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Use of 2" Eyepieces
The Bintel Dobsonian comes with 2” focuser (Figure
9) as standard equipment. Because of its large size,
it is able to accept eyepieces that have a 2" barrel
diameter. 2" eyepieces are desirable because they
allow for a wider field of view than 1.25" eyepieces.
Many observers own at least one 2" eyepiece to yield
the largest field of view possible for looking at wide
deep sky objects, such as open star clusters and
gaseous nebulae. 2" eyepieces can also give nice
views of the Moon. To use a 2" eyepiece, simply
remove the 1.25" eyepiece adapter from the end of
the focuser drawtube. You must first loosen the two
thumb screws on the 2" eyepiece adapter to do this.
Now, insert a 2" eyepiece into the focuser, and
secure it with the two thumb screws. Focus the
eyepiece as described previously. You’ll be stunned
at the wide fields of view that 2" eyepieces can
deliver. It makes you feel like you’re floating through
space!
Eyepiece Selection
By using eyepieces of varying focal lengths, it is
possible to attain many magnifications with the
Bintel Dobsonian. The telescopes come with two high
-quality Plossl eyepieces: a 15mm, which gives a
magnification of 80x and 83x, and a 9mm, which
gives a magnification of 139x and 133x as well as a
wide angle 2” eyepiece, either 26mm or 32mm. Oth-
er eyepieces can be used to achieve higher or lower
powers. It is quite common for an observer to own
five or more eyepieces to access a wide range of
magnifications. This allows the observer to choose
the best eyepiece to use depending on the object
being viewed. At least to begin with, the three
supplied eyepieces will suffice nicely. Whatever you
choose to view, always start by inserting your lowest
power (longest focal length) eyepiece to locate and
center the object. Low magnification yields a wide
field of view, which shows a larger area of sky in the
eyepiece. This makes acquiring and centering an
object much easier. Once you’ve centered the object
in the eyepiece, you can switch to higher
magnification (shorter focal length eyepiece), if you
wish. This is especially recommended for small and
bright objects, like planets and double stars. The
Moon also takes higher magnifications well.
Deep-sky objects, however, typically look better at
medium or low magnifications. This is because many
of them are quite faint, yet have some extent
(apparent width). Deep-sky objects will often
disappear at higher magnifications, since greater
magnification inherently yields dimmer images. This
is not the case for all deep-sky objects, however.
Many galaxies are quite small, yet are somewhat
bright, so higher power may show more detail. The
best rule of thumb with eyepiece selection is to start
with a low power, wide field, and then work your
way up in magnification. If the object looks better,
try an even higher magnification. If the object looks
worse, then back off the magnification a little by
using a lower power eyepiece.
Objects to Observe
Now that you are all set up and ready to go, one
critical decision must be made: what to look at?
The Moon
With its rocky surface, the Moon is one of the easiest
and most interesting targets to view with your
telescope. Lunar craters, marias, and even mountain
ranges can all be clearly seen from an average
distance of 350,000 kms away! With its ever
-changing phases, you’ll get a new view of the Moon
every night. The best time to observe our one and
only natural satellite is during a partial phase, that
is, when the Moon is
NOT
full. During partial phases,
shadows are cast on the surface, which reveal
more detail, especially right along the border be-
tween the dark and light portions of the disk (called
the “terminator”). A full Moon is too bright and de-
void of surface shadows to yield a pleasing view.
Make sure to observe the Moon when it is well above
the horizon to get the sharpest images.
Use the included Moon filter to dim the Moon when it
is very bright. It simply threads onto the bottom of
the eyepieces (you must first remove the eyepiece
from the focuser to attach a filter). You’ll find that
the Moon filter improves viewing comfort, and also
helps to bring out subtle features on the lunar
surface. Some coloured filters also help to cast
different shadows.
The Sun
You can change your nighttime telescope into a
daytime Sun viewer by installing an optional
full-aperture solar filter over the front opening of a
Bintel Dobsonian. The primary attraction is sunspots,
which change shape, appearance, and location daily.
Sunspots are directly related to magnetic activity in
the Sun. Many observers like to make drawings of
sunspots to monitor how the Sun is changing from
day to day.
Important Note:
Do not look at the Sun with any optical
instrument without a professionally made solar
filter, or permanent eye damage could result.
Field of View
One of the most important considerations with
eyepieces is also the most overlooked consideration
when making eyepiece selection . In all star charts
or catalogs the suggested power is never mentioned
Deep sky objects are always shown by their size.
The size is nearly always shown in arc minutes.
To determine the field of view of any given eyepiece
in arc minutes follow this simple formula.
Each type of eyepiece has an apparent field of view
ie (Plossl = 50° Apparent field)
Divide the Apparent field by the Magnification
The answer will the Real field of view in degrees
Multiply this answer by 60 and you will have the field
of view in arc minutes.
Now pick the eyepiece that the object fits into and
enjoy the view. It will be much better than just
going by power alone.
