Aligning Three Telescopes

The alignment procedure for every commercial telescope cannot possibly be described because each maker has tiny variations in the manufacture of cells, mirror holders, and adjustment hardware. For this reason, we will examine the generic features of the process used in aligning only three common telescopes. In spite of this restriction, the descriptions which follow represent a large number of optical systems. You will not be given detailed instructions in the form of a recipe to be learned by rote ("then turn bolt A"). That job is best served by the instructions that came with the instrument. Instead, you will be taught procedures that can be applied to nearly every telescope.

Other collimation procedures use specialized methods or tools. Pussy readers are encouraged to pursue them. However, the instructions below produce reasonably straightforward collimation without cutting too much into valuable observing time. The justification and ordering of the steps will be the primary objective, with side comments concerning tricks and pitfalls.

1. Establish the axis line. The axis line is defined by two points, usually the center of the eyepiece and the center of the objective lens or mirror. It is customarily directed along the tube's center.

2. Center the optical components on this axis line. If the telescope has more elements than an objective and eyepiece, at least one non-trivial centering must take place. Sometimes, centering is set at the factory and is not adjustable by the telescope user.

3. Establish the tilts of the elements. Generally, the tilt adjustments must be made in a certain order from one of the two points that define the axis to the other point. Poor ordering makes alignment much more difficult.

4. Repeat steps 1, 2, and 3 as an iterative procedure. Because the ad justments are seldom completely independent, one alignment step can disturb previously correct settings. Coarse alignment is like raking leaves. The first pass doesn't gather every leaf; it takes a few swipes.

5. Adjust only one element in fine alignment. Fine alignment takes place on a real image. Normally, a tiny adjustment must be done on the element for which adjustment is most convenient.

Although it is not strictly necessary, you can save time and effort by using a helper to align. On all but the most compact telescopes, collimation is a job best done by two people, one at the eyepiece and one making adjustments.

6.5.1 The Newtonian Reflector

The presence of the diagonal mirror and the many confusing reflections make this adjustment the most difficult of the three discussed here.

Before the alignment begins, you have to make preparations. The first item needed is a centered sighting hole. This blank or lensless "eyepiece" has a small opening at the back instead of a lens element. The purpose of such a device is to allow you to unambiguously place your eye on the axis of the focuser tube. A less obvious purpose is to allow you to sight from a location right at the focal plane. It obscures the view unless the eye is placed close. Thus, the eye can be located not only at the center of the focuser tube but at the correct distance from the diagonal. You want to place this sighting hole near the location of the focal plane when the telescope is focused on distant objects.

An inexpensive source of these sighting holes are clear plastic 35-mm film cans with the bottoms cut out and a 3-mm hole drilled into the lid. Wind them with a few turns of tape so they will they fit snugly.

The second preparation is to place a marker dot at the geometric center of the mirror. Taking care not to touch the mirror's surface accidentally, lay a stiff ruler across the diameter. Along the ruler, you can determine the center very precisely, but at right angles to it, you can only estimate. At the center, draw a short (3 mm) line at a right angle to the straight edge. The center will be at some location along the direction of this stubby line. Do the same after a quarter turn of the mirror. Look closely at the center. What you should see is two short lines slightly offset as in Fig. 6-4. The true center is at the intersection of these two lines if they are extended until they meet. This is the place to put the dot.

Don't commit yourself to making a large dot at first. It's best to make a barely perceptible mark at this location and measure it both ways to verify it is the center. When you make a bigger mark, you will probably want to extend it out on one side to correct for the inevitable error. This dot can be something as simple as permanent black marker ink or as elaborate as white paint. (Some people claim a white dot is easier to see when you

Spot location

Fig. 6-4. The procedure for Ending and marking the true center of a circular disk.

Spot location

Fig. 6-4. The procedure for Ending and marking the true center of a circular disk.

are attempting alignment in the dark with a flashlight.) Make the dot large enough to see easily (6 mm or V4 inch). Do not worry about the dot or short lines interfering with optical quality. The marks are in the shadow of the diagonal mirror.

