To test this Schmidt-Cassegrain, recall that the telescope diverts rays beyond the edge of the Airy disk at about 48 m (157 ft), or about 24 times the focal length. (Table 5-2 results in an incorrect distance because the internal focusing mechanism compromises the optical correction.) You decide to increase separation distance to at least 100 m (328 ft). A source at this range is 2.5 times the 20 focal lengths recommended after Table 5-2, so it requires 2.5 times the 0.134-mm pinhole size of Table 5-3, or 0.335 mm. The reflective sphere should thus be 100 mm in diameter. You don't have a 4-inch sphere, but you can find a convex mirror that would be 7 inches in diameter if it were a complete sphere.
The pinhole can expand a factor of 2 before it exceeds the Airy disk size. Seven inches, though, is a bit close to the limit. Then you recall that if you test with the Sun directly to your back, with the glitter point centered in the sphere, the divisor is closer to 450 than 300. This condition would permit a sphere at least 6 inches across, and 7 inches is not too much more.
You set up the test early one morning with the Sun low on the eastern horizon and the sighting range to the west. A quick look in the eyepiece confirms that the image is too bright. After hurrying inside to get a 2-inch tree ornament, you set the new sphere at a range of 70 meters and point the instrument again. A peculiar tube current elongates the secondary shadow on one side of focus, but it goes away after a few minutes.
First comes alignment, a relatively straightforward operation since it involves only adjusting one element. Slipping on a deep-yellow filter, you do the snap test, but you can't say for sure whether the instrument snaps adequately. Focus seems soft, but not seriously defective.
You detect a small amount of spherical aberration, but you can't tell if it is undercorrection or overcorrection. The telescope is refocused with the regular high magnification eyepiece and yellow filter, the set screw loosened, and the eyepiece withdrawn 1/5 inch (5 mm). Checking Tables 5-1a and 5-1b, you see that this amount corresponds to about 12 wavelengths outside
5.3. Performing the Test
focus. The edge of the pattern is stronger than the inside, so the system is overcorrected.
Since a 33% obstruction is already in place, you look for the first appearance of the secondary shadow. It appears only about 1.5 times as far on the other side. Since the instrument is expected to appear slightly overcorrected anyway (with the close source position), this amount is very mild.
In looking for roughness, you find an unusual amount of turbulence even this early in the morning. You'll have to test again when it is quieter.
Assessment: The instrument shows signs of being excellent, but ground turbulence is too severe. Observation of the planet Saturn that night showed Cassini's division looking sharp and black between bouts of bad seeing. You will try the test again at night using a flashlight source.
A straightforward way of greatly improving a telescope's image is to align the optics. One of the most ignored optical problems, misalignment is also one of the most curable. The improvements derived by aligning previously neglected instruments are always noticeable, and sometimes the enhancement is startling.
The star test cannot only be used to diagnose misalignment, it is also useful to achieve fine alignment. Once you become familiar with the star-test method of making that last small adjustment on the collimation, you will make it a standard procedure during every observing session. With practice, star-test alignment becomes easy.
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