Category Archives: Astro-Knowlege

Brief descriptions of astronomical concepts needed to understand casual observational astronomy.

An Interesting Astronomical Observation Project

Most people intuitively know that days are shorter and nights are longer during the winter months, and days are longer and nights are shorter during the summer months. For those people who don’t know why, but would like to develop an understanding of why that is, here is an interesting observation project that begins to tease out the answer.

As I write this on December 23, 2019, we are just two days past the winter solstice, or the astronomical first day of winter. This event coincides with the shortest day and longest night of the year. And, at around this time, the sun rises and sets at its southernmost point on the horizon. This is a great time to begin the project, and observe the rise and set points move northward over the next six months.

This project can work with observing just sunrises, just sunsets, or both. After deciding when events you can observe, the first thing to do is select one or two observation points that are readily and repeatedly available. One should be toward the east in the mornings for sunrises, and the other toward the west in the afternoons for sunsets.

For the most casual observer, make a mental note of the sunrise or sunset point of the horizon, and the time. This first observation is your baseline. Make this observation every couple of days, and compare them with your baseline observation. During the period between the winter solstice (around December 21st) to the summer solstice (around June 21st), an observer should note that the rise or set point moves northward as the winter and spring progress. The sunrise time should be earlier and the sunset time should be later during this progression. By the time you get to June, you’ll be surprised at how much the rise or set points have moved, and how much the time has changed as well. From June 21st back to December 21st, the rise or set point should be moving southward on the horizon while the rise time gets later and the set time gets earlier.

More sophisticated observers can use a compass to record the azimuth of the sunrise or sunset, and record their observations in a spreadsheet for later analysis. A magnetic compass that can read degrees or a smartphone app can do the job. If you’ve never used a compass, they are not difficult to learn, and there are many online resources. Also, be sure to record the time in Universal Coordinated Time (UTC) to eliminate any confusion that daylight savings time might impose.

As an example for us in the eastern time zone, to convert standard time (EST) to UTC, add five hours. To convert daylight savings time (EDT), add four hours. Be aware that from early May until mid-August, sunset times of 8 pm EDT (and later) will use the next days date. For instance, sunset at 8:36 pm on June 21st might be recorded as 2020-06-21 20:36 EDT. Converting this to UTC by adding four hours pushes the time past midnight, and results in 2020-06-22 00:36 UTC.

If anyone actually does the observations, and would like an interpretation of the results, I would be glad to work with you, or write more on this topic, just let me know.

Southern Constellations

The southern constellations are those that fall within the Southern Celestial Zone, which is located at the bottom, or southern, portion of the sky map below. This zone is bounded by the celestial equator (0° declination) to the north, and the southern horizon to the south.

Stars in this region tend to hang low in the southern sky,  never rising very far above the horizon. From our N39° latitude on the terrestrial sphere, these constellations generally rise in the southeast, make a brief appearance above the horizon, and then set in the southwest. As a the southern declination of a star increases, the amount of time that it will appear above our horizon decreases.

To gain a sense of how a star’s declination affects its time above the horizon, examine the -20° and -40° declination lines in the chart below. The -20° declination lines spans from the 0th hour (0h) of right ascension line to the 10th hour (10h) of right ascension line, which means that a star traversing the sky at -20° will be above the horizon for ten hours. Now consider an object just 20 degrees farther south on the -40° declination line. This line spans from the 2nd to the 8th hour of right ascension, which means that the star will appear above the horizon for only six hours. The effect accelerates as the declination increases even more. At five degrees farther south, or -25° declination, we approach the southern limit of our view from N39° latitude. An object at this declination would only appear above a perfect southern horizon for just a few minutes.

In contrast, objects that have a more northerly declination remain above the horizon increasingly longer. At +39.14° declination and higher on the celestial sphere, which corresponds to my N39.14° latitude on the terrestrial sphere, objects remain above the horizon all night!

Sky_Map_Celestial_Zones

Northern Constellations

Northern Constellations are those that fall within the Northern zone of the celestial sphere, which is zone that spans from east to west across the central area of the sky chart below. This zone is bounded on the north by the circle that defines the Circumpolar region, and on the south by the equator, or 0° declination, that spans from the east to west points on the horizon.

Stars in this zone generally rise in the east, pass directly or nearly directly overhead, and then set in the west. These are the most easily observed constellations when they are on the meridian, because views in this region are the least affected by the atmospheric haze that degrades views near the horizon. This region is also the least likely to be blocked by obstructions such as trees or buildings on the horizon.

Sky_Map_Celestial_Zones

© James R. Johnson, 2015.

Cardinal Directions for Casual Stargazers

Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.
Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.