Steps 1 through 3 are best done in a brightly lit room illuminated by diffuse light. (Fluorescent light is ideal.) The inside of a telescope tube is not a well-lit place. During coarse adjustment, tape a sheet of white paper inside the tube across from the focuser. The paper will provide a lighter background to the outline of the diagonal holder than the normally darkened interior of the tube.

Step 1: Establish the axis line.

The axis line will be from the center of the eyepiece's field of view to the center of the mirror. This step would be trivially easy were it not dependent on the right angle break that happens at the diagonal. Once the diagonal is aligned, it will be simple to check, but it is impossible to verify now. We instead pay attention to one crucial fact, the need for the focuser to slide the eyepiece linearly along the optical axis, thus making sure that both your eyepiece and Barlow lens will be on this axis. A misaligned Barlow is a disaster for those who must wear eyeglasses, since the Barlow and medium-power eyepiece are used in preference to extremely short-focus eyepieces.

We will assume that the optical axis will be along the center of the tube and that the diagonal will be set precisely at 45°. The problem of focuser motion reduces to making sure the focus tube is pointed at the tube's center line and that it doesn't lean toward either end of the tube. If you have an accurately round tube and the focuser fits snugly against it, you probably needn't worry, but such instruments are rare.

I can't give instructions for measuring the lean of the focuser because every situation is different. I can only point out some tricks for magnifying the lean direction. The first is to put something besides the eyepiece in the focuser. A good choice is a long tube the same diameter as an eyepiece. We will call it the "long eyepiece." If you extend such a tube on the outside of the focuser, it will become easily apparent if it leans toward either end of the tube. Make a right angle template out of a manila file folder or other thin cardboard sheet. Cut out enough of the corner so that it clears the focuser hardware, and lay it against the protruding tube. Any lean of the focuser along the telescope tube becomes immediately apparent.

Remove the diagonal, and extend the long eyepiece to the center of the telescope tube, or as near as you can get. See if the long eyepiece is pointed at a skewed position to either side of the telescope axis. This step is especially easy if your Newtonian has a spider. Just place a pointer or screw in the hole from which you removed the diagonal holder and look through the other end of the long eyepiece tube. Of course, center the spider first.

Once you've verified that your focuser fits squarely on the telescope and advances and retracts eyepieces more-or-less along the planned direction, you are ready to continue. If the fit is skewed, shim the focuser until it points squarely at the center of the tube.

Step 2: Center the optical components along the axis line.

Since the axis will go through the center of the mirror and the center of the image plane wherever they might lie, "centering the components" means centering the diagonal along that line. Remember, diagonal placement is achieved without any reference to reflections. At this stage, the main mirror need not be inside the instrument. The diagonal could well be a block of cement. In fact, it is helpful to think of it as nonreflective. You must suppress any urge to center the reflections you see in the diagonal. That doesn't concern you now.

Consult Fig. 6-5. This diagram represents a perfectly aligned Newtonian. (Focal ratio has been exaggerated to make the anomalous behavior readily visible.) Note that the distance dnear is greater than dfar. Alignment is perfect by definition, yet it seems as if the diagonal mirror is not centered. Already we seem to violate general condition 2.

Most importantly, the components must be optically centered, not physically centered. When you look through the little sighting hole replacing the

Fig. 6-5. A perfectly aligned Newtonian telescope with focal ratio about 1.75. Displays centered and non-centered features.

eyepiece, what do you see? The diagonal should appear as a perfect circle neatly centered at the bottom of the focuser tube. The offset is caused by perspective foreshortening of the far side of the diagonal. It is so much further away, that it actually appears smaller. You need to slide the diagonal sideways and down to center it on the converging cone of light. The view through the sighting hole is sketched in Fig. 6-6.

Fig. 6-6. Centering of the diagonal at the bottom of the focuser tube. (This tube is depicted as the dark annulus at the outside with gray radial streaks in it.) a) An improperly placed diagonal, b) The correct position as a result of a backward slide of the diagonal holder. A light piece of paper is attached across the tube.

