Effects of Misalignment

If alignment isn't precise, any number of bad effects can result. Although we will calculate patterns only for misaligned Newtonian reflectors, the behavior depicted here qualitatively describes a wide variety of systems and is generally useful.

Off-axis, a Newtonian reflector exhibits a mixture of two pure aberrations, coma and astigmatism. In this case, astigmatism isn't ground into the glass permanently. It is caused by viewing the mirror at an oblique angle. Similarly, coma is induced merely by the tilt of the mirror.

As misalignment worsens, astigmatism overtakes coma to become the dominant aberration. Coma worsens linearly with collimation error, while astigmatism increases as the square of the distance to the optical axis. This behavior is illustrated in Fig. 6-1. For all practical cases of misalignment, the crossover point is always beyond the level of even coarse alignment. Coma is always stronger than induced astigmatism for Newtonian telescopes.

The bright parts of a comatic image develop wing-shaped appendages, and some of the light is smeared away from the optical axis. Coma effects are the same inside or outside focus. In each case, the aberration stretches the image away from the optical axis.

Misalignment of the image in Newtonians is easier to perceive outside of focus. The skewed optics no longer have a centered obstruction, and outside focus the off-center shadow of the diagonal adds to the stretching caused by coma. The offset shadow makes coma seem even worse. Inside focus, the offcenter shadow leans in a manner that partially counterbalances the coma stretch. Alignment on either side of focus is possible, but the preferred method is to pull the eyepiece back.

Astronomical telescopes have such a constricted field of view that coma caused by the objective is generally not visible unless the optics are mis

1 Tilted-component telescopes don't follow such a simple sequence. They require a procedure matched to the particular instrument.

Ol c

Ol c

Distance off-axis

Fig. 6-1. Astigmatism and coma aberration as misalignment becomes worse in Newtonian reflectors.

Distance off-axis

Fig. 6-1. Astigmatism and coma aberration as misalignment becomes worse in Newtonian reflectors.

aligned and the image is viewed at high magnification. Any off-axis smearing of the image in low-power eyepieces is primarily a fault of the ocular.

Unfortunately, coma and astigmatism in composite form are common errors in poorly collimated Newtonian reflectors and catadioptrics. Well-designed refractors and advanced reflectors use the multiplicity of surfaces to reduce or eliminate the coma error. Good refractors chiefly show misalignment through astigmatism.

Alignment is probably the single greatest contributor to the undeserved shabby reputation of Newtonian reflectors. These instruments are commonly made at very low focal ratios. Their owners don't realize their telescopes must be kept in razor-sharp alignment. A fast Newtonian usually spends its entire existence in a wretched state of collimation. Of course, if low magnifications are used, the zone of excellent alignment will usually be found somewhere within the field stop of the eyepiece. However, at high magnifications (where optical quality must be at its best), the focal-plane region of good quality can be at one edge of the field stop or possibly outside it entirely.

As an example, let's look closely at the common 10-inch (250-mm) f/4.5 Newtonian reflector. Schroeder (p. 96) says that f/4.5 paraboloids can tolerate misalignments of about 1.8 arcminutes. Superb images are thus confined to a region at the focal plane only 0.05 inches (1.2 mm) in diameter. This tolerance is a bit tight, however. "Decent" imaging would allow a misalignment of about 3 arcminutes, a circle on the focal plane of 0.08-inch (2 mm) diameter. The passable region is a circle V5 the diameter of the full moon. This alignment is ruined with only about 1/10 turn of a main mirror adjustment screw.

Fig. 6-2. Aberration function of misaligned wavefront just after it has passed through the aperture.

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