CAMDEN Dobsonian Instruction Manual

Lending Telescope Collection

Dobsonian Telescope Instruction Manual

Warning: Never look directly at the Sun through any telescope or finder scope – even for an instant
– as permanent eye damage could result. Do not point the telescope at the Sun, as parts will melt.
Children should use this telescope only with adult supervision.

Transporting the Telescope.

The telescope kit comes in three parts:

i) The Optical tube assembly in a padded bag,

ii) The hard case containing eyepieces and accessories,

iii) And the timber base mount.

Special care needs to be taken while transporting the telescope. Please plan your transport of the
telescope to prevent it moving around and being damaged. The optic tube is quite heavy and over
1m in length. Please secure with a seat belt or other method if travelling with this across the back
seat of a car. The base mount is also quite large (50cm x 50cm x 70cm).

Care and Maintenance

Cleaning the lenses or mirrors

Do not clean the lenses or mirrors. Let the library know if they seem dirty. Eyepieces can get dirty
fairly quickly. Please don’t poke them, as it’s very easy to scratch the coatings. However in the case
of minor smudges, you might like to use the Lens pen, provided in the kit. First brush away any dust
with the Pen. Then use the cleaning tip on the pen to clean the eyepiece. Be sure to keep the dust
caps on the telescope when not in use.

Aligning the mirrors

Please don’t make any adjustment to the telescopes mirrors. If it seems the telescope is in need of
adjustment please contact the library.

Fog on the mirrors and eyepieces

It is normal for dew to form on all parts of the telescope while stargazing. The telescope is fitted
with a fan unit to help minimize dew formation. Eyepieces will often have dew form on them, and
are unusable until they warm up and dry out. If the eyepiece fogs up, wrap your hands around it for
a few moments. As it warms up the fog should disappear. DO NOT wipe the lens with a tissue or
cloth as it has special coatings on it that could be damaged if you do.

At the end of the night, you can take the telescope inside, and leave unpacked. This will allow any
moisture that is in the telescope to dry out.

Getting Started

Assembling the telescope.

Please refer to the video for assembling the telescope. It can be found on the library website:

www.library.camden.nsw.go.au

or on the DVD included in the telescope kit.

Using the telescope

It is best to get a feel for the basic functions of the telescope during the day, before observing
astronomical objects at night. This way you will be able to use the equipment with ease when it is
dark and make the most of the night sky. Find a spot outdoors where you have plenty of room to
move around the telescope, and where you have a clear view of an object or vista that is at least
500m away. It is not critical that the base be exactly level, but it should be place on flat ground or

pavement to ensure smooth movement of the telescope. Although we recommend familiarizing
yourself with the equipment during daylight hours, it is critical that you never point the telescope at
or near the Sun.

Altitude and Azimuth – base mount

The base allows the telescope to move along two axes: Altitude (up/down) and azimuth (left/right).
The telescope can be positioned by holding the optical tube and moving it left or right so the base
rotates. And up and down so altitude side bearing rotate in the base’s cradle.

Move the telescope gently. You can adjust / tighten the side movement, from the side of the base.
Please ensure it’s adjusted to move smoothly, but not be loose or unstable.

Viewing with eyeglasses

If you wear eyeglasses, you may be able to keep them on while you observe if the eyepieces have
enough eye relief to allow you to see the whole field of view. You can try this by looking through the
eyepiece first with your glasses on and then with them off, and see if the glasses restrict the view to
only a portion of the full field. If they do, you can easily observe with your glasses off by just
refocusing the telescope the needed amount. If you suffer from severe astigmatism, however, you
may find images noticeably sharper with your glasses on. Then look through the telescope’s
eyepiece to see if that point is centered in the field of view. If it is, the job is done. If not, make the
necessary adjustments.

Aligning the Finderscope

The finder scope must be aligned accurately with the telescope for proper use. To align it, first aim
the main telescope in the general direction of an object at least 500m away – a telegraph pole or a
tree for example. Position the object in the center of the telescopes eyepiece.

Now look in the finder scope. Is the object visible? Ideally, it will be somewhere in the field of view.
If it is not, some course adjustment of the two finder scope alignment thumbscrews will be needed
to get the finder scope roughly parallel to the main. With the image in the finder scope’s field of
view, you can now use the alignment screws to center the object on the intersection of the
crosshairs.

By loosening one alignment thumbscrew and tightening another you change the line of sight of the
finder scope. Continue making adjustments to the various alignment thumb screws until the image
in both the finder scope and the telescope’s eyepiece is exactly centered. Check the alignment by
moving the telescope to another object and fixing the finder scope’s crosshairs on the exact point
you want to look at.

Then look through the telescope’s eyepiece to see if that point is centered in the field of view. If it is,
the job is done. If not, make necessary adjustments until the two images match up. The finder scope
alignment needs to be checked before every observing session.

