Collimation

Not only must the optical elements of each optical tube be collimated, but the optical axes of both tubes must be aligned. They must not only be aligned to each other, but also to the hinge or other axis about which interpupillary distance is adjusted. If this latter criterion is not met, the result is a phenomenon called conditional alignment in which the two optical axes are only aligned at the inter-pupillary distance that was set during collimation and will get progressively out of alignment for other interpupillary distances. This may be acceptable if only one person uses the binoculars.

The permitted divergence of the optical axes from true parallelism is determined by the ability of the eyes to accommodate divergence and by the magnification of the binoculars. If these limits are exceeded, either it will not be possible to merge the images from each optical tube or, if they can be merged, eye-

Table 2.1. Collimation Tolerances

Magnification

Step

Convergence

Divergence

x7

2arcmin

6.5arcmin

3 arcmin

xl0

1.5arcmin

4.5arcmin

2arcmin

xl5

1.0arcmin

3.0arcmin

1.5 arcmin

x20

0.75arcmin

2.25arcmin

1.0 arcmin

x30

0.5arcmin

1.5arcmin

0.67arcmin

x40

0.38arcmin

1.13arcmin

0.5arcmin

strain and its attendant fatigue or headache results. Acceptable tolerances in the apparent field of view are:

• Vertical misalignment (step, dipvergence): 15arcmin

• Horizontal convergence:2 45arcmin

• Horizontal divergence: 20arcmin

To ascertain the real tolerances, you need to divide these by the magnification to obtain the collimation tolerances listed in Table 2.1.

There are two ways in which the optical axes of the binoculars can be aligned. In almost all binoculars, the objective lenses are mounted in eccentric rings. These can be adjusted to move the optical axis in relation to the body of the binocular. In many other binoculars, the prisms are adjustable, either by grub screws (set screws) that are accessible from the outside or by being housed in a cluster whose adjustment screws are accessible by removing the cover plate on the prism housing. (See Chapter 5 for advice on how to collimate a binocular.) Collimation by eccentric rings on the objectives is preferable because tilting the prisms will result in the introduction of more astigmatism.

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