Binoviewers are becoming a very popular accessory for telescope users.
They offer several advantages over single-eyed viewing, at the cost of a few
disadvantages. Some of these effects are subtle, while others are more
characteristics of binoviewing are the same as with
conventional binocular use. The primary feature of binoviewing
is that it fits the way our brains process visual information better than
single-eyed (Cyclops) viewing. This is true both of binoviewers
and binoculars, and is one reason that conventional binoculars are so popular
in astronomy. Most users find that objects or starfields
appear three dimensional. Because of the narrow spacing of the objectives,
binoculars do not provide true depth perception; the brain is fooled into
thinking itís there. The same effect is observed with binoviewers
for the same reason. Another effect is that bringing the same information into
both eyes permits the brain to process the information differently, making
subtle details more apparent. User comfort is also a very important feature of
any two-eyed viewing system. Itís much more relaxing to observe with both eyes
than to either ignore or close one eye.
binoviewer characteristics arenít shared by
binoculars. One feature of binoviewers thatís not generally
a factor with binoculars is the suppression of ďfloatersĒ when a small exit
pupil is being used. Floaters are bits of organic matter floating within the
fluid in the eyeball. At small exit pupils these can be distracting. With binoviewers the brain can receive the same celestial view
from both eyes and cancel out the floaters in each. Astigmatism is another
effect noticed by many observers when using small exit pupils that can be cancelled
during processing. Less technical factors exist also. Itís easier and less
expensive to achieve larger apertures with a binoviewer
setup than with binoculars. The ability to change magnifications by switching
eyepieces or adding focal multipliers or reducers is another advantage.
disadvantages of binoviewing are obvious. The added
cost of doubling an eyepiece collection is difficult to ignore. Modern systems
from Denkmeier and Siebert minimize this effect by
permitting multiple magnifications. This makes each EP pair more useful and
reduces the number of pairs required. Splitting the light into two paths and
running it through more optical elements results in less light reaching each
eye from a given object than in Cyclops mode.
Most users seem to find this is offset by the increased efficiency with
which the images are processed by the brain. Some find it to be a disadvantage for deep sky use. Choosing a binoviewer unit with efficient prism design and/or coatings
can reduce the losses.
obvious disadvantages exist. The light
path through a binoviewer is generally between four
and five inches. Some telescopes (nearly all refractors and Newtonians) donít
provide sufficient focus range to accommodate this added length. Correctors are
available to compensate for this, but they add cost and complexity to the
system as well as increasing light loss to some extent. The prisms used in
common binoviewer designs restrict the light path.
Some units have clear apertures of 26mm or greater (Denkmeier,
Tele Vue), while some restrict the lightpath to 22mm or less (Celestron, Burgess, Siebert, StellarVue). Itís desirable to avoid using eyepieces with
field stops larger than the clear aperture of the binoviewer
being used, which means that the smaller units are generally less desirable for
use at lower magnifications. The Siebert Echelon series offers clear apertures
of up to 35mm, but these havenít been generally available and I havenít seen
one. They also offer 2Ē units which I also havenít seen and canít comment on.
So whatís it gonna cost?
Binoviewing is much less expensive than it has traditionally been. Very
good performance can be provided by the recent crop of imported units by
Burgess, StellarVue, and Celestron for $200 or so
including an eyepiece pair. Midrange units in the $400 class (Denkmeier, Siebert) can do even better. Higher-end units by
Denkmeier, Siebert, or Televue
are available in the $1000 range.
Burgess unit is a clear winner in the entry-level binoviewer
class. For $200 you can find out whether binoviewing
is for you, and if you decide you donít like it (or like it so much that you
want to upgrade) itíd be hard to lose much money disposing of it. If you find
the Burgess unit unavailable at a given time (they suffer chronic supply
problems) the StellarVue is comparable. In the midrange, the Denkmeier
Big Easy package is an attractive unit. For $400 you receive a unit with 26mm
of clear aperture and self-centering EP holders, plus correctors to make it
work with nearly any telescope at multiple magnifications. For a little more,
the Siebert Black Night provides 22mm of clear aperture and a corrector, but
adds a pair of eyepieces and a case. The most popular unit in the $1000 class
is the Denkmeier II. They have recently switched
their line around quite a bit and offer a great many options including their PowerXSwitch so itíd be prudent to check with your dealer
for current package pricing. The 2Ē Siebert units are in the same range and
show promise but Iím reluctant to recommend something I havenít seen.