Choosing the right Glasspath Compensator for your Telescope for use with a Baader Planetarium Binoviewer

♦ Choosing a Glasspath Compensator

The Baader Glasspath Compensator™ serves multiple functions.  In addition to correcting the color 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 below 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.

For example, our own Takahashi Sky90 SV has 160mm of back-focus as measured from the rear surface of it's stock 2"eyepiece clamp to the focal point.  From Table 1, you can see that the Sky90 will be able to reach focus without a glasspath compensator at all, even with our Amici correct-image star diagonal.  In this case, we would choose either the 1.7X or 2.6X compensator.  This would give us the ability to use the Maxbright with no compensator (for low-power wide field viewing), and the higher powered compensator provides us with the optimum image quality  for planetary observations.

Table 1: Maxbright Binocular Viewer Path-Lengths

♦ Back-Focus

Sometimes referred to as In-Focus or In-Travel.  Back-focus is an important factor to consider when choosing a binocular viewer, or any other long accessory (cameras, Herschel Wedge, etc).  Back-focus is simply the distance from your telescope's focal point to the surface of it's focuser (when fully retracted).   Any accessory inserted into the focuser consumes some of this back-focus (star diagonals, eyepiece adapters or reducers, cameras, eyepieces, etc).  In order to reach focus, the focal plane of an eyepiece (or camera) must be able to be positioned at the telescope's focus.  If the stackup of parts are longer than the telescope's back-focus, then it will not be possible to reach focus.   Unfortunately, telescope manufacturers do not follow any consistent guidelines for the amount of back-focus a telescope should provide.  Further, few manufacturers even provide the back-focus specification for their scopes.   As a result, it is left up to the user to find out for themselves.

Newtonians typically have the least amount of back-focus.  In order to extend the focal point well past the focuser surface, a larger secondary mirror is required.  This increases the central obstruction (loss of contrast) and increases cost.  As a result, many Newtonians have only a small amount of back-focus.   For this reason, we offer a very special glasspath compensator that is designed for Newtonians.  This compensator also corrects the off-axis coma that affects fast Newtonians.

Schmidt-Cassegrain and Maksutov telescopes can  typically provide large amounts of back-focus, owing to their movable primary mirrors.  This allows many of them to accommodate binoviewers even without any glasspath compensator (also, depending on the size of star diagonal used).   SCT users will find the 1.25X compensator is likely to be the best match to their scopes, providing all of the glasspath compensating benefits at moderate  magnifications.  Refractors vary considerably in the amount of back-focus they provide, sometimes within the same model.  

How do I determine how much back-focus my telescope has?

The best way to determine your telescope's back-focus is to measure it yourself.  If you are intending to use a binoviewer with your scope, you will want to measure the back-focus using any adapters or star diagonals that will be present when using the binoviewer. 

The easiest way to measure back focus is to point your scope at the Moon  (with accessories, but No Eyepiece) and project its image onto a flat sheet of paper.  The focuser should be fully retracted.  Hold a white sheet of paper behind the empty focuser or above the star diagonal and find the position where the image of the moon comes to sharp focus.  This position is your telescope's focal plane.  Using a scale, measure the distance from the paper to the end of  the focuser or star diagonal.  This is the amount of available back-focus for additional accessories (ie, Binoviewer plus eyepieces).  It is possible to make a more accurate measurement with the actual eyepieces to be used with the binoviewer, but for most purposes this white paper projection method should be adequate to help select the necessary glasspath compensator.

In some cases, you may find that your scope does not have sufficient back-focus to work with any of the glasspath compensators we provide.  If you are using a 2" star diagonal in your system, you may want to consider using a shorter 1.25" star diagonal.  The Maxbright Binocular Viewer has been especially designed to close-couple to our T-2 Diagonals, in order to provide the minimum path length possible.  Newtonian users may want to consider shifting their primary mirror forward (or shortening their truss tube poles) in order to increase their telescope's amount of back-focus.  

♦ Configurations

The Baader Maxbright Binocular Viewer is able to be configured many different ways.  The front rotating ring of the Maxbright has an internal T-thread, which allows it to be directly coupled to any externally T-threaded accessory.  The configurations shown below are just some of the more popular combinations. 

	Maxbright Binocular Viewer with 1.25" Nosepiece - Maxbright Binocular (# BINO-11, 12, or 13)   (glasspath compensator slips into nosepiece prior to attachment of nosepiece to binocular viewer)
	Maxbright Binocular Viewer with 2" Nosepiece - Maxbright Binocular (# BINO-21, 22, or 23)   (glasspath compensator slips into nosepiece prior to attachment of nosepiece to binocular viewer)
	Maxbright Binocular Viewer with T-2 Star Diagonal (Prism, Amici, or Mirror) (This setup provides the shortest path-length and highest quality images possible) - Maxbright Binocular (# BINO-21, 22, or 23 - use  BINO-11, 12, or 13 for 1.25" focusers) - T-2 Diagonal (# T2-01, AMICI, or MAX-1)   (glasspath compensator threads into the top of the star diagonal prior to attachment of the diagonal to binocular viewer)
	Maxbright Binocular Viewer with 1.7X Newtonian Glasspath Compensator - Maxbright Binocular (# BINO) - 1.7X Newton Corrector  (# ORJ)   (To attach The 1.7X corrector, first thread the included T2-07 Change Ring onto the Maxbright.  The corrector then mounts do the Change Ring by its integral dovetail clamp)