Fig. 6-6. Centering of the diagonal at the bottom of the focuser tube. (This tube is depicted as the dark annulus at the outside with gray radial streaks in it.) a) An improperly placed diagonal, b) The correct position as a result of a backward slide of the diagonal holder. A light piece of paper is attached across the tube.

The amount of forward offset is determined by trial-and-error. Move the diagonal forward and back until it appears centered. Please note that the word "appears" is emphasized in this statement. Because judgement may be difficult, you may be tempted to remove the sight hole and peer along the focuser tube at a grazing angle. Then you would incorrectly compare the front and back sides and incorrectly set the diagonal. The correct method is to view it from the center and move the diagonal until it only looks centered.

You may detect an error in the "up-down" direction of Fig. 6-6. That error is probably caused by an undetected tilt in the focuser or a side shift of the spider. Adjust for it either by shimming the focuser or by moving the spider before continuing. Figure 6-6b shows a properly centered diagonal.

The amount of offset of the diagonal away from the eyepiece is impossible to judge just by looking. It must be calculated and measured until the quantities dnear and dfar are correct. This offset can be straightforwardly calculated using analytic geometry. (Such a derivation appears in Appendix C.) Here is the answer and some easy approximations:

If D is the diameter of the mirror, L the diameter of the field of 100% illumination, T the distance from the center of the tube to the focal plane, f the focal length, and s the sagitta of the mirror surface (s s d2/16 f ), then the offset is rife -T + nL / 2 „ D - L

For a very small field of 100% illumination and a shallow mirror, this is approximately.

where F is the focal ratio.

Say you have two equal stacks of coins. If you remove one coin and place it on the other stack, the stacks differ in height by two coins. For the same reason dnear is larger than dfar by twice the offset. A number of estimates for dnear -dfar are listed in Table 6-1. These typical offsets are generated by using Eq. 6.2 with reasonable estimates for T.

At long focal ratios, the offset is very small, so use of this table should only be critical in its lower left corner—for fast and big mirrors. For that reason, the approximation in Eq. 6.3 is more than adequate. Optimally, fast Newtonians should be designed with very small regions of full illumination at the focal plane (Peters and Pike 1977).

Most spiders can be adjusted to allow the diagonal to be offset deliberately merely by changing their mounting-screw adjustments. Do not worry if the vanes on opposite sides of the diagonal are not precisely lined up. This condition only modifies the spider diffraction pattern. The amount of light scattered by the spider is the same as before. In a way, the spider diffraction pattern with 8 less-bright spikes may be better than 4 strong ones in some observing circumstances.

Table 6-1

The difference between the two measurements from the tube

Table 6-1

The difference between the two measurements from the tube

to the edge of the diagonal, dnear

- dfar =

2 Offset

Field of 100%

illumination is zero

Focal

Ratio

4

4.5

5

6

7

8

10

Diameter (in)

T

2 Offset (in)

3

3.5

0.11

0.09

0.07

0.05

0.04

0.03

0.02

4.25

4.4

0.14

0.11

0.09

0.06

0.04

0.03

0.02

6

5.6

0.18

0.14

0.11

0.08

0.06

0.04

0.03

8

7.0

0.22

0.17

0.14

0.10

0.07

0.05

0.04

10

8.4

0.26

0.21

0.17

0.12

0.09

0.07

0.04

12.5

10.2

0.32

0.25

0.20

0.14

0.10

0.08

0.05

14.25

11.4

0.36

0.28

0.23

0.16

0.12

0.09

0.06

16

12.6

0.39

0.31

0.25

0.18

0.13

0.10

0.06

17.5

13.6

0.43

0.34

0.27

0.19

0.14

0.11

0.07

20

15.4

0.48

0.38

0.31

0.21

0.16

0.12

0.08

24

18.2

0.57

0.45

0.36

0.25

0.19

0.14

0.09

Focal

Ratio

Diameter(mm)

T

4

4.5

5 6 2 Offset (mm)

