Inch f12 Apochromatic Refractor

Because this telescope is expected to perform well under the most difficult circumstances, it will be tested in a comparatively harsh manner. Twenty times the focal length is 120 feet, or 37 meters. To avoid vignetting from the baffling and to avoid an erroneous estimate of spherical aberration, you will push this distance to 80 m. Interpolating Table 5-3, you see that a pinhole diameter of about 0.16 mm would be correct at 37 meters, but you're going twice that far, so you want one twice as large. Three hundred pinhole diameters of 0.32 mm yields 96 mm, or about 4 inches.

You only have a 50-mm tree ornament, but since you will be doing this test at night it's easy to move the 1-cm masked flashlight to about 60 cm from the sphere instead of the usual 1 m. The 60-cm distance means that the hole will subtend a little less than a 1° angle as viewed from the sphere. You back the flashlight mask with an 80A "tungsten" camera filter in order to achieve better color balance for the chromatic aberration tests.

Taking the telescope to your usual observing site, you hang the sphere about 250 feet from the telescope. The flashlight is pointed at the sphere from a couple of feet away on the near side. Because the fully assembled telescope is inconveniently high when directed toward the horizon, you place it between the seats of two sturdy "movie director's" folding chairs. You will sit on the ground.

You attempt to point the telescope by moving the rear chair. The telescope is directed at the feet of the tripod, so you elevate the front by slipping in a magazine. The sphere is now slightly too low.

It seems easier to move the target than the telescope, so you walk to the source and move the sphere higher. You rearrange the flashlight, verifying that the brightest reflection is back toward the telescope.

The image needs only a jiggle to center it. You slip in a higher magnification eyepiece. The first thing to examine is color correction. The disk has a slight magenta or reddish fringe inside focus and a green fringe outside focus. In focus, there is no apparent color haze. No red dot appears just outside of focus, but since you are testing an apochromat, none is anticipated. Rainbow smearing is not apparent in any direction, which indicates that decentering or wedge error is absent. A brighter image would be helpful, so you move the flashlight to about 30 cm from the sphere.

Now, the Airy disk is noticeably bloated, but no color haze is visible. You return to the flashlight and move it back.

Putting a green filter on the eyepiece, you look for astigmatism or stretching as an indicator of misalignment. None can be seen. Defocusing either way, no apparent difficulty with correction appears. The telescope snaps well. You defocus a long distance and look for zones. None are seen. Turned edge doesn't show, but this is a refractor. The lens cell obscures the far edge.

You are disturbed by the lack of contrast in the diffraction rings. This could indicate a problem with roughness. Then again, your eye just may be unaccustomed to the delicacy of the rings. You put in a deep-red filter, but that cuts out too much light, so you turn once again to the green filter.

The in-focus image seems to have several asymmetric thickenings in the rings, but that could be caused by slow-moving air currents between the image and you. You watch long enough to decide that the pattern is fixed.

Centering the 33% paper obstruction on the sewing-thread web, you search for small correction difficulties. You are unable to detect any difference.

Assessment: This telescope may suffer a slight medium-scale roughness, which would compromise the images on perfect nights. Such a small amount of aberration would have gone unnoticed in the other instruments. Nevertheless, it is worrisome in a lunar-planetary refractor. However, you decide to do the formal star test again and evaluate it a number of nights on planetary images. Roughness is a difficult aberration to unambiguously separate from turbulence, and you could have misdiagnosed it.

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