Choosing The Apropriate Eyepiece; Calculating The Fi Eld Of View - Dörr DANUBIA DELTA 20 Notice D'utilisation

Pointing at an object
Pointing at an object, for example to the South (Fig.p), can often be achieved with the optical tube positioned
on either side of the mount. When there is a choice of sides, particularly when there could be a long observing
period, the East side (Fig.p2) should be chosen in the Northern Hemisphere because tracking in R.A. will move
it away from the mount's legs. This is particularly important when using an R.A motor, because if the optical
tube jambs against the mount's legs, it can result in damage to the motor and/or the gears.
Telescopes with long focal lengths often have a "blind spot" when pointing near the zenith, because the eyepi-
ece-end of the optical tube bumps into the mount's legs (Fig.q1). To adapt for this, the optical tube can be very
carefully slipped up inside the tube rings (Fig.q2). This can be done safely because the tube is pointing almost
vertically, and therefore moving it does not cause a DEC.-balance problem. It is very important to move the
tube back to the DEC.-balanced position before observing other sky areas. Something which can be a problem
is that the optical tube often rotates so that the eyepiece, fi nderscope and the focussing knobs are in less
convenient positions. The diagonal can be rotated to adjust the eyepiece. However, to adjust the positions of
the fi nderscope and focussing knobs, loosen the tube rings holding the optical tube and gently rotate it. Do
this when you are going to view an area for while, but it is inconvenient to do every time you briefl y go to a new
area. Finally, there are a few things to consider to ensure that you are comfortable during the viewing session.
First is setting the height of the mount above the ground by adjusting the tripod legs. You must consider the
height that you want your eyepiece to be, and if possible plan on sitting on a comfortable chair or stool. Very
long optical tubes need to be mounted higher or you will end up crouching or lying on the ground when looking
at objects near the zenith. On the other hand, a short optical tube can be mounted lower so that there is less
movement due to vibration sources, such as wind. This is something that should be decided before going
through the e ort of polar aligning the mount.
08 | CHOOSING THE APPROPRIATE EYEPIECE
08.1 CALCULATING THE MAGNIFICATION (POWER)
The magnifi cation produced by a telescope is determined by the focal length of the eyepiece that is used with
it. To determine a magnifi cation for your telescope, divide its focal length by the focal length of the eyepieces
you are going to use. For example, a 10mm focal length eyepiece will give 80x magnifi cation with an 800mm
focal length telescope.
Focal length of the telescope
Magnifi cation =
Focal length of the eyepiece
When you are looking at astronomical objects, you are looking through a column of air that reaches to the
edge of space and that column seldom stays still. Similarly, when viewing over land you are often looking
through heat waves radiating from the ground, house, buildings, etc. Your telescope may be able to give
very high magnifi cation but what you end up magnifying is all the turbulence between the telescope and the
subject. A good rule of thumb is that the usable magnifi cation of a telescope is about 2x per mm of aperture
under good conditions.
08.2 CALCULATING THE FIELD OF VIEW
The size of the view that you see through your telescope is called the true (or actual) fi eld of view and it is
determined by the design of the eyepiece. Every eyepiece has a value, called the apparent fi eld of view, which
is supplied by the manufacturer. Field of view is usually measured in degrees and/or arc-minutes (there are
60 arc-minutes in a degree). The true fi eld of view produced by your telescope is calculated by dividing the
eyepiece's apparent fi eld of view by the magnifi cation that you previously calculated for the combination.
Using the fi gures in the previous magnifi cation example, if your 10mm eyepiece has an apparent fi eld of view
of 52 degrees, then the true fi eld of view is 0.65 degrees or 39 arc-minutes.
Apparent Field of View
True Field of View =
Magnifi cation
To put this in perspective, the moon is about 0.5° or 30 arc-minutes in diameter, so this combination would be
fi ne for viewing the whole moon with a little room to spare. Remember, too much magnifi cation and too small
a fi eld of view can make it very hard to fi nd things. It is usually best to start at a lower magnifi cation with its
wider fi eld and then increase the magnifi cation when you have found what you are looking for. First fi nd the
moon then look at the shadows in the craters!
800 mm
e.g. = 80x
10 mm
52°
e.g. = 0,65°
80x
22
Fig.o
Examples of the telescope moved in R.A. and DEC.
Fig.p
Telescope pointing South
Fig.q
Telescope pointing at the Zenith