Fig. 6-10. The 3 arcminute misalignment of Fig. 6-9 as defocusing is adjusted from 1 wavelength to 4 wavelengths. The misalignment is clearly shown.
to correlate the pattern you are seeing with a cell adjustment screw.
A Newtonian has two mirror reflections, including one at a right angle. Perhaps some people can visualize the three-dimensional situation well enough to determine which screw to turn purely by logic, but they are few. The best technique to find this screw is to decide which clock angle you want to move the image—say, to 7 o'clock. Then rack the eyepiece a long distance out of focus. Placing your hand in the optical path of the telescope and noting where the shadow intrudes on the defocused disk, you can decide what part of the mirror corresponds to a certain clock angle. You can bring the hand shadow either to 7 o'clock or 1 o'clock. Then follow this orientation back along the tube to the mirror cell.
When you trace this line back to the mirror, you will emerge either close to a screw or across the tube from one. Give that screw a small adjustment in either direction. It is not important which way you turn it, just that you remember the modification. Recenter the star and see if the situation is worse or better.
If it's worse, undo the damage and turn it the other way. Look at the image again. It should now improve. Pick a new adjustment direction, and repeat the whole process. Always recenter the star before deciding on the next step. If the cell adjustment bottoms out and you cannot tighten a screw, remember that loosening the other two is equivalent.
Fig. 6-11. The adjustment triangle of a three-sided cell, and the first few steps in fine alignment on a star-test image.
As you home in on a fine-aligned mirror, you should be following a path like that in the example of Fig. 6-11. You can only tilt the mirror along the edges of the adjustment triangle. Please remember, what you are really changing is the position of the optical axis, not the image, so you want to move the image in a reverse direction to all of these adjustments.
I personally find image movement very confusing, so I don't even watch the image shift in the field of view. I just decide the clock angle and use
Fig. 6-11. The adjustment triangle of a three-sided cell, and the first few steps in fine alignment on a star-test image.
Tolerance region the trial-and-error method described above. Again, it is more important to move slowly and methodically than to understand every twist and turn. The path of Fig. 6-11 is not the only solution to this initial condition or even the most efficient, but it succeeds.
Most smaller, less-expensive refractors cannot be collimated because the makers prefer locking down the factory adjustment. Reasoning that incompetent users cannot botch the alignment, they don't fit the telescopes with adjustments. Fortunately, small refractors use a long-focus design that can tolerate large misalignments, so the absence of adjustments doesn't often affect the image.
In recent years, partly because of the resurgence of interest in big refractors and the advent of new apochromatic and advanced glass designs used at low focal ratio, refractors are being supplied with adjustable cells once again. If your refractor isn't adjustable, you can still check it using the method described here. Unfortunately, telescopes lacking adjustments might have to be returned to the maker for collimation. So-called "telephoto" designs may also need to be returned. These instruments are permanently fitted with a hard-to-reach telenegative amplifier as the last lens group, far down the tube.
Refractors are not difficult to align. Geometric alignment is usually sufficient because their advanced optical designs yield wide, well-corrected fields. The problem is one of technique and equipment. A refractor is aligned sitting on a table with the lens cap on.
The usual device used to inject light into the darkened tube is called a Cheshire eyepiece (Sidgwick 1955, p. 185), a modified version of the sighting hole used to align the Newtonian. In that case, ordinary room light on the translucent film-can cap was sufficient to provide enough backlight to see the dot. For refractors, a great deal more light is needed. Ordinary glass only reflects about 5% of the energy that strikes it, transmitting the rest. Coated lenses reflect even less. The Cheshire eyepiece is designed to provide a target bright enough to see, even after inefficient reflections. You can obtain one commercially or make it yourself.
Figure 6-12 shows one such alignment tool. It has a long tube with a porthole drilled in the side to allow light to enter. A dowel cut at 45° is inserted in the end of the eyepiece and drilled so that you can see through it. The oblique side of the dowel is at least painted glossy white, and if its surface is polished metal, so much the better. The inside of the sighting hole is carefully blackened. The purpose of the defining stop is to put a crisp edge on the target. Such a refinement is not really necessary if the
Port In sic1"
Defining Annular appearance stop down the tube
Fig. 6-12. The Cheshire "eyepiece." The Cheshire is just an illuminated sighting hole; it contains no curved or high-quality optical surfaces. The reflective target pattern is shown at right.
porthole is carefully placed.2
Taylor advocates the use of a white card tilted at 45° into which a sighting hole is placed, a "Cheshire" without the "eyepiece" (Taylor 1983). This suggestion, however, presumes that a great deal of care is taken with mounting the card and centering the sighting hole. Blacken a small elliptical area around the hole so you will see a circle at 45°.
