Beginners' corner
Beginners who never observed the sky through a telescope often consider making one. It is a good way to learn I experienced myself.
What optical layout will you choose?
The simpliest
is the Newton reflector telescope. As it is the most known, information
is easy to find. Simple and efficient.
What mirrors?
The primary
mirror has a parabolic shape and the secondary mirroir is plane. Better
buy both together because the manufacturer will supply the suitable secondary
miror.
Diameter (D): the bigger it is, the better the resolution
is.
Focal length (F): this is the distance between the center
of the primary mirror and the focus where rays perpendicular to the surface
of the primary mirror converge. It is given by the manufacturer by 10%.
F/D ratio: the smaller it is, the shorter and the wider the
telescope is. It also gives more light and the parabola is deeper. F/D
= 4, 5 or 6 is suitable for deep sky.
Quality of the parabola: lambda/12 is standard. Dobson's mirrors
are of lower quality (lambda/4 or lambda/8). A better quality (lambda/16
or more) is only usefull if the sky is good enough.
Eyepiece
Several technologies are available
: Kellner, Huygens, orthoscopic, wide-field... Better choose orthoscopic
quality as a minimum to avoid being desappointed.
Focal length (f): 15, 30 or 50mm depending on the magnifying
you wish.
Magnifying (m = F/f): the more you magnify, the smaller the
field is. This makes observation more difficult because telescope defects
are amplified and you often lose the easiest objects (Moon, planets...).
I recommend a magnifying ratio of 30 to 50 times to start.
Apparent field (a): 50° minimum with an orthoscopic eyepiece.
A wider field is comfortable, but you will have to bent your head to see
on the sides.
Real field (r = a/m): part of the sky seen in the eyepiece.
Example: F = 750 mm, f = 15 mm, a = 50°, m = 750/15 = 50 times, r
= 50/50 = 1° i.e. twice the Moon.
Barrel diameter: it is the diameter of the shiny part machined
to 24.50 mm, 31.75 mm (1.25") or 50.80 mm (2"). I would advise 31.75
mm (1.25") because you will have more choice and it will be cheaper and
less cumbersome than 50.80 mm (2").
Finder
It is a small 30 to 50 mm in diameter refractor telescope, whose
magnifiying power is 7 to 8 times
(finder 7x50 or 8x50 for example). Its real field is 6 or 7°. You
can start without it to limit the initial cost. It is traditionally fixed
very close to the optical tube, but it is more comfortable to fix it 5
or 10 cm from the tube (blue bloc between the black tube and the blue
finder). You must add an anti-dew tube, a plastic cylinder which goes
on the finder, a rolled X-ray photo for example.
Optical tube
It is traditionally a white and closed cylinder. But I chose to make
it black, open and square. On the first hand, it is easier to make a square
tube using wood, on the second hand, it is easier to fix instruments on a
square tube. White is considered best to reduce telescope turbulence, but
black is OK for visual observation. An open tube is lighter, optical adjustment
and handling is easier. If optical part are not well adjusted in the beginning,
images even look focussed but resolution is reduced.
Focuser
Every time you touch it
you may move the telescope. With the hydraulic focusing
system
, you avoid touching it directly. It must be precise and without perceptible
play because the eyepiece must be positionned by about 20 microns. This figure
depends on the mirror, focal length, magnifying power, observation style,
visual or photographic, but it shows a mere threaded rod is not suitable.
If you prefer to buy a capstan or a rack-and-pinion focuser, be careful the
screw should turn very gently. If it is too hard, the telescope will move
and you will lose what you are looking at.
You can
see a barlow lens on the picture (white spot) and the eyepiece (red spot).
The use of the barlow lens is to double magnifying power when atmospheric
conditions are excellent. It is rarely useful.
Mount
Vibration test: give a slight jerk with your finger on the tube.
Damp out time: it is the time necessary to damp out the oscillations
produced by the previous jerk. For a 115/900 telescope with a standard
mount, this time is approximately 5 seconds. Amateur astronomers consider
to be good a telescope that has a 2 second or less damp out time.
Oscillation frequency: if the telescope oscillates slowly, your
eye can follow observed sky objects. If it oscillates fast, stars will
appear as lines. A wood mount gives slow oscillations (about 1 Hz). A metal
one gives high frequency oscillations (10 Hz or more).
Friction: two kinds of friction can be told apart: static friction
and sliding friction. If you push gently an object on a table, you have to
apply a certain force before it starts sliding: this is static sliding. Once
this object has started moving, there is still a friction, but it is lower:
it is sliding friction. In astronomy static friction has to be reduced as
much as possible, but sliding friction is good because it avoids telescope
to move alone. If static friction is too important, you will have to push
the tube to follow a star, but if you push too hard, you will go too far and
loose the star. When changing eyepiece, telescope is unbalanced and it may
move if friction is too low.