Focusing the Finder scope

If when looking through the finder scope, the images appear somewhat out of focus, you will need
to refocus the finder scope for your eyes. Loosen the lock ring located behind the objective lens cell
on the body of the finder scope. Back the lock ring off by a few turns. Refocus the finder scope on a
distant object by threading the objective lens cell in or out on the finder scope body. Precise
focusing can be achieved by focusing the finder scope on a bright star. Once the image appears
sharp, retighten the lock ring behind the objective lens cell.

Aiming / Pointing the telescope

With the finder scope aligned, the telescope can be quickly and accurately pointed at anything you
wish to observe. The finder scope has a much wider field of view than the telescope’s eyepiece, and
therefore it is much easier to center an object in the finder scope. Then, if the finder scope is
accurately aligned, the object will be centered in the telescope’s field of view.

Start by once again moving the telescope until it is pointed in the general direction of the object you
want to see. Some observers find it convenient to sight along the tube to do this. Now, look in the
finder scope. If your general aim is accurate, the object should appear somewhere in the field of
view. Make small adjustments to the telescope’s position until the object is centered on the finder’s
crosshairs.

Focusing the telescope

Insert the low power 26mm eyepiece into the focuser and secure it with the thumbscrew. Move the
telescope so the front (open) end is pointing in the general direction of an object at least 250m
away. Now, slowly rotate one of the focusing nobs until the object comes into sharp focus. Go a little
bit beyond sharp focus until the image starts to blur again, then reserve back to focus – just to make
sure you have hit the exact focus point.

If you have trouble focusing, rotate the
focusing knob so the drawtube is in as far as
it will go. Now look through the eyepiece
while slowly rotating the focusing knob in the
opposite direction. You should soon see the
point at which focus is reached. On the
underside of the focuser there are two metal
thumbscrews. The thumbscrew closest to
the body of the scope will lock the focuser
position; the other thumbscrew will adjust
focuser tension.

Observing Tips

Site selection

Pick a location away from streetlights and bright yard lighting. Avoid viewing over rooftops and
chimneys, as they often have warm air currents rising from them, which distorts the image seen in
the eyepiece. You should also not observe out an open window from indoors for the same reason.
The best options would be out of town, away from light pollution. Most importantly, make sure that
any chosen site has a clear view of a large portion of the sky.

Seeing and Transparency

Atmospheric conditions play a huge part in the quality of viewing. In good conditions, star twinkling
is minimal and objects appear steady in the eyepiece. Seeing is best overhead, worst at the horizon.
Also seeing is generally better after midnight, when much of the heat absorbed by the Earth during
the day has radiated off into space.

Typically, seeing conditions will be better at sites that have an altitude over 3000 feet. Altitude helps
because it decreases the amount of distortion causing atmosphere you are looking through.

A good way to judge if the seeing is good or not is to look at bright stars about 40◦ above the
horizon. If the stars appear to twinkle, the atmosphere is significantly distorting the incoming light,
and views at high magnification will not appear sharp. If the stars appear steady and do not twinkle,
seeing conditions are probably good, and higher magnifications will be possible.

Good transparency is especially important for observing faint objects. It simply means the air is free
of moisture, smoke and dust. Which all tend to scatter light, reducing an object’s brightness.

Cooling the telescope

All optical instruments need time to reach thermal equilibrium to achieve maximum stability of
lenses and mirrors, which is essential for good performance of the telescope. When moved from a
warm indoor location, outside to cooler air a telescope needs time to cool to the ambient
temperature.

A cooling fan is at the base of the telescope to assist in the cooling down period prior to use. The
battery pack for the fan is attached with Velcro to the base of the telescope. The fan should be
switched off after the telescope has cooled down, and before observing.

Let your eyes adjust to the dark

Do not expect to go from a lighted house into the darkness of night outdoors and immediately be
able to see faint objects. Your eyes take about 30minutes to reach approx. 80% of their full dark­adapted sensitivity. Many observers notice improvements after several hours of darkness. As your
eyes become dark adapted, more stars will glimmer into view and you will be able to fainter details
in objects.

To see what you are doing in the darkness use the red filtered flashlight – there is one included in
the kit. Red light does not spoil your eyes dark adaptation. Beware too that nearby porch and
streetlights and car headlights will spoil your night vision.