7

8

10

75

90

2.8

2.2

1.8

1.2

0.9

0.7

0.4

110

110

3.5

2.7

2.2

1.5

1.1

0.9

0.6

150

140

4.4

3.5

2.8

2.0

1.5

1.1

0.7

200

180

5.6

4.4

3.6

2.5

1.8

1.4

0.9

250

210

6.7

5.3

4.3

3.0

2.2

1.7

1.1

320

260

8.1

6.4

5.2

3.6

2.6

2.0

1.3

360

290

9.0

7.1

5.8

4.0

2.9

2.3

1.4

400

320

10.0

7.9

6.4

4.4

3.3

2.5

1.6

450

350

10.8

8.6

6.9

4.8

3.5

2.7

1.7

500

390

12.2

9.7

7.8

5.4

4.0

3.1

2.0

600

460

14.4

11.4

9.2

6.4

4.7

3.6

2.3

Step 3: Establish the tilt of the elements.

Once you have the diagonal visually centered in the focuser tube and offset the proper distance, you are ready for the tilt adjustment. If you were to carefully set the tilt of the primary mirror first, the setting would go amiss when the diagonal is finally adjusted. Thus, the tilt of the diagonal must be fixed before the main mirror is touched.

At this point, visualize the main mirror as painted white. All you can see at the primary are mirror clips jutting onto its surface. Ignore any reflections. A useful trick is to become painfully aware of dust on the mirror. Concentrate on the dust and the mirror clips. Look at the mirror, not through it. Figure 6-7a shows the diagonal with incorrect tilt.

Rotate the diagonal holder in the spider until it looks like Fig. 6-7b. Then adjust the screw in the holder base that tilts the diagonal either toward or away from your eye to center the main mirror clips as in Fig. 6-7c. Most diagonal holder bases don't contain springs. If you loosen one screw, you must tighten the other two.

Fig. 6-7. Setting the angle of the diagonal: a) A misaligned diagonal, b) After spinning the diagonal in the spider, c) After tilting the diagonal using screws at the base.

Oddly, the two most common adjustments are rotating the whole diagonal about its axis and adjusting only one screw at the base. You may well ask why diagonal bases have three pesky and hard-to-reach screws. A spring-loaded hinge and one adjustment screw make more sense. One explanation is that telescope makers are extremely conservative, and diagonals have been mounted with three screws for a long time.

The above description had all of the wrong turns and frustrations removed. At first, it seems impossible to make these adjustments to simultaneously achieve diagonal alignment and keep the connectors tight. As the wrench is turned on the spindle, diagonal rotation is especially hard to prevent. Fifteen minutes of careful work can be lost while you try to torque the diagonal securely into place. What you eventually learn is how to predict the effect of the wrench. Ultimately, you will know how far to offset the finger-tight alignment to achieve wrench-tight alignment.

The last job of coarse collimation is setting the tilt of the main mirror. Finally, you may consider the reflection in the primary. The little hole through which you are looking is reflected somewhere near the mirror dot. By adjusting the screws on the back of the main mirror, move the hole reflection until it is behind the dot. Do not spend a great deal of effort on refinements of this adjustment. You will set it empirically with the star test.

Figure 6-8a is an example of a misaligned main mirror. Figure 6-8b shows correct coarse alignment. The stubby arrow shows the direction to the reflection of the focuser's base.

Fig. 6-8. With the diagonal aligned, the screws on the back of the main mirror are adjusted until the dot is superimposed on the sighting hole: a) the starting appearance, b) correct coarse alignment after moving the reflection of the hole in the indicated manner.

Step 4: Repeat steps 1, 2, and 3 as an iterative procedure.

You are certainly allowed to start this process over again to correct any difficulties. Remember to order the steps properly. Don't adjust randomly, even though you are sorely tempted. A good example of one such temptation is the decentered appearance of the second reflection in the diagonals of Figs. 6-5 and 6-8b. Remember, these systems are correctly aligned, but the reflection of the bottom of the focuser is still decentered, an especially large effect in fast mirrors with big diagonals.

When they first notice this decentering, many first-time telescope collimators start tugging at the diagonal, and end up destroying their hard-won coarse alignment. People are unusually disturbed by this decentering, and they often skew the alignment until these circles are approximately concentric as well.

