Performing the Test

Because people wanting to test telescopes don't usually have access to towers or convenient topography to elevate the sphere, they must test through ground turbulence. Daytime testing is probably best done during the early morning and over a grassy field, but different locations and times have their own behavior. Often, a quiet time of non-turbulent behavior can be found to briefly persist near sunset. Go ahead and try the test anytime and anywhere; you may be pleasantly surprised. Also, try to place the Sun at your back to shade the eyepiece and ensure an approximately round reflection in the sphere.

You can mount your sphere in a stiff piece of poster board, poking the hanger through a hole cut in the board. If you don't use the board and wish to hang the sphere on a bush or a tree, then be sure to paint, tape, or otherwise obscure the hanger region of the ornament. It has nonspherical curvature that may present a second interfering point of light. If you are using a curved rear-view mirror, be sure to obscure the edge if it is shiny. (See Fig. 5-1.)

Fig. 5-1. Spheres are illuminated by a large spotlight to exaggerate the glitter renection. A convex rear-view mirror is shown connected to the tripod head. The stray reflection points have yet to be obscured by tape.

One might think that a black poster board would work best, but a uniform color of dark green also works well. The uniformity is more important than the color, although bright colors should not be used.

Most telescopes require moderate distances, but large Newtonians of low focal ratio demand long, clear testing fields. Long distances that meet other requirements are sometimes hard to find. You may be able to locate straight stretches of country road, perhaps straddling a convenient valley. Be sure to set up your testing range over grass on the upwind side of the road. You don't want the heat from asphalt to disturb the test. Also, avoid optical paths that cross over building roofs. Public parks are ideal, since they feature large expanses of grass and might even be relatively deserted early in the morning.

If your telescope has a thin mirror, be prepared for astigmatism. Think of the thin mirror as flexible. Upended, it sags. If your telescope has a heavy mirror (thick or thin), also anticipate some warping. This is particularly true if you aren't supporting the mirror gently in a sling, although it appears occasionally even then.

Such patterns will not resemble the neatly symmetrical diagrams appearing in the pinched optics chapter because only two supports are likely to squeeze. If you can't eliminate this effect by careful mounting, you will have to test at elevated angles on real stars.

You may find the alignment of the telescope is changed when the instrument is pointed at the horizon. Because optics are mounted loosely, try to set up a situation with a slight upward tilt of the optical axis. With luck, the optics will lean back to rest against their natural supports. Either choose a testing range with a natural upward slope or mount the source higher. At worst, you may need to do temporary fine-alignment for the test. One suspects that glass power-line insulators are so popular as curved reflectors only because they are conveniently mounted on towers.

Then try a "snap test." Rock the focuser on either side of the sharpest image and see how difficult it is to set the focus. In the most desirable situation, the focus seems to snap into place, and no matter where you halt, you are always convinced that the limiting factor in focusing is your inability to stop your hand from turning precisely at the crispest image. (Much of this depends on the focal ratio and how steadily the telescope is mounted.) The least-desirable situation is one where the focus looks about equally good over a range of focuser travel. You drift through the region of best focus, unable to decide. Your hand-eye coordination is far from being the limiting factor (Suiter 1990). If your visual power of accommodation is strong, then you must provide a dominant field object to hold the eye's focus while you vary the focuser. A reticle on the field plane of the eyepiece provides such an object. If you have an illuminated reticle eyepiece of 12 mm or below for photographic guiding, you can use it (possibly with a good Barlow). If you don't have a guiding eyepiece, stretch a scrap of black electrical tape halfway across the field stop of a high magnification eyepiece (the stop is the hole on the underside). If you have placed the tape close to the best focus of the ocular, you will see half of the eyepiece's field occluded by a sharp-edged shadow. Place the point source image close to a straight edge of the artificial pattern. Your eye will naturally focus on the large, high-contrast edge. Then you can vary the image focus at will while the focus of your eye is held as if it were in a vise. This psychological trick is common in darkroom enlarger focusing aids.

Look for individual aberrations following the instructions in the chapters that deal with them. In testing refractors for optical errors having nothing to do with color correction, you may find it helpful to use a deep yellow or green filter on the eyepiece. In fact, using a colored filter is a good policy for all telescopes, whether reflector or refractor. Even though reflectors have no overt color errors, the star test still suffers confusion from the finite bandwidth of white light. For example, red light of wavelength 630 nm may be 10 wavelengths out of focus, while deep blue light of wavelength 420 nm is 15 wavelengths out of focus. A colored filter reduces the range of contributing wavelengths in the image.

Another helpful tool is a 33% obstruction mask for your telescope. This provides a uniform obstruction for critical spherical aberration tests. A mask is easily made to fit over a reflector's spider. Just draw a circle one-third the diameter of your primary mirror on thin cardboard about the thickness of a manila file folder. Cut it out and fold it into quarters. Then cut a slice about 6 mm to 10 mm long near the vertex of the "V" in the folded center. Open it back up, and smooth it out. The center cut becomes a cross. Fit it over the protruding rear side of the spider. A mask is not necessary on f/10 Schmidt-Cassegrains because their obstruction is about 33% anyway. A refractor mask can be cut out of paper. Suspend the obstruction on a web of sewing thread taped to the dewcap. Of course, if your obstruction is over 73 the aperture already, this option is not open to you. Such telescopes are usually specialty instruments anyway, for which the extra obstruction is accepted to secure some other advantage.

As an example of how to use these tables from beginning to end, as well as a road map of the procedures and pitfalls you may encounter in star testing telescopes, the rest of the chapter describes the testing procedure for four imaginary instruments.

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