How to Collimate your binoculars
Bring your binoculars back to life with Stephen Tonkin’s step-by-step guide to correcting misalignment
The increasing availability of inexpensive astronomical binoculars has opened this side of our hobby to tens of thousands of people who would otherwise have been priced out of it. But with that have come some compromises, one of which is that low- cost binoculars can easily lose collimation.
Collimation, as far as binoculars are concerned, means that the images from the two optical tubes must merge within very tight tolerances. The night sky is very demanding of optical systems, so a slight misalignment that you may not notice in daylight can become especially apparent under the stars. If your binoculars are badly out of collimation they’ll give a double image which is, at best, very annoying. However, even if collimation is only slightly outside acceptable tolerances, your brain will attempt to compensate for it, putting strain on the muscles that move your eyes, which can lead to headaches or nausea.
If your binoculars arrive out of collimation or if they’re still under guarantee, you should return them to the vendor to be remedied, because the measures described here will immediately invalidate any warranty. If the guarantee has expired, you may wish to have them collimated by a professional repairer, but for many inexpensive binoculars this will cost more than the price of replacing them. In this case, you have nothing to lose by attempting the job yourself.
It takes specialist equipment and a lot of skill to perform a full collimation, so what we’ll do here is what’s known as a ‘conditional alignment’, where we align the optical tubes with each other in order to get a single image, but we won’t attempt to align them with the hinge as well. As a result, they’ll only be in good alignment for the interpupillary distance at which you align them; this is the ‘condition’ in conditional alignment.
Aligning the prisms
The typical cause of Porro-prism binoculars losing collimation is being dropped or receiving a knock that shifts one of the prisms. In the most common form of inexpensive binoculars, each prism is held in place with a spring clip that tensions it against a screw that tilts the prism. This arrangement means the prisms are vulnerable to being dislodged by impacts, but it also means that you can usually correct the problem by adjusting the tilting screws.
First of all, thoroughly examine your binoculars to make sure that any miscollimation isn’t due to external physical damage, such as an objective tube being bent out of alignment. Fixing this kind of defect is beyond