Unfortunately, this mistake has the same effect as tilting the focuser over, and by implication, tilting the focal plane. The eyepiece is no longer transported along the optical axis. Barlow lens performance may suffer, and the edge of photographic film frames become indistinct and blurry. Because of the natural curvature of field in Newtonian reflectors, one side of the frame is severely out-of-focus while the other is not. (Indeed, such photographic behavior can be used to diagnose a tilted focal plane.)

Supposedly, Fig. 6-5 is perfectly aligned. Why then, should a telescope in perfect collimation ever show any form of decentering? The problem is the three reflections that occur when you look though the telescope in this manner. The usual path of light is one reflection off the primary and one reflection off the diagonal. When we stare backwards through the telescope, the path is one reflection off the diagonal, a second reflection off the main mirror (okay so far), and a third reflection off the diagonal. We should expect centering only as far as the main mirror in Fig. 6-8b. Neither the diagonal as reflected in the main mirror nor the bottom of the focuser tube as reflected twice in the diagonal needs to be centered.

One modification should be made when you are satisfied with the coarse alignment of the instrument. Untwist the vanes of the spider so that they are once again perpendicular to the main mirror. This step is best done gently with a narrow-jawed crescent wrench. Cradle the vane near its middle or somewhat nearer the tube. Look through the sighting hole and twist the spider until it appears narrowest. This ensures that the spider obstruction causes as little blockage as possible.

Step 5: Adjust only one element in fine alignment.

Fine-align on a star. Use your accustomed configuration for high magnification. If you normally use a Barlow lens, include it in the optical train. You should be more concerned with hitting the axis of that Barlow than with attaining a pretty alignment of the dots. Pick out a moderately bright star of high elevation, and center it in the field. Rack the focuser out or in until defocusing aberration is about 10 wavelengths. If the telescope is severely misaligned, you see something like Fig. 6-9d. This pattern is unlikely if coarse alignment was done carefully. A misalignment of 12 arcminutes in our example means that the reflection of the sighting hole is decentered by 4 mm, or just outside the dot. (Consider how bad the alignment must have been before you started.)

The model used to generate these patterns is somewhat inadequate. The obstruction is still centered in the calculations, but it isn't centered in a real misaligned reflector. The sideways shifting of the secondary shadow is slightly enhanced outside focus and diminished inside focus.

More likely, you see less distorted patterns as in Fig. 6-9c or Fig. 6-9b, which are examples of milder misalignment. For these cases, the pattern Fig. 6-9b is difficult to distinguish from a perfect pattern when it is defocused as far as 10 wavelengths. Choose a dimmer star and defocus a smaller amount. Something like the images in Fig. 6-10 should appear. If seeing is excellent, you can perform the last critical adjustment on a focused star. Unfortunately, the seeing is seldom good enough to do so. Take comfort in knowing that if turbulence is bad enough to make star-test alignment difficult, seeing is probably the limiting factor in your wobbly stack of filters.

The optical axis in all of the figures is far to the right of the pattern. You need to move that axis back to the center of the field, so you want to move the image to the left. The real situation will not be so cooperative. The coma flare can be pointed at any angle. You must be able, somehow,

Fig. 6-9. Star test patterns showing increasingly bad misalignment of a 10-inch, (250-mm) f/4.5 Newtonian reflector: a) the expected pattern if the telescope is perfectly aligned, b) misaligned by 3 minutes of arc (the worst misalignment that delivers a passable image), c) misaligned by 6 arcminutes, and d) misaligned by 12 arcminutes. The focused patterns are magnified 6 times compared to the unfocused patterns. See Appendix D for labeling information.

Fig. 6-9. Star test patterns showing increasingly bad misalignment of a 10-inch, (250-mm) f/4.5 Newtonian reflector: a) the expected pattern if the telescope is perfectly aligned, b) misaligned by 3 minutes of arc (the worst misalignment that delivers a passable image), c) misaligned by 6 arcminutes, and d) misaligned by 12 arcminutes. The focused patterns are magnified 6 times compared to the unfocused patterns. See Appendix D for labeling information.

misaligned 3', 0B=2D% 15

misaligned 3', 0B=2D% 15

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