonal and mirror are not off to one side or part of the edge cuts off unevenly. That is all there is to it.
A real life example is with my 16” scope. To transport it I remove the diagonal but leave the mirror in place. When I set up, I mount the diagonal and align it with my focuser, racked out. I have an eyepiece tool that I slip onto the focuser drawtube. It is just a piece of metal the size of a 1 ¼” eyepiece with a small hole in the center. I line up the diagonal with the hole and then align the mirror to that. It is crude but effective. Sometimes the inside nut (the one closest to the diagonal) that holds that side of the diagonal rod in the spider drifts a little and I have to make adjustments to get the diagonal to center in the focuser. Many times, I do not even have to align the mirror. I always kick the bottom of the tube (very low tech) to make sure it will not drift and if it does, make a minor mirror cell adjustment. This entire process takes less than five minutes.
Before closing, I should mention a few words about adjusting the mirror. Most mirror cells adjust at three points. Getting used to the movement may be a challenge. After over 40 years of doing this, I still cannot get the orientation right. I will just tweak one nut until the mirror moves, and if it moves the wrong way, I reverse it and try another. It still never takes more than a few minutes to adjust the mirror even with all that stumbling around. However, I get an awful lot of exercise during this procedure. I have to walk back to the mirror cell, tweak it, and walk up to the focuser to check movement. If your scope tube is longer than your arm can reach, you will go through the same thing. It is always nice to have a friend to help!
For those of you inclined to expensive toys, there are a few that aid in obtaining the best collimation. Collimating the optics by eye works well, but if you are a perfectionist, or are going to do some photography, you may want the alignment to be more precise. This is especially relevant for short focal length telescopes in the f/4 to f/5 range. The older set of toys is the Cheshire collimating tool set. This is a set of three eyepiece-sized devices that work on either Newtonian or compound telescopes. The set costs (if I remember right) around eighty to a hundred and thirty dollars. I also believe you may purchase the tools individually as some of the tools are for compound scopes versus the Newtonian design. The second option is the laser collimator. Now mister El-Cheapo here may want to spring for one of these toys someday. They range in price for about sixty dollars on up. You plug the laser into the focuser and follow the light beam. Adjustment is a lot easier and more precise.
In conclusion, collimation is an essential part of your telescope setup. It does not need to be a chore and is certainly not rocket science. You should do it so that you get your mirrors at least in the ballpark and do not have to observe stars that look like comets!

Date: Mon, 10 Mar 2008 04:42:42 -0700From: xsmatt81 at gmail.comTo: Lvas at lvlug.orgSubject: [Lvas] collimationWho can I talk to or get help with collimation of a f/5 5' newt? Ive been trying to get it done with the cheshire/sight tube..but having alot of trouble with the spider vane screw's..I dunno if it's just poor quality or what..but it wont center no matter what. Should I attend the meeting and bring my scope along? I hate to ask these thing's, but it's getting to a point where I think I will need a hand doing my first real collimation.Thanks people-- sincerely  -Matt 
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