Orion SKYVIEW 9877 User Manual

®

Astronomical

Telescope

User Guide

WARNING!!

Never point the telescope directly at or near the Sun at any time.
Observing the Sun, even for a fraction of a second, will result in
instant and ir reversible eye damage. Please ensure minors ar e
supervised by an adult conversant with this real danger when

using telescopes or binoculars.

OPTICAL VISION LIMITED

www.opticalvision.co.uk

Types of Telescope...

f all the many and varied telescopes
available for use by amateur

O

astronomers and nature watchers, all
can be categorised into three types: the
re f r a c t o r, the reflector & the catadioptric.
Each have their relative strengths and weak­nesses, but they all have a common function:
to gather and focus light from distant objects
to produce a bright image that may be magni­fied. In this respect it is the aperture (i.e. the
diameter of the main mirror or lens) of the
telescope that performs a critical function.
With larger apertures, more light is gathered
so fainter objects may be perceived and the
resolving power (i.e. the ability to see fine
detail) is increased.

When comparing telescopes of similar type
but of differing apertures, an instrument that
has a main lens or mirror twice the size of
another gathers four times as much light, not
twice. We are comparing the collecting areas
of the larger and smaller telescopes, which is
proportional to the squares of the apertures.
So, for example, a 120mm telescope gathers

2.25 times (225%) as much light as an instru­ment of 80mm aperture (120/80 = 1.5, 1.5
squared = 2.25).

When it comes to seeing fine detail in an
image, we are simply comparing apertures:
a 120mm telescope will enable you to
perceive lunar craters, for example, half the
size of those visible in a 60mm instrument.
No amount of magnification applied to the
smaller telescope will show you that which
will be visible in the larger instrument, though
both images may appear equally sharp. The
larger telescope merely forms its images out
of smaller 'dots'.

However, the resolving power (as it is cor­rectly termed) of even the smallest telescope
is awesome: a 60mm telescope is capable of
resolving detail as small as a £1 coin at a dis­tance of 2.35 km - nearly 1.5 miles!

< The refractor:

This is the type of
instrument that the lay­man thinks of when
conjuring up a mental
picture of a telescope.

At the end of the tube
furthest away from the
o b s e rver there is an
objective lens (or rather
two glass elements
sandwiched together
for reasons to be ex­plained in a moment)
that gathers light from
t h e object under scru t i n y,
to form an image at the
other end which is
viewed by means of an
eyepiece.

The objective lens can­not be made of a single
piece of glass since
such an element is
incapable of bringing
light of differing wave­lengths to a common
focus, introducing a
prismatic effect that
causes bright objects
to be surrounded by
false rainbow colours.

This undesirable quality
of refractors is virtually
eliminated by making
the objective out of two
glass elements with
optical characteristics
that effectively 'cancel
out' the false colour.

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Consequently, an objective lens is a piece of
optical equipment that is difficult to manufac­ture and explains why refractors are the most
expensive form of telescope, aperture for
aperture. However, a refractor can deliver
exquisite images that are very well corrected
and extremly high in contrast, suitable for
observing fine lunar and planetary detail, or
for separating difficult double stars.

Refractors are well suit­ed for people on the
move since it is
e x t remely difficult for
the optical components
to come out of align­ment. Also, the lenses
will not need to be
recoated in a lifetime's
use and maintenance is
minimal.

< The reflector: This

is the cheapest form of
telescope, aperture for

a p e rt u re, that money
can buy. In the form

most encountered, the
Newtonian (after Isaac
Newton's design in

1668) uses a concave

mirror at the base of the

tube to gather
and focus the
light from the
object under
s c ru t i n y. The

light is interc e p t e d

b e f o re coming to a

focus near the mouth of

the tube by a small, flat
mirror inclined at 45° to

the optical axis which

relays the image out of

a hole in the side of the tube to a waiting
eyepiece that magnifies the image in the nor­mal fashion. Since the light is not refracted in
any way, and a mirror reflects light of all
colours equally, there is no false colour from
a reflecting telescope.

