© Baader Planetarium GmbH | 2020
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focuser should be fully retracted. Hold the card behind the empty focuser or above the
star diagonal and find the position where the image of the Moon comes to sharp fo-
cus. This position is your telescope's focal plane. Measure the distance from the card
to the end of the focuser or the top of the star diagonal. This is the amount of available
back-focus for additional accessories – for example, a binoviewer plus eyepieces.
Armed with this information, refer to the first row of the table below. First, subtract
the recommended 5-10 mm from your back-focus value to allow for differences in
eyepiece design and vision. Then, find the first column with a value less then your
adjusted back-focus. The heading of this column indicates the appropriate Glasspath
Corrector.
Item Nr.
Configuration with various
Baader Glasspath Correctors
No
GC
T-2
1,25x
GC
T-2
1,7x
GC
T-2
2,6x
GC
2" 1,7x
Newton
GC
2" 1,8x
RT/SC
GC
Straight-through
(only with nosepiece)
110
92
77
44
31
n/a
# 2456005
T-2 Prism Diagonal 32 mm
148
131
116
83
n/a
n/a
# 2456095
T-2 Deluxe Zeiss Prism Diagonal
151
134
119
86
n/a
n/a
# 2456103
T-2 BBHS
®
Mirror Diagonal
153
136
121
88
n/a
n/a
# 2456130
T-2 Deluxe Amici-Prism, 90°
(for upright images)
158
141
126
93
n/a
n/a
# 2456100
T-2 / 90° Maxbright Mirror Diagonal
153
135
120
87
n/a
31
Table 1: Combinations of Glasspath Correctors (GC) and the MaxBright
®
II Binoviewer. Required back-focus in mm.
n/a = not applicable
Please note: The lengths are approximate values; allow for some variation e.g. for diopter adjustment.
Some telescopes don't have enough back-focus for any of our glasspath correctors.
Try to avoid 2" star diagonals, because they have a long optical length which requires
a longer GC and results in a higher minimum magnification. You can achieve lower
magnifications (and thus a larger field of view) by using our very compact star diago-
nals which were designed especially with binoviewing in mind.
Users of Newtonian telescope may get the most out of the typical short back-focus
by chosing either the 2.6x glasspath corrector or, if a 2"-focusser is available, by us-
ing the even shorter 2" GC for Newtonians. You may also try to move the main mirror
closer to the focuser by about 15 mm, so that they may use the next-smaller GC.
Choosing a Glasspath Corrector
The Baader Glasspath Corrector fulfills two critical roles. First, it corrects the chro-
matic aberration (colour error) caused by the long light-path through the binoviewer
glass prisms. Second, the corrector magnifies and extends the focal point of the main
telescope.
The focal extending aspect is vital to using a binocular viewer with most telescopes.
Due to the basic design of all binocular viewers, light must travel an additional 110mm
or more before reaching the focal plane of the eyepieces. Many telescopes, especially
Newtonian reflectors, do not have sufficient back-focus (in other words, not enough
focuser in-travel) to achieve focus with such an optically long accessory. By using a
Glasspath Corrector, the focal point of the telescope can be extended sufficiently to ac-
commodate the length of the binoviewer.
