Testing a lens

Every lens, especially one bought secondhand, should be tested as soon as you get it. The stars make excellent test targets.

What you need is a piggyback exposure, with good tracking, of a rich star field. I often use the field of a Persei or the Pleiades.

Exposures need not be long, nor do you need a dark country sky. Take several 30-second exposures of the star field, both in focus and slightly out of focus. On the pictures, look for:

• excessive vignetting;

• excessive degradation of star images away from the center;

• internal reflections (showing up as large haloes or arcs);

• miscollimation (one side of the picture in focus and the other side not);

• out-of-round star images in the center of the field.

The last of these is the most serious, and over the years, I've encountered it with three different lenses from reputable makers. Figure 7.5 shows what to look for, and the cause is an off-center lens element, either from imprecise manufacturing or because of mechanical damage.

It is normal for star images to be asymmetrical at the edges of the picture, but those in the very center should be perfectly round. If out of focus, they should be the shape of the lens diaphragm, which may be round or hexagonal. If you get distorted star images, judge the severity of the problem by comparing one lens to another, since no lens is perfect. Also, check the star atlas; don't let a nebula, cluster, or multiple star fool you into thinking you've found a defect.

Spherical and chromatic aberration, both resulting in round haloes around the stars, are not necessarily serious problems in deep-sky work. They can even help the brighter stars stand out, making up for the lack of sideways diffusion of light in the digital sensor compared to film.

Figure 7.5. Asymmetrical star images (enlarged) from lens with decentered element. At center of field, even out-of-focus stars should be round.

Optical defects usually diminish when the lens is stopped down. Telecentric-ity issues (p. 75) stay the same or get worse. Thus, if there are "comet tails" on stars around the edge of the picture, and they persist at f /5.6 or f /8 as well as wide open, then what you are seeing is fringing within the sensor. With film or with a different sensor, the same lens may perform much better.

Particularly with lenses 20 or 30 years old, quality is partly a matter of luck; there's a lot of unit-to-unit variation. One reason newer lenses have more elements is so that a slight error in the curvature of one surface will have less total effect. Compare a Newtonian reflector, whose quality depends entirely on one perfect paraboloid, to a refractor, which has four optical surfaces and is more tolerant of slightly incorrect curvature in each of them.

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