Choosing a 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
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
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.
|Straight Through (w/ nosepiece only)
|with T-2 Prism Diagonal
with Deluxe Amici Diagonal
(for correct image
Table 1: Maxbright Binocular Viewer
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
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
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.
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. Of course, you are
free to invent your own, using any of our
Astro T-2 System components.
Maxbright Binocular Viewer
with 1.25" Nosepiece
(# BINO-11, 12, or 13)
compensator threads into nosepiece prior to attachment of nosepiece to
Maxbright Binocular Viewer
with 2" Nosepiece
(# BINO-21, 22, or 23)
compensator slips into nosepiece, along with provided space ring, prior to attachment of nosepiece to
Maxbright Binocular Viewer
with T-2 Star Diagonal (Prism, Amici, or Mirror)
provides the shortest path-length and highest quality images possible)
(# BINO-21, 22, or 23 - use BINO-11, 12, or 13 for 1.25"
T-2 Diagonal (# T2-01, T2-01C, AMICI-DX1, or MAX-1)
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
Corrector (# ORJ)
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)
last updated 08/04/12