Being able to identify the four cardinal directions – north, south, east, and west – is necessary so that stargazers can correctly orient a star chart to permit a desired constellation might be found. Close is good enough for our purpose, and once any one of the cardinal directions is determined, the other three are easily derived by imagining our body as a compass rose. I will describe a compass rose stance and how to use it, and I will present three easy methods to determine at least one cardinal direction. Many more methods can be found by a Web search on “finding cardinal directions without a compass.”

Assume a compass rose stance by standing up, and extending the arms up and straight out from the sides, and horizontal to the ground. Assume that you already know which way is north and that you are facing in that direction. Looking at the compass rose above, we can see that the face is the north point of the rose, the right hand is east, the left is west, and the south point is the back of the head. If we can determine any one of the four cardinal directions, and turn our self until the correct point of the compass rose stance is aligned in that direction, then we can determine the other three cardinal directions off of the the other three points of our compass rose stance.

The easiest method to find one cardinal direction is based on the Sun, and can be used if one knows where the Sun rises and sets. The Sun rises in the east, and it sets in the west. That orange glow on the horizon before sunrise and after sunset also indicates which directions are east and west, respectively. To illustrate, if I am observing in the early evening and can still see the sunset orange glow is brightest at one point on the horizon, I have determined which direction is west. Assuming my compass rose stance with my left hand pointing toward the west, I know that north is straight ahead, east is to my left, and south is behind me.

A compass is just as easy as working of off the sunrise and sunset points if one is available. Since the red arrow will always point north, an observer can face in direction of the red arrow and derive the other three directions by extending the hands toward the horizon. The left hand points west and the right hand points east. South, of course, is behind the observer.

Once north has been determined, it is easier to find Polaris, which can be used in the middle of the night from any location. Read Polaris, The North Star to learn how to find this star and use it to determine which direction is north. I recommend that any serious stargazer invest the time in learning this method.

One of these processes or a similar process must be used to determine the cardinal directions the first time that a stargazer observes from a new location. If a stargazer is to make repeated observations from the same point, then the cardinal directions need only be determined once and committed to memory or noted in an observing log. For instance if an observer remembers that a distinct tree on the distant horizon that is just a little left of North, then using that tree as a future reference, the observer readily knows which direction is north on subsequent visits to that spot.

Knowing the cardinal directions is an inherent part of observing the sky. Learning how to determine these directions is quite easy, and becomes second nature with just a little practice, and after a while the compass rose stance will no longer be needed.

© James R. Johnson, 2015.
jim@jrjohnson.net

The Circumpolar Region of the Celestial Sphere

A circumpolar object (star, constellation, deep space object) is one that never sets below the horizon during the Earth’s daily rotation. Any point in the sky that has a declination (degrees of separation from the celestial equator) greater than the observer’s latitude (degrees of separation from the terrestrial equator) will never set below the horizon. If for some reason the Sun ceased to illuminate the sky, a circumpolar object could be seen to circle Polaris once in about every 24 hour period. Cepheus, for instance, is a circumpolar constellation that is oriented with the top of the house-like asterism nearest Polaris. In the September evening sky, Cepheus is high above Polaris, and the “house” appears to be upside down. Over the course of 24 hours, Cepheus will circle Polaris. In six hours, the “house” is horizontal with the top pointing west, in 12 hours it appears upright, but below Polaris, in 18 hours it appears vertical again, this time with the top pointing east. Six months from now in March, Cepheus’ orientation at dusk is the same upright appearance as Cepheus’ 12-hour position in September.

The circumpolar region of the celestial sphere is indicated on the annotated sky map below as the nearly circular region. Two points define this region, the north celestial pole, and the north point on the horizon. The north celestial pole is at the center of the region’s circular border, and all stars on the map circle around this central point as a result of the earth’s daily rotation about its axis. Stars near the pole make small tight circles around the pole, while those out near the drawn circle, but still within the circle, make wider sweeping circles. Those stars outside of the circle also circle the pole, but will dip below the the northern horizon, and are therefore not circumpolar stars.

Sky_Map_Celestial_Zones

© James R. Johnson, 2015.

The Speed of Light

Light travels in photons that are either emitted from hot objects like light bulb filaments, or reflected off of cooler objects, like the walls. The Sun and Moon are also emission and reflection objects, respectively. Photons travel from their source to our eye or camera sensor at, well, the speed of light. We can express this speed in terms with which we are already accustomed, like miles per hour (mph). Since light travels at 186,282 miles per second, that equates to 670.6 million mph. That is incredibly fast! Since mph has a distance component (miles), we can use the speed of light, or the speed of anything else, to measure distance. If something is 670.6 million miles away, we can also say that it is one light-hour away.