Most adjustable lens cells use some variant of the push-pull system depicted in Fig. 6-13. This lens cell, when precisely adjusted and locked down, is extremely stable. A telescope flange is fixed on the tube, and the lens cell floats on three adjustable "push" screws threaded into that flange. Because the push screws are not sufficient to prevent the cell from dropping off, a matching pull screw associated with each push screw is added, making 6 screws in three groups around the tube. Since no springs are used, as one screw of each pair is loosened, its partner must always be tightened. Figure 6-13 shows a wide separation of the lens cell and the telescope flange to demonstrate the function of a push-pull pair. In fact, the starting configuration should always have the lens cell fitted nearly against the telescope flange. The adjustment will be longer-lived if the gap is small. Also, most real mirror cells carefully tuck the adjustment screws out of the way so they will not disturb the clean lines of the telescope. Some designers are so clever at hiding these screws that the instrument may not at first even appear adjustable.
If you can make this adjustment with a helper, you are strongly advised to do so. A couple minutes of alignment with two people quickly expands to an hour when you do it yourself. Put the Cheshire in the focuser
2 If you use a Cheshire to align Newtonians, make sure you have sufficient in-focus travel to allow the eye to be placed close to the focal plane, and with fast mirrors make certain that the long sighting hole does not obscure the outside of the optics.
and shine light from the side, carefully shielding your eyes from the light source. If possible, use a low-magnification, close-focusing telescope looking directly through the rear hole. This ploy removes your eye from the bright sidelighting and expands the reflections so you can easily see them.
In one apochromatic refractor, the pattern of Fig. 6-14a was visible through a close-focusing finder telescope behind the sighthole of the Cheshire. The big annulus was a bright steely gray and was just about the size of the Cheshire. It must have been reflected from an air-to-glass surface of low curvature. The next annulus inward was pastel blue and somewhat dimmer. It was smaller, so it must have originated at a more sharply curved surface. The smallest ring was a very dull red or magenta and was not even observable with unmagnified vision. It perhaps relied on two or more of the inefficient reflections at coated interfaces.
This apochromat had six air-to-glass surfaces, and only three reticle reflections were seen. Fewer reflections are anticipated in a doublet. In fact, the Cheshire tool may not be very useful with some cemented doublets or refractors which use optical couplants (oils or gels) between the lenses. Only one reflection might be bright enough to see, and you wouldn't be able to compare it to others.
By adjusting the push-pull pairs, you can quickly make the reflection look like Fig. 6-14b. Turn the telescope over and check the pattern again. You will probably discover that the lens rattle designed into the cell compromises the centering. This condition is nothing to worry about; simply adjust it until it is about equally misaligned at each orientation. Even with the situation in Fig. 6-14a, misalignment was not noticed in the image.
After you achieve this grade of alignment, you are usually done. You can check the out-of-focus image but you probably won't be able to detect any astigmatism caused by misalignment (although other causes are still possibilities). The well-corrected field of a typical refractor is enormous.
If you do notice some astigmatism, you can certainly try to adjust it out using the star test. The direction of adjustment is less clear than it was with more coma present, since the direction of the optical axis can either be along the short dimension of the out-of-focus stellar disk or along the long dimension. For example, the optical axis may be found at 4 o'clock, 10 o'clock, 1 o'clock, or 7 o'clock, depending on whether you're inside or outside focus. With coma present, the angle was unique.
If the eyepiece is set inside focus, the optical axis can be found on either side of the short axis of the astigmatic oval. If the eyepiece is outside focus, the optical axis can be found along the long axis. For obvious reasons, you should decide on a certain side of focus and stick with it.
When the cell is adjusted at 90° to the proper direction, the stretch direction of astigmatism rotates rapidly. Also, undoing the adjustment and going an equal distance on the other side doesn't improve anything; it just reverses the rotation.
In star-test alignment, only a tweak of the push-pull screws will be enough. After all, the telescope should be very nearly collimated. Tiny changes at the screws mean enormous changes at the focal plane. If you are unable to remove the astigmatism by collimating it out, your telescope may be suffering from pinched optics or a true cylindrical deformation ground into the glass.
Let's review the general steps involved in alignment and see how they applied to refractors:
1. Establish the axis line. It was defined as the center of the tube.
2. Center the optical components on this axis line. Since most refractors have only one closely-spaced group of lens elements held in an accurately machined cell, this step was automatic. The focuser is assumed to transport the eyepiece along the axis. (In small, inexpensive refractors, this condition is not always met.)
3. Establish the tilts of the elements. This step was accomplished by centering the reflection of the annular reticle pattern of the Cheshire eyepiece.
4. Repeat steps 1, 2, and 3 as an iterative procedure. Check the align ment with the refractor turned over, and adjust until the Cheshire reflection looks about equally misaligned at all orientations.
5. Adjust only one element in fine alignment. This step was probably not needed, but if it were, it would have taken place on the objective.
