We find in Table 5-2 that the source distance for 1/8-wavelength error is 2x12/ or 24 times the focal length of 4 feet. Multiplication yields a source distance of 96 feet (29 m). Table 5-3a or 5-3b says the source diameter is about 0.10 millimeter, or about 0.004 inch.
This pinhole diameter—multiplied by 300—indicates a solar reflection in a 30-mm sphere. Since this is reasonably close to the 1-inch (25-mm) ornament bulb, we will use that. Don't do a lot of extra work matching these parameters precisely. You don't need to measure a 29-m testing range carefully. The sensitivity of the star test is not helped or hindered by this sort of precision. Pace the distance off; thirty-five steps should be enough. If the only available spherical reflector is 2.5 inches in diameter, you can easily go twice as far to about 200 feet (60 m). You are in control, not the test.
The range is set up over a grassy lawn. It is about 8 A.M. with the sphere placed to the south-southwest. The Sun is over the tester's left shoulder.
We note from Tables 5-la and 5-lb that the focusing travel necessary to achieve a defocusing aberration of 12 wavelengths is 1.9 mm (0.075 inches). The focuser withdraws 3/4 inch per rotation, so we will turn the focuser less than about ±:/10 of a turn for most aberration checks.
The first thing you notice when viewing the defocused image is the seeming appearance of severe misalignment. When you set the telescope up, you checked the coarse alignment, and it was fine. You replace the sighthole eyepiece. The mirror's dot is still on the center. What is happening?
Then you notice that the focuser is racked back 50 mm farther than usual. In fact, you had to dig in your eyepiece box to find the focus extender tube. You look in the sighthole again and this time see the problem. At this focus position, the edge of the diagonal mirror cuts off the outer portion of the mirror. The vignetting is a little worse on one side than the other, which explains the off-center diagonal shadow. The out-of-focus disk isn't really that far misaligned, it just isn't completely illuminated.
You walk out to the sphere, pick it up, and take it another 30 steps farther back. Returning to the telescope, you put in an eyepiece and focus; it is about an inch closer to the tube. Pulling the eyepiece out and putting the sighthole back in, you can now see the whole mirror reflected in the diagonal. Offhand, you realize that you could have used a bigger sphere at this new distance. You decide to give this one a try anyway; it seems bright enough.
This time you notice that the secondary shadow leans slightly to the left side of the defocused image. This appearance indicates real misalignment. You twist the appropriate screw on the primary mirror cell (see Chapter 6) and then check the image again. It's worse. Returning to the adjustment screw and undoing the previous adjustment, you give it a slight turn in the opposite direction.
Checking again, you see much better alignment. A few more minor adjustments and alignment is finished. Collimation will probably have to be redone before the telescope is used on elevated objects because the tube assembly is unusually strained for this horizontal angle. At this point, you look for pinching or astigmatism. You see no such effects, but then this mirror is small, full-thickness, and gently mounted.
Performing the snap test, you see that the image goes through focus rapidly. That's good news. You stare at the defocused image and try to detect a stationary pattern indicating surface roughness in the slight turbulence. You don't see any, but you will test for this condition later in the dark. It's difficult to detect roughness in any turbulence, even the slight amount troubling the instrument now. One good point is the smooth appearance of the diffraction minima; they would be coarse or broken if
Chapter 5. Conducting the Star Test roughness were severe. You put a yellow filter on the eyepiece. It seems as if the filter isn't deep enough because many colors can still be seen (in fact, color error seems worse). You put on a green filter. Now the minima are more visible.
Roll the focus back and forth equal distances on either side of focus to look for spherical aberration. Recall that this is exhibited by a hollow center on one side of focus and a bright center that diminishes toward the edge on the other side. Some tendency in this direction is visible, but it is not severe. You put the standard 33% obstruction mask on the protruding bolt at the rear of the spider and very carefully find best focus. Then you see how far you have to move the focuser inward and outward for the shadow of the diagonal to appear equally at the center. The darkness appears almost instantaneously with inward focus, but it hangs up briefly with outward motion.
This is a subjective judgement, so you replace the green filter with a neutral density filter to change conditions somewhat and try it again. Using a red filter, you obtain a fresh estimate. You remove all filters and try once more. It seems as if the average ratio of these motions is as large as 1:2 or 1:3. It may mean you have a marginally undercorrected mirror (see Chapter 10).
Taking the mask off, you next defocus the instrument from far inside focus to far outside focus, all the while looking for dips or extra bright rings that would indicate zones. You see no tight circular structure.
The last test is for turned edge. The way you normally look for this is to put on a deep-colored filter (red is good) and inspect the visibility of diffraction minima inside of focus compared with outside of focus. If turned-down edge is the only aberration, the rings are strong and crisp on the outside and weak or washed out on the inside. However, the effects of turned edge are competing with the effect of undercorrection, which also tends to wash out the diffraction rings on the outside of focus. You peer through the filtered eyepiece and can't really decide this point. One aberration fogs another.
Assessment: This telescope should perform passably on the planets, but it could do better. It is right at the edge of specifications, so you shouldn't complain to the maker. The optics don't seem to be severely rough, but a test for roughness will have to wait for a dark field of view and less turbulence. The telescope was acquired for general-purpose use, a task it should perform well.
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