^ The catadioptric:

The goal of the telescope designer is to pro­duce an optical system that delivers an image
as free from any aberrations (defects) as
possible in a package that is compact and
manageable. This has led to designs incorpo­rating both reflecting and refracting elements
to produce an instrument that combines the
best attributes of both systems.

Thus, we now see many commercial tele­scopes similar to the Maksutov-Cassegrain
system illustrated above that packs a long
focal length into a physically short tube, while
preserving the high-contrast imagery associ­ated with refractors of the same aperture.

Catadioptric variations of the Newtonian tele­scope are currently very much in vogue,
offering compact tube assemblies with conve­nient viewing positions. Since these systems
often have optical windows sealing the end of
the tube (which would otherwise be open in a
conventional Newtonian), the internal optical
components are far better protected from the
elements and dust.

Remember that no particular design of tele­scope is intrinsically better than another
– each is well suited to a wide variety of
observational subjects. The important consid­eration is that the optics should be accurately
m a n u f a c t u red and be precisely collimated
(aligned).

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Telescopes & Mounts...

The three legs should be attached to the tripod
head one by one by the three bolts and wing
nuts provided. They should not be overtight­ened, but just made finger tight. Pay part i c u l a r
attention to the orientation of each leg prior to
assembly such that the tripod tray bracket is

ALT-AZ

ALT-AZ

f all the many and varied telescopes
available for use by the amateur

O

astronomers, the mountings that sup­port them fall into two types - the alt-azimuth
and the equatorial. The 'Using your telescope'
section of this booklet that follows will tell you
more about these terms and the actual use of
your instrument, so we will concentrate on
the actual initial assembly and adjustments of
your telescope here.

facing inward. The tripod may now be placed
on the ground with the legs splayed enough
for the accessory tray to be attached. Adjust
the height of each leg so that the tripod head
is initially kept low for maximum rigidity,
taking note that the thumb screws holding the
legs in place are securely tightened prior to
the next stage.

Next, attach the alt-azimuth/equatorial mount
(depending on your particular model: see the
ALT-AZ or EQ label with the accompanying
photographs) to the tripod head, followed by
the accessory tray between the tripod legs if
you haven't already done so.

If your telescope mount is fitted with flexible
slow motion controls (e.g. AZ-3, EQ-1, EQ2
and EQ3-2) these may now be attached to the
two small chrome shafts on the mounting via
the thumb screws on the ends of the cables,
taking care to locate the screw in the
machined 'notch' on each shaft.

EQ-5

The initial assembly of your telescope is best
undertaken in daylight with plenty of room to
lay out the components and to familiarise with
the accompanying images to see how the
assembled instrument should look.

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EQ-2

EQ-1

Balancing the Telescope: To eliminate

stresses on the EQ equatorial mounts and to
ensure smooth, judder-free motion of the
telescope the instrument needs to be
balanced about both the declination and polar
(right ascension) axis. This is especially im­portant if you propose to use a motor drive
for astrophotography at a future date.

screw firmly - the telescope is now balanced
about the polar (right ascension) axis.

(ii) With the counterweight bar horizontal,
ensure that the right ascension clamp is
locked and unlock the declination clamp and
turn the telescope tube so that it is also hori­zontal. As before, GENTLY release your grip
on the tube and note is there is a tendency for
one end to rise in the air. If so, slowly loosen
the clamps holding the tube and slowly slide
the tube towards the end that rises. Lock the
tube clamp rings once more and see if the
telescope remains horizontal when so posi­tioned. Repeat this process until the telescope
remains wherever you place it.

