Collimating a Schmidt-Cassegrain Telescope

Telescope collimation refers to alignment of the elements in its optical system. Perfect alignment is required to achieve optimal performance of any optical system. Otherwise, contrast and detail are lost. This guide focuses (pun intended) on the Schmidt-Cassegrain Telescope (SCT) design, which is susceptible to misalignment. Periodic collimation is required for casual observing to correct misalignment that results from normal handling and transportation of the telescope. Even more frequent collimation is required of the perfectionist who wishes to account for collimation errors caused by pointing the SCT at different objects at different positions. This is what can occur when a telescope “leans” in another direction after collimation has been perfected. When seeking high resolution at higher magnifications, one should ideally collimate on every object observed or photographed. This might seem like an excessive burden, but it becomes second nature and can be quickly accomplished with practice. The superior result produced by perfect collimation is worth this price.

Few people, other than astronomers, appreciate the multiple skill sets that are required to plan, set up, and execute an observing or imaging session. Collimation is just another skill that an astronomer must master, because the difference between mediocre and great collimation is astounding! I cannot imagine why an astronomer would put so much effort into a session without checking collimation, and adjusting if necessary. My goal in writing this article is to shorten the learning curve to achieving really good collimation with the hope that astronomers will be more inclined to adopt collimation as a regular part of their set up.

This guide begins with a description of the phenomenology that is evaluated to determine if precise collimation is achieved. Next is a description of an SCT optical system, and how it is adjusted to achieve precise collimation. After the phenomenology involved and the SCT optical system are understood, the basics of collimating an SCT can be explored along with the rational that explains the details behind each concept. With a firm grasp of the basics, all that is left is to provide the procedures that are used to actually collimate an SCT.

The Phenomenology

A collimation process can be followed with great results without really understanding the telescope or the phenomenology that is being exloited, but that is seldom satisfactory for astronomers. They often want a deeper understanding of what is really going on in order to permit application of expert judgement in their tradecraft. In order to understand what is really going on, an explanation of some phenomena and concepts is required. The key phenomena associated with collimation is the concept of a pinpoint light source, and how it interacts with an optical system. In addition to collimation and a pinpoint light source, I will also cover the Airy disk and diffraction rings.

The essence of collimation is how photons travel with respect to one another. Light from a point source arriving at an optical system is considered collimated if all photons are travelling parallel to one another. A collimated optical system transmits collimated light to a single point at the focal plane. Generating a collimated light source for a microscope might be tricky, but Astronomers are lucky. They have access to a sky full of collimated pinpoint light sources. Each star is so far away that it cannot be magnified larger than a single point, and thus all of the photons arriving at the telescope are collimated. Consider an object like Jupiter than can be resolved to a disk with a telescope. Details on the disk can be observed because every point on the disk is a separate pinpoint light source. A telescope’s job is to preserve all of these pinpoint sources through the optical path to the focal plane. A telescope can only achieve this feat when correctly collimated.

The Airy disk is named after George Biddell Airy, who in 1835 provided the first theoretical description of how an optical system resolves a pinpoint light source. The Airy disk is the smallest disk that is formed by a pinpoint light source in a given optical system. All other things being equal in two telescopes, the telescope with the larger aperture is said to have more resolving power, because it is capable of resolving a pinpoint light source into a smaller Airy disk. This smallest disk implies perfect focus, and any other focus position produces an enlarged blob of light. Surrounding the Airy disk is one or more alternating light and dark rings of equal spacing known as diffraction rings. These rings can be seen only under the very best seeing conditions. Diffraction rings are formed as the result of light waves interacting with the circular aperture of the telescope. The spacing of these rings is a function of wavelength of the light.

The SCT Optical System

The optical path of an SCT from entry to exit consists of a corrector plate, a primary mirror, and a secondary mirror. For this discussion, front means the end of the telescope pointing toward the observed object, and back means the other end of the telescope where the gathered light is focused onto an eyepiece or sensor. Light arriving at an SCT first encounters the corrector plate at the front of the telescope. The corrector plate is so gently curved that it has the appearance of a flat pane of glass. The purpose of its optical curvature is to correct for spherical aberration. This is necessary because the next optical component, the primary mirror, is spherically curved, which means that it is incapable of bringing light rays to a single point of focus without a corrector. The aperture of an SCT is determined by the diameter of the primary mirror, which is located at the back of the telescope. It reflects light forward toward a focal point. This mirror is perforated at its center, and it can be adjusted forward or backward to achieve focus. The next element in the optical path is the secondary mirror, which is mounted in the center of the corrector plate. It is usually spherical in shape, which continues to focus the light. This is the final optical component that the light encounters before achieving focus just to the rear of the center perforation in the primary mirror.