Schmidt-Cassegrains of effective focal ratio f/10 have a primary mirror of about f/2 multiplied by a five-power convex secondary mirror. The center of curvature of the primary mirror must be behind the center of the secondary. Since the main mirror is not adjustable by the user on most Schmidt-Cassegrains, that adjustment must be set properly at the factory, or the telescope cannot be collimated.
An unacceptable main mirror adjustment is difficult to diagnose, but some clues exist. First, go through the rest of this collimation procedure to the best of your ability. Then, using a sighting hole (described above under Newtonian alignment), look back through the optical system. If you don't see absolutely concentric circles, rings within rings, your best alignment may be a kind of compromise. You will be offsetting the secondary to partially compensate for the aberrations induced by a misaligned primary. Still, the main mirror misalignment must be fairly serious before you are really able to detect non-circularity in these tiny reflections.
The front side of the instrument is an easier location to detect misalignment of the primary mirror. For 200-mm Schmidt-Cassegrains, place your eye a couple of feet from the front (about / meter) and center the biggest reflection of the secondary outside the back of the secondary. By carefully adjusting the placement of the eye, you are able to see the reflection of the secondary as a thin annulus outside the true secondary. You are now nearer the alignment axis of the main mirror. If the mirror is seriously misaligned, it should be obvious that this axis does not coincide with the axis of the tube because the inside of the telescope will look tilted.
Another clue is derived from the way these primaries are mounted. The focusing action actually transports the mirror forward. The mirror's center is glued to a plate on the front of this axial focuser. Often, these mirrors get out of adjustment because of some sort of mechanical fault in the focuser. (Perhaps it has taken an enormous jolt during shipping.) As you focus the instrument, the image is not seen to defocus in a fixed location but reels or loops across the field. In any case, such anomalous focusing behavior, if severe enough, demands factory service.
For now, let's assume you have a well-aligned primary mirror. The only free adjustment is the tilt of the secondary mirror. If you have a severely misaligned Schmidt-Cassegrain, you may need to coarsely align by looking through it with a sighting hole. Center the reflection of the primary mirror in the secondary. Usually, this step will be unnecessary.
The final step is fine alignment with the star test. You can align the telescope in the daytime on an artificial star or at night on a real star. The Schmidt-Cassegrain is the most convenient of the example telescopes to align because of its compactness. If your arms are long, you can actually reach the adjustment screws while your head is behind the eyepiece. Of course, collimation is still easiest with two people, one calling out instructions and the other trying to obey them.
The secondary cell of a Schmidt-Cassegrain mount is a variation of the diagonal mount of the Newtonian. In both cases, loosening one screw is counterbalanced by tightening the other two. In changing the tilt of the secondary mirror, you must achieve alignment while keeping the mirror cell screwed tight.
It may seem expedient to overly tighten one screw as collimation is approached. Avoid such a shortcut. The secondary mirror is mounted in glass, and you might break the corrector. Also, the secondary mirror is held on a stiff plate, but this plate can be bent and the mirror strained. Finally, you might jerk the wrench out of the socket when forcing it and end up scratching the corrector. Tighten it snugly, but don't force it. If you have to move it a bit more, loosen the other two screws instead.
A misaligned Schmidt-Cassegrain will generate the same sort of star test behavior as depicted earlier for a Newtonian. Perform the star test without a 45° elbow. You will have one less potential source of aberration. and you will be able to observe the angle of the optical axis. The correct screw to turn is straightforwardly determined. Soon, you can center the shadow of the secondary in the image. (Use the same method as was used in the Newtonian.) Refine the alignment by turning to a dimmer star and defocusing it less or, if seeing is excellent, leave the telescope in focus and
6.5. Aligning Three Telescopes
adjust the image for symmetry.
To review, the steps involved in aligning a Schmidt-Cassegrain were as follows:
1. Establish the axis line. That line, by definition, is coaxial with the tube.
2. Center the optical components on the axis line. Centerings are factory-set and hence are not adjustable.
3. Establish the tilts of the elements. The tilt of the corrector is, to first order, unimportant. The tilt of the primary is a factory setting and depends strongly on the condition of the focusing mechanism. Only the tilt of the secondary may be adjusted.
4. Repeat steps 1, 2, and 3 as an iterative procedure. Because so many of the coarse alignment adjustments are out of the owner's hands, iteration is impossible.
5. Adjust only one element in fine alignment. This step is combined with step 3. It is done on a star or artificial pinhole source placed at around 50 meters or farther; only the secondary is adjusted.
If your telescope still displays asymmetric images at the end of these steps, then it will have to be returned to the manufacturer. The primary minor is probably tilted. The lack of adjustments on the main mirror is perhaps the weakest feature of commercial Schmidt-Cassegrain designs.
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