The telescope is now balanced about both the
declination and polar (right ascension) axes,
In due course you will add various acces­sories to the telescope that will slightly alter
the balance position particularly if it is a cam­era for astrophotography) in which case you
will have to go through processes (i) and (ii)
again, but for now you may care to mark the
balance points on the counterweight bar and
the telescope tube with small pieces of tape
for fast assembly in future.

(i) Locate the right ascension clamp and
loosen it whilst holding the telescope tube.
Turn the telescope about this axis until the
c o u n t e rweight bar is approximately horizontal.
GENTLY release your grip on the tube and
note if the counterweight has a tendency to
fall or rise: if it RISES, unlock the counter­weight locking screw and slide it away from
the telescope; if it FALLS , slowly slide the
counterweight towards the telescope side of
the mount. Repeat this process until the
counterweight bar remains in one place with­out support and clamp the counterweight

AZ-3

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EQ3-2

Attaching the finderscope:

An essential prerequisite for the easy location
of objects on both land and in the sky is the
correct location and alignment of the finder­scope that attaches to the tube of the tele­scope. It is, in fact, a smaller version of the
main telescope that is designed to have a low
magnification and a wide field of view so that
the desired target may be easily located. The
eyepiece of the finder scope is equipped with
cross-hairs much like a gun sight marking the
centre of the field of view.

Aligning the finderscope:

(i) Setup the telescope on its mount outdoors
in the day time and ensure that it is balanced
as previously described. You may wish to
extend the tripod legs and securely lock them
again once you have the telescope at a com­fortable working height. Enlisting the help of
a friend will aid this process. Select the lowest
magnification eyepiece in the set (this is the
one with the largest number engraved on the
cap: usually 20mm or 25mm) and place it in
the focuser drawtube as previously described.

(ii) Select a distant prominent object such as
the tip of an electricity pylon or the tip of a
c h u rch spire, though any distant, well-defined
object will do: the further away the better. In
the case of an equatorial mount loosen the
declination and right ascension clamps, or the
altitude and azimuth locks on the alt-az
mount. Sight along the tube and position it
such that the telescope is pointing towards
the distant landmark.

(iii) This first attempt will call for a little trial
and error since the telescope will not be cor­rectly focused, so as soon as one sees some­thing blurred through the eyepiece lock the
declination/right ascension clamps (equatorial
mount )or the altitude and azimuth clamps
(Alt-az mount). Use the slow motion controls
as necessary to centre the object in the field
of view.

(iv) Turn the focuser knobs back or forth until
the image becomes crisp and sharp. Do not
be concerned that the image appears upside
down (reflecting telescope) or reversed in a
left to right plane (refractor with zenith prism)

- this is perfectly natural for an astronomical
telescope since there is no 'up' or 'down' in
the sky and you will soon get used to it. With
the image correctly focused you may wish to
use the slow motion controls to perfect the
alignment on the distant target.

(v) Now we can proceed to the Finder. You
will note that the image it offers is upside
down for the same reasons as that of the
main telescope. Depending on the design of
the telescope that you possess, the Finder will
be attached to the main telescope by a brack­et that will have either three or six radially
spaced adjusting screws: if yours is of the latter
type, adjust the front three screws to permit
the tube of the Finder to be held centrally in
the bracket. Once this has been achieved,
check that the target is still central to the field
of view of the main telescope then slowly
adjust the rear three screws of the finder
bracket until the same object is positioned in
the centre of the cross-wires in the finder's
eyepiece. Again it may help to have a friend to
help you with this procedure. Once achieved,
ensure that the lock nuts of the Finder adjust­ing screws are secure. Now you can loosen
the locks on the polar (/azimuth) and declina­tion (/altitude) axes and practice locating
objects during the day.

Telescope basics:

Calculating the telescope's magnification

The magnifying power of any given telescope
and eyepiece is given by a simple formula that
requires a knowledge of the instrument's
focal length and that of the eyepiece. As we
have seen, the focal length of an eyepiece is
usually engraved on its cap: 10mm or 25mm,
for example.

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