The Basics

This guide offers a couple of techniques rough collimation techniques should be used before proceeding to the precision collimation procedure, unless a precise collimation was obtained during a recent observing session. Before proceeding to the details of either a rough or precise collimation, There are some basics that apply to all collimation techniques. Do not attempt to follow these basic steps, but do read them carefully to understand the concepts before proceeding to and actually following the rough or precise alignment procedures. Before collimating, always allow the telescope to thermally stabilize, which could take an hour or more.

Tilting the secondary mirror with respect to the primary mirror is the only mechanical adjustment required to collimate an SCT. Looking at the front of the telescope, the secondary mirror housing is located in the center of the corrector plate. If a central screw is present on the front of this housing, it should never be adjusted. Somewhere between the center of the housing and its outside edge are three adjustment screws situated 120° apart. The mirror itself is located on the back side of this housing. The adjustment screws control the tilt of the secondary mirror, and they hold it in tension against a central pivot. The tilt of the mirror is changed by loosening and tightening the adjustment screws, thereby pivoting the mirror about the central pivot. A single adjustment requires that all three screws be turned in order to change the tilt AND maintain the tension. For instance, if a certain adjustment screw must be tightened to achieve a desired effect, then the other two screws must first be loosened to prevent the adjustment without over tightening the first screw. Conversely, if a screw must be loosened, then loosen that screw first and complete the adjustment by tightening the other two screws to keep the mirror snug against the central pivot. Most SCTs come with adjustment screws, but after market adjustment knobs that are easier to use are available. That knobs can effectively hold collimation is the subject of debate. I recommend screws if collimation is only checked periodically to ensure that collimation remains acceptable over a longer period of time. Knobs, being easier to adjust, are probably just fine when collimation is checked frequently. Did I say that the central screw, if present, should never be adjusted?

A defocused pinpoint light source is used for rough collimation, and in the initial step of a precise collimation. When defocusing a pinpoint light source, three things become apparent. First the pinpoint becomes a diffuse blob. All of the same photons are still there, they are just spread over a larger area. It becomes easier to detect a collimation error the more the telescope is defocused. Next, notice that the center of the blob will blacked out. This is the shadow of the secondary mirror. And lastly, defraction circles will surround the central black blob. The central shadow and the defraction rings viewed in a perfectly collimated telescope will appear perfectly round, and perfectly centered within one another. Anything else is an uncollimated telescope, and adjustment is required to achieve perfect collimation.

In order to adjust collimation, the adjustment screws or knobs are turned in small increments. The result is then checked at the eyepiece. If the adjustment improves collimation, continue to make small adjustments in that direction until achieving perfection. If collimation worsens, adjust back to the original position, and make a small adjustment in the opposite direction. The adjustment screws should be snug, not tight or loose. If a screws should seem overly tight or loose while adjusting, slightly adjust the other two screws in the opposite direction.

Short arms vs. long telescope. If the person doing the collimation has long arms and a short telescope, it may be possible to turn the adjustment screws while observing the effect in the eye piece in real time. More often, this is not the case. Two people working together might be effective, especially if these two people work together often, and know how to collimate with minimal communication. Usually, an experienced collimator will be able to get the job done quickly working alone. As a guide to determine which screw to adjust, a person working alone can observe the direction in which collimation is off, and determine which screw to adjust by putting a finger or other thin object in front of the objective in that direction. Once the finger and the collimation error are aligned, it will be pointing to the screw that requires adjustment.

Some steps will require that the telescope be purposefully defocused. The out of focus position can be either inside of focus, or outside of focus. It really does not make any difference technically. A habit of always defocusing in the same direction simplifies the collimation process by making the results of a given action predictable. This permits one to quickly develop a sense of which adjustment screw to turn, and in which direction it should be turned.