The amount of time involved essentially becomes the yardstick. The Moon is about two light-seconds away. The Sun is eight light-minutes away. A light-hour does not get us all the way to Saturn, and the closest star is four light-years away. Galactic sizes are on the scale of hundreds of thousands of light-years, and intergalactic distances are on the scale of millions and even billions of light-years. There’s nothing like the distances across the vast emptiness of space to make light seem to flow like molasses.

An implication of light’s incredibly slow speed is that our eyes are essentially time machines. The point at which Saturn appears in the sky is actually where it was over an hour ago. Similarly, The Andromeda Galaxy, our closest galactic neighbor, is seen as it was 2.5 million years ago. A supernova detected today in a galaxy located 11 million light-years away actually happened 11 million years ago.

The Virgo Galaxy Cluster

The distribution of galaxies within the Universe tends to be in clusters. The one nearest our local cluster is the Virgo Galaxy Cluster. It contains about 1500 galaxies, it is roughly circular when viewed from Earth, and its width is about 8 degrees, or about the width of 16 full moons. Like the stars in a star cluster, the galaxies in a cluster are gravitationally related to one another. Because of the cluster’s large size, and because even the brightest galaxies are quite dim, it is not observable at all with the unaided eye, and even the best telescope can see but a portion of the cluster at one time. The best way to observe the cluster is photographically in a wide field of view. Here is my attempt to photograph the cluster last year: http://www.jrjohnson.net/pages/image_template.php?ID=11.

There are about 30 galaxies in this inverse image, and they can be identified with their Messier catalog number (e.g. M86) or their New General Catalog (NGC) number. In a good quality, properly adjusted display, each of the galaxies can be seen as a smudge inside the circle near the M or NGC labels. The source data for this image is 24 full frame, single exposures that were stacked together to reduce the noise (graininess). Each of the individual images were exposed for 30 seconds at f/5.6, ISO 800 using a 70mm lens. Although my camera was guided, an unguided exposure of field this wide should have only very minor star trails, which are caused by the Earth’s rotation during the 30 seconds that the camera shutter was open. The galaxies could be brighter, and perhaps more of them could be seen if a lower f/ number or a higher ISO were used. Experiment with various camera settings, and consult a star chart on line to determine where to point the camera.
http://en.wikipedia.org/wiki/Virgo_Cluster

The Inclination of Saturn’s Rings

As Saturn cycles through its 29 1/2 year orbit about the Sun, we on Earth will alternatively see the top (north face) of Saturn’s rings for roughly half of its orbit, and the bottom half (south face) of Saturn’s rings for the other half. This occurs as a result of how the rings are inclined with respect to the plane of Saturn’s orbit. As the view transitions between upper and lower halves of the rings, our view will be edge on, and the rings will seem to have temporarily vanished. The last edge-on view was in 2009, at which time our current view of the north face of the rings began opening up. Even before Saturn’s rings fully open in 2017, our present view is pretty spectacular.

Polaris (The North Star)

Circumpolar star trailsPolaris’ special significance is that it is the Pole Star, located very near the point at which the Earth’s northern axis intersects the celestial sphere. As a result of this unique location in the sky, Polaris will appear to remain stationary all night while all of the other stars will appear to rotate around it. You may have seen star trail images that illustrate this effect. Another implication of Polaris’ unique location is that it is a measure of one’s latitude. From Ashton, MD for example, Polaris appears 39.15° above the point on the horizon in the direction of due north, which corresponds to Ashton’s latitude on a map or globe. Polaris marks the end of the bear’s tail in Ursa Minor, or the end of the handle of the Little Dipper.

The best place to start when looking for Polaris is the Big Dipper which is part of the constellation known as Ursa Major. The two stars at the end of the bowl farthest from the handle are pointer stars. Follow an imaginary line from the pointer star at the  bottom of the bowl, through the one at the top of the bowl. The 2nd magnitude star that is about five times the distance between the two pointer stars is Polaris.

The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.
The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.

Polaris has not always been the Earth’s Pole Star, nor will it always be the pole star. As a result of the procession of the equinox, the axis about which the Earth rotates will trace out a large circle every 26,000 years, as illustrated below. This sounds like a long time, but when the Egyptian pyramids were built 5,000 years ago, Thuban in Draco was the pole star, and thus some north facing entrances of the pyramids were aligned on this star. In 8,000 years from now, Deneb in Cygnus will be the pole star, and the Earth’s axis will be aligned with a point near Vega in about 12,000 years from now.
http://en.wikipedia.org/wiki/Polaris.

© James R. Johnson, 2014.
jim@jrjohnson.net

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.