20
Manual: MaxBright
®
II Binoviewer
Addendum
Addendum 1: The right Glasspath Corrector
Back-Focus
Sometimes referred to as “in-focus” or “in-travel”, back-focus is an important factor to
consider when choosing a binoviewer or any other long accessory (cameras, Herschel
Wedge, etc). Back-focus is simply the distance from a telescope's focal point to the
surface of its fully retracted focuser. Any accessory inserted into the focuser consumes
some of this back-focus – star diagonals, eyepiece adapters, reducers, cameras, eye-
pieces, etc. In order to reach focus, the focal plane of an eyepiece or camera must be
positioned at the telescope's focus.
To aid in determining the total path length when using various components, our list-
ing of Baader Astro T-2 System
TM
includes the optical path length for each product. If
the stack-up of parts is longer than the telescope's back-focus, it will not be possible
to reach focus.
Unfortunately, there is no consistency in the telescope industry for the amount of
back-focus a telescope provides.
Very few manufacturers provide the back-focus
specifications for their scopes . Thus, the best way to determine your telescope's back-
focus is to measure it yourself .
To reduce the possibility of errors, it is best to measure
the back-focus using all adapters or star diagonals that will be present when using the
binoviewer.
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 would be required,
or at least moving the primary mirror up the tube – which of course creates its own
issues, too. This increases the central obstruction resulting in a loss of contrast and
increased cost. For this reason, we offer a large 2" 1.7x Glasspath Corrector that is de-
signed specifically for Newtonians. This compensator also corrects the off-axis coma
that affects fast Newtonians and reduces the required back-focus to 31 mm!
Most Schmidt-Cassegrain and Maksutov telescopes provide large amounts of back-fo-
cus due to their movable primary mirror. It may be possible to use a binoviewer without
a compensator, though our 1.25x Glasspath Corrector is recommended for its colour
correction and to keep the focal length of the telescope closer to its optimal design.
Refractors vary considerably in the amount of back-focus they provide, sometimes
even within the same model line. Even if a refractor will come to focus without a com-
pensator, it is again recommended that at least the 1.25x Glasspath Corrector should
be used for colour correction or to allow the use of longer focal length eyepieces while
still providing higher magnifications.
How to measure the back-focus of my telescope?
The best method for finding out the back-focus of your telescope is: Do a
measurement!
Don't forget to attach all the adapters you will need for the binoviewer
before measuring the back-focus.
The easiest way to measure back focus is to point your scope at the Moon (with all
accessories, but without an eyepiece) and project its image onto a white card. The