Tracking Celestial Objects

The Earth is constantly rotating about its polar axis, completing one full rotation every 24 hours; this
is what defines a “day”. We do not feel the Earth rotating, but we can tell that it is at night by seeing
the apparent movement of stars from east to west. This movement translates into a rate of .25° per
minute, or arc-seconds per second. (There are 60 arc-minutes in 1°, and 60 arc-seconds in one arc­minute.) This is called the sidereal rate. When you observe any astronomical object, you are
watching a moving target. This means the telescope’s position must slowly be updated over time to
keep an object in the field of view. To keep the object in the telescope’s field of view (to “track” it),
the telescope must be moved by small increments every now and then, in the direction the object is
moving. This is easy to do with a Bintel Dobsonian because of its buttery smooth motion on both
axes. As the object moves off toward the edge of the field of view, you just lightly nudge the
telescope to bring it back to the center. You will notice that it is more difficult to track objects when
the telescope tube is aimed nearly straight up. This is the inherent to the basic design of the
Dobsonian, and stems from the fact that there is very little azimuth when the tube is in a near­vertical position.

To gain more leverage, try grasping the tube close to the altitude side bearings with both hands.
Also, when looking overhead, if the telescope cannot be moved any more in altitude, rotate the

telescope 180° in azimuth to continue motion. Remember that objects appear to move across the
field of view faster at higher magnifications. This is because the field of view becomes narrower.

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 15m, which gives a
magnification of 80 and 83x, and a 9mm, which gives a magnification of 139x and 133x, as well as a
wide angle 2” eyepiece 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 just 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 an 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 as 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.

Field of View

One of the most important considerations with eyepieces is also the most overlooked consideration
when making an 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 arc­minutes. To determine the field of view of any given eyepiece follow this simple formula:

Each type of eyepiece has an apparent field of view i.e. (Plossl = 50° Apparent field)

Divide the Apparent field by the magnification.

The answer will be 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.

Use of 2” Eyepieces

The Bintel Dobsonian comes with 2” focuser 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 1.35” eyepieces. Many observers own at least one 2” eyepiece;
simply remove the 1.25” eyepiece adapter from the end of the focuser drawtube. You must first
loosen the two thumbscrews on the 2” eyepiece adapter to do this. Now insert a 2” eyepiece into
the focuser, and secure it with two thumbscrews. 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.

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 350,000kms 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 between the dark and light portions of the disk (called
the “terminator”). A full moon is too bright and devoid 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 Planets

The planets don’t stay put like the stars, so to find them you should refer to our website
(www.bintel.com.au/newsletter.html), or to charts published monthly in our newsletter Night Sky or

astronomy magazines like Sky and Space, Sky and Telescope. Venus, Mars, Jupiter and Saturn are the
brightest objects in the sky after the Sun and the Moon.

Your Bintel Dobsonian is capable of showing you these planets in some detail. Other planets may be
visible but will appear star-like. Because planets are quite small in apparent size, optional higher
power eyepieces are recommended and are often needed for detailed observations. Not all the
planets are generally visible at any one time.

Jupiter The largest planet, Jupiter, is a great subject for observation. You can see the disk of the
giant planet and watch the ever-changing positions of its four largest moons – Io, Callisto, Europa,
and Ganymede. Higher power eyepieces should bring out the bands on the planet’s disk.

Saturn The ringed planet is a breathtaking sight when it is well positioned. The tilt angle of the rings
varies over a period of many years; sometimes they are seen edge-on, while at other times they are
broadside and look like giant “ears” on each side of Saturn’s disk. A steady atmosphere is necessary
for a good view. You will probably see a bright “star” close by, which is Saturn’s brightest moon,
Titan.

Venus At its brightest, Venus is the most luminous object in the sky, excluding the Sun and the
Moon. It is so bright that sometimes it is visible to the naked eye during full daylight! Ironically,
Venus appears as thin as a crescent, not a full disk, when at its peak brightness. Because it is so close
to the Sun, it never wanders too far from the morning or evening horizon. No surface markings can
be seen on Venus, which is always shrouded in dense clouds.

Mars The Red Planet makes its closest approach to Earth every two years. During close approaches
you’ll see a red disk, and may be able to see the polar ice cap. To see surface detail on Mars, you will
need a high power eyepiece and very steady air!

The Stars

Stars will appear like twinkling points of light. Even powerful telescopes cannot magnify stars to
appear as more than a point of light! You can, however, enjoy the different colours of the stars and
locate many pretty double and multiple stars. The famous “Alpha Cenaturi” in the constellation
Centaurus and the gorgeous two-colour double star Albireo in Cygnus are favourites. Defocusing a
star slightly can help bring out its colour.

Deep-Sky Objects

Under dark skies, you can observe a wealth of fascinating deep-sky objects, including gaseous
nebulas, open and Globular star clusters, and a variety of different types of galaxies. Most deep-sky
objects are very faint, so it is important that you find an observing site well away from light

pollution. Take plenty of time to let your eyes adjust to the darkness. Do not expect these objects to
appear like the photographs you see in books and magazines; most will look like dim gray smudges.