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How To Adjust a Binocular Viewer
Submitted: Friday, 13th January 2006 by Graeme Clement

A number of members have binocular viewers for their telescopes.  Other members who look through these but who have little experience using binocular viewers can find merging the two images difficult.

I had the same problem and resolved it only after using a binocular viewer in daylight, installed on a spotting scope (terrestrial telescope), enabling me to see clearly what I was doing with it.  This led me to purchase a binocular viewer of the same model.  Before attempting to use it with my telescope I then handled it in bright artificial light separately from a telescope and eyepieces, rotating all its moving parts through their entire ranges of movement in all combinations, to familiarise myself with the instrument.  Afterward I used it in the dark of night without any trouble.

Because the mechanical design of a binocular viewer and its mode of deployment differ from those of “stand-alone” binoculars, it is easy to underestimate the precision required in one’s adjustment of the instrument for one’s eyes.  I constantly made the mistake of trying to adjust my posture to a binocular viewer as it had been adjusted for the previous observer’s eyes and posture; it just doesn’t work.

In view of the foregoing, I have come up with a technique to help newcomers succeed in the art of adjusting a binocular viewer.  I outline a description of this procedure below.

To achieve a merger of the two images the following requirements must be met:-

  1. the interpupillary distance must be exactly right for your eyes;  and
  2. the optical axes of the two eyepieces (see below) must be in exactly the same plane (see below) as the optical axes of your eyes.

The optical axis of an eyepiece is its longitudinal axis – the centreline of the light path that passes through it.  The optical axis of an eye is similar, and can perhaps best be described as the line that passes through the centres of the cornea, the pupil and the lens of your eye and through the centre of your retina.

By “the same plane as the optical axes of your eyes” I refer not to the altitude (or elevation) of the optical plane in front of you, but to the lateral (“sideways”) “alignment” of your eyes – that is, the plane of the eyepieces must not be even very slightly “lower” on the left and “higher” on the right than your eyes’ optical axes, or vice versa.  Each of the eyepieces must be precisely aligned with your corresponding eye; and in view of human physical constraints (to which the comfort factor is an excellent guide) the binocular viewer should be adjusted to conform with the positions of your eyes and not the reverse.

The latter [(2), above] is a stumbling block for some (and perhaps most) first time users and, to make the job a little easier perhaps, precedes the former [(1), above] in my description of my method, below, of adjusting a binocular viewer.

A binocular viewer will often require re-adjustment for a person to observe the same object with the same telescope immediately after another person, because of individual physical differences, as well as their different stances. Indeed, if you change your own stance it can require re-adjustment of the binocular viewer; hence the need first to find a position that will remain comfortable indefinitely.

Additionally, a binocular viewer may have to be re-adjusted any time a new object is brought into view in the telescope.  This is because, as the telescope is moved from one object to another, the binocular viewer is moved to a (perhaps only slightly) different angle in relation to the observer’s body.  Thus, you often must assume a different posture to bring your eyes to the eyepieces in the viewer.  This means the optimally comfortable position of your head (see below) may change from one object to the next.  My method is designed for application in any such position.

The technique is comprised of the following four steps:-

  1. After finding the object with the telescope, you should front up to the binocular viewer in such a way as to assume a natural, comfortable position with your eyes looking straight down the centreline of the body of the binocular viewer, and around which centreline the two eyepiece holders pivot. (That is, the centreline should pass through a point midway along a line joining the centres of your pupils: the bridge of your nose.) This may require being comfortably seated, standing on a ladder, or rotating the o.t.a.  (fodder for ‘orses: oatey ‘ay J) of the telescope in its rings, et cetera.  (O.T.A. is an acronym for Optical Tube Assembly, perhaps better known to you simply as the “tube”);
  2. Without moving your head, individually rotate the eyepieces (with their holders) around the binocular viewer’s centreline as required, to bring the optical axes of both eyepieces into exactly the same plane as those of your eyes.  To achieve this result you may find it necessary also to rotate the body of the binocular viewer in the telescope’s focuser, and then re-adjust the eyepieces. Your eyes need not be close to laterally horizontal (horizontal to your left and right), but should be in whatever position is most comfortable for you for observing through the viewer, within the constraints of its angle at the azimuth and elevation to which the telescope is pointed.  When you find such a position, you should adjust the eyepieces precisely to match the angle of your eyes.  (See below.);
  3. Then, while maintaining the optimally comfortable position of your head, adjust the interpupillary distance until no black area appears between the fields of the two eyepieces and you have a clear view of these fields.  (Most people find it easiest first to bring the interpupillary distance to its minimum adjustment [close the eyepieces right in as close together in their eyepiece holders as they will go] and slowly widen the distance until you achieve the correct distance as described above.)  As you do so, you should make compensatory adjustments to restore the plane of the eyepieces to that of your eyes, as explained in step 2, above.  (See below.);
  4. When you have got the adjustment right, your brain will effortlessly merge the two images; if it does not, the binocular viewer needs to be adjusted more precisely as described above – perhaps only a slight tweak or two.  Repeat steps 1 to 3, above, until you succeed.  At first, you may find in the first cycle or two (of steps 1 to 3) you will achieve only very roughly the adjustment you need, but with every repetition of the cycle you should approach more closely to the precision required.

The adjustments I describe in steps 2 and 3 above are both made by moving the eyepieces in the same manner, and must be performed together.  However, they are difficult to describe clearly in combination and should for similar reasons be considered separately by the observer when about to observe with a binocular viewer until he/she becomes familiar with the procedure, when it will have become intuitive; hence my separate treatment of these above.

Be aware that you should bring the focuser of the telescope into play as part of the procedure outlined above.  When you have succeeded in merging the two images (and prior to that as well, if necessary) you should finely adjust the focus for each eye.  First adjust the focuser of the telescope optimally for one eye (close the other) and then rotate the eyepiece holder of the binocular viewer to finely focus for the other eye (closing the first eye to do so).

If your experience is anything like mine, you will find handling your binocular viewer separately from a telescope and eyepieces, rotating all its moving parts through their entire ranges of movement in all combinations, in broad daylight or in bright artificial light, will be very helpful before first attempting this procedure in darkness in the field.  One critical reason for this is that you need to be aware of the limits of your binocular viewer’s ranges of movement, so you can gauge how much your body must compensate for these when assuming your optimally comfortable physical pose.

Word has it that approximately ten percent of people cannot merge the two images in a binocular viewer under any circumstances.  I believed I was one of these until I succeeded with the Arctic Fox Optics model which is available at BTOW.  (The one I first used with success is that installed in the Arctic Fox Optics spotting scope on display and available for demonstration at BTOW.  I recommend you should try that for yourself – in daylight so you can see what you are doing with it.)  If you have not yet succeeded, perhaps you are not one of the unfortunate ten percent, but simply need a more favourable introduction to it as I did.

Article by Grame Clement (The Cosmic Sentinel). Discuss this article at the IceInSpace Forums.

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