The Baader Glasspath Compensator™ serves multiple functions. In addition to correcting the colour error caused by the long light-path through the binoviewer glass prisms (glasspath), the compensator also magnifies and extends the focal point of the telescope. This focal extending aspect is key to being able to use a binocular viewer in most telescopes. Due to the basic design of all binocular viewers, the distance that light must travel through the viewer is on the order of 4.5" or more (the Maxbright is the shortest viewer we know of, at 110mm). Many telescopes, especially Newtonians, do not have sufficient back-focus in order to reach focus with such an optically long accessory (ie, not enough focuser in-travel). By using a Baader compensator, the focal point of the telescope can be extended sufficiently to accommodate the length of the binoviewer.
A secondary benefit of the compensator is to provide additional magnification - ahead of the binoviewer. For high magnifications (ie, planetary observation), it is generally preferable to magnify the image before the binoviewer, rather than to use shorter focal length eyepieces. By magnifying prior to the viewer, the effects of any optical tolerances and misalignments in the centering of the eyepieces are reduced. A compensator enables the use of longer focal length eyepieces to achieve high magnifications, which tend to have longer eye relief (greater comfort) than short focal length eyepieces. It is also less expensive to purchase an additional compensator (to provide additional magnifications), rather than to purchase additional sets of eyepieces. In general, for binoviewing use we recommend eyepieces with focal lengths of 8mm or longer.
For these reasons, many users find it is very useful to have more than one compensator. We recommend first choosing the lowest powered compensator that will allow your scope to reach focus. This will permit the widest possible fields of view. In addition, one or more higher powered compensators can be added to give a greater range of magnifications.
The first step in choosing a compensator is to determine how much back-focus your telescope has available (please see the section on back-focus). Once you have determined the back-focus of your telescope configuration (ie, with any adapters or star diagonals in place), simply use the following table to determine which compensator (s) provide a path-length that is less than your telescope's available back-focus. Please note: the distances in the following table are approximate and are measured from the front mounting flange of the binocular viewer or nosepiece, to the top surface of the eyepiece holders. Many eyepieces have their focal points located ahead of, or behind, their shoulder. As a result, the effects of an eyepiece's focal point location are not taken into consideration here, as the distance required to reach focus will usually vary from these values. Users that are near or far-sighted will also find that more or less back-focus is needed to accommodate their needs. For these reasons, we recommend that you allow for at least 5-10mm extra back-focus.