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3 The Telescope
the translation from encoder position to actual sky position. Once a GoTo telescope
is set up for the night, finding a target is just a case of picking a target from a list
held on the computer or telescope handset and pressing a button, although you may
occasionally need to re-align the telescope during the night, as position errors can
creep in. Gone are the days of finding a bright star first and then using a star map to
hop between stars to find a faint target.
3.3 Eyepiece
It is likely that for most of your observation time, you will not be looking through
the telescope. This is especially true for remote and robotic observations. However,
sometimes you do need to use your eyes, for example in resolving a problem or for
outreach and public engagement events at your observatory.
You cannot just look through a telescope. You need additional optics to bring the
image to focus on your retina, namely an eyepiece.
Eyepieces come in a bewildering array of types, but all do the same thing. They
take the light from the telescope and bring it to focus for an observer, although
occasionally you might need an eyepiece for an instrument if it cannot operate near
the primary focus. In general, eyepieces come in a range of barrel sizes. The barrow
is the part that slides into your focuser and is typically chromed. Standard sizes are
1.24 and 2 in., although 1 in. and sizes greater than 2 in. exist. Not that the size listed
is the size of the barrel, since many top-end eyepieces are wider than this in their
main body (this is to fit in all the optics). Adapters are available if your eyepiece is
too large or too small for the focuser barrel.
On the eyepiece, there should be a number, normally in millimetres, noting the
focal length of the eyepiