Telescope Reviews: A comment on collimating binoculars

As Bill has often affirmed, the binoculars collimation is a procedure that gets the perfect alignment of both optical axes with the mechanical axis (the hinge). If this is not done, if the optical axes are aligned but not with the mechanical axis, then the alignment obtained between the axes will be simply lost when changing a bit the interpupillary distance (IPD).
If you want to check the proper alignment of a binocular, the target must be at infinite or at something very similar. If the target is at 1000 m. and we try to collimate a binocular with about 20 cm of distance between the centre of the barrels, then, when the optical axes will be perfectly aligned to the target, they will be not parallel, but converging a bit. This bit is about 40 arc seconds. When pointing the binocular to a star, the two images of a star will be diverging 40 arc seconds. If you consider that the tolerance we could reasonably accept for divergence is 20 minutes of apparent field of view, it means that in a 20x the divergence tolerated between the two axes is 1 minute. Thus, if we collimate using a target at 1000m without taking in mind this, the alignment obtained will be already grazing the divergence tolerance limit. This horizontal misalignment produced by a not enough distant target can affect also the vertical alignment when the IPD will be changed. Let’s imagine what happens if we use a target at 100 m: optical axes convergence will be 10 times higher, thus not acceptable even in a roof binocular of very moderate magnification.

About a star as target. In my opinion a very far target in daylight is better. If the field of view is completely dark and you are looking at a very bright point, eyes are more prone to merge the images. This is at least my experience.

About collimation by tilting porro prisms. Before tilting prisms, these should be positioned not only to square them, but also to get already some alignment of the axes, in order to avoid the need of tilting prisms excessively, which would deteriorate the image quality. This sort of pre-collimation is useful also in binoculars with adjustments on the objective lenses, it order to avoid that the adjustment range allowed by eccentrical rings and objective cells becomes not sufficient to correct a too bad position of the prisms.

About the Oberwerk suggestions on collimation: this is the umpteenth site explaining how to get what is euphemistically called conditional alignment, i.e. the alignment of the axes without alignment to the hinge.
Sorry, this isn’t collimation.
Alii Service Notes and Seyfried’s book are good to know a bit of overhauling, but they don’t give enough information on collimation of the optical axes to the mechanical axis, which is indispensable for alignment at all the interpupillary distances. One of the three tomes of the Amateur Telescope Making does it, it is a very important text, but it explains how to do it with collimating equipments where the binocular is strongly attached to a mount (that generally is only for glasses with “nacked” hinge). For the moment being I have not yet found texts on collimation without collimating equipment.

Ed, I don’t understand how “perfect field alignment and perfectly merged star images do not occur at the same time”. If prisms are squared and alignment of axes and mechanical hinge is obtained, and if the field stops of the eyepiece are correctly positioned, alignment should be on the whole field of view . Am I wrong?

Bill, how can I get OPTICS AND OPTICAL INSTRUMENTS?

Claudio