Whether or not to collimate with a star diagonal in the optics path is another point of debate. A high-quality diagonal in the optics path is acceptable if this configuration is contemplated for visual observation. Keep in mind that the directional effects of a certain adjustment to which one has become accustomed my be reversed. There is no good reason to place a diagonal in the optical path for astrophotography, so remove the diagonal when collimating for astrophotography. So finally, here are the basic collimation steps:

  1. Thermally stabilize the telescope
  2. Center the collimation target in the eyepiece
  3. Adjust the collimation screws to achieve the desired result
  4. Re-center the collimation target in the eyepiece
  5. Repeat until there is no need to re-center in the eyepiece

When collimating for absolute perfection, the best collimation target is always a star near the object to be observed in a dark sky under good seeing conditions. For a more casual collimation, there are some alternative targets that can be used for periodic or rough collimation. A star is still the best point source of light, and a dark sky with good seeing conditions is still desired. For even better seeing, select a star near the zenith.

Any alternative target must simulate the pinpoint light source that a star presents, and it must be placed sufficiently distant from the telescope that the telescope can be focused. Commercially manufactured laser targets are probably slightly better than the others that will be mentioned here. Because of the added cost and the added calibration steps of a laser target, and how easily an SCT might loose perfect collimation, this is not be an attractive alternative for collimating an SCT. Other alternative targets are deceptively simple. I have never used it, but the idea that I like is a chrome ball bearing in direct sunlight. There is only one pinpoint spot on the ball bearing that reflects sunlight back toward the telescope. A glass Christmas tree ball works as well. Another is a bulb in a box, with the open end facing the telescope, and covered with a pin-pricked sheet of aluminum foil.

That covers the basics, so now on to the actual collimation procedure. I divide the procedures into rough and precise. In the precise procedure, I further call out what is needed for the occasional collimation or for even more precise collimation that would be conducted before every observation or image capture.

Collimation Procedures

Rough Collimation.

  1. Select an eyepiece that provides a magnification of roughly the aperture of the telescope in millimeters. E.g., an 11″ telescope aperture is 280mm, so use an eyepiece that provides about 280x magnification. For an 11″ F/10 telescope, which has a 2800mm focal length, use a 10mm eyepiece (magnification = telescope focal length/eyepiece focal length)
  2. Select a bright target star of 1st magnitude or greater, or use one of the alternative targets previously described
  3. Defocus as much as possible to reveal the collimation error, if any. Proceed to Precise Collimation if no error can be detected.
  4. Determine which screw will correct the error, and adjust 1/8 turn in the required direction
  5. Return to the eye piece, recenter the target, and readjust if necessary
  6. Repeat the last two steps until recentering and readjusting is no longer necessary

Precise Collimation.

  1. Select an eye piece magnifies 2x the telescope aperature in millimeters. Using the 11″ telescope example, 560x, which would require at least 5mm focal length eye piece
  2. Select a target star of 2nd (e.g. Polaris) or 3rd magnitude near the zenith for casual/infrequent collimation, or near the object to be observed for a one-time collimation
  3. Defocus as much as possible to reveal the collimation error, if any
  4. Determine which screw will correct the error, adjust 1/8 turn in the required direction
  5. Return to the eye piece, recenter the target, readjust if necessary. Proceed to Step 7 if no error is detected.
  6. Repeat the last two steps until recentering and readjusting is no longer necessary
  7. Precisely focus the star, and observe first diffraction ring in the airy disk pattern. Note, the airy disk will not be visible under marginal seeing conditions.
  8. Adjust, if necessary, using 1/16th turn at a time until the first diffraction ring around the disk is complete and uniform

Conclusion

This completes our examination of SCT collimation for casual visual observation or precise astrophotography. First, we explored the phenomenology that make collimation possible, and the SCT optical system that is of interest to us. Following next was the the a detailed treatment basics of collimating an SCT, along with the rational that explained the details behind each concept. Lastly, detailed procedures were provided to guide a knowledgeable astronomer through rough and precision collimation of an SCT. I frequently refesh my memory be reading through the entire article, and to make sure that I do not miss anything, I actually open and use these procedures when I collimate. You are welcome to do the same, and your feedback and suggestions will be valued.

© James R. Johnson, 2014.

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