## Eccentric Rings

Binoculars differ from telescopes in that collimation is achieved by lateral movement of the objectives, not by tilting them. Moving the lens one way will move the image in the eyepiece the other way. However, it is all but impossible to move an

Figure 5.12.

Collimation screws.

Figure 5.12.

Collimation screws.

Figure 5.13.

Eccentric rings. Rotating these rings moves the lens laterally. Note that it is marked to enable its return to the original position.

Figure 5.13.

Eccentric rings. Rotating these rings moves the lens laterally. Note that it is marked to enable its return to the original position.

objective only either laterally or vertically, and collimation with eccentric rings can be monumentally frustrating until you get the hang of it. It can be a tough test of perseverance and patience—you have been warned!

First, mark the positions of the rings (see Figure 5.13) so that if you do not manage to improve the image, you can at least set them to their original position. Next, set the rings so that there is no eccentricity (i.e., the narrowest part of the inner ring aligns with the widest part of the outer ring and vice versa). Rotate the objective lens assembly a small increment—about 10 to 15 degrees—at a time until it has made one revolution and see whether there is any movement of the image and, if so, if it is sufficient to bring the images into proper alignment. If it is not, slip the inner ring about 10 degrees and repeat the rotation of the lens assembly. Repeat this until the images are aligned as well as possible. If there is not sufficient eccentricity in the rings, you will need to adjust the prisms.

Note: It is usually better to set the binoculars so that there is a discernible amount of divergence (i.e., so the optical paths from the binoculars are converging) (see Chapter 2, note 2), and then to gradually collimate from there than it is to approach collimation from the other way. This is because the eyes are more sensitive to divergence than to convergence.

For more detailed accounts of collimation see Seyfried's Choosing, Using, & Repairing Binoculars, which gives a more detailed account of conditional collima-tion, including building bench-testing apparatus, and the Naval Education and Training Program Development Center's (NAVEDTRA) Basic Optics and Optical Instruments, which gives accounts of full collimation with bench test apparatus.

Bibliography

Dismantled Porro-prism binocular: http://www.actionoptics.co.uk/disdbin.htm.

The Naval Education and Training Program Development Center. Basic Optics and Optical

Instruments. New York: Dover, 1997. Seyfried, J.W. Choosing, Using, & Repairing Binoculars. Ann Arbor: University Optics, 1995.

CHAPTER SIX

 Holding and TV/frm ntiniT iVIU Ullllllg |

When you decide how to mount your binoculars, there are two considerations that you must take into account: stability and comfort. Both of these play a significant part in determining how much you will be able to see. If the binoculars are not held in a manner that is reasonably stable, in order to eliminate shake, the amount of detail you see will be severely reduced. If you are not comfortable when you observe, you will quickly tire, and tiredness is always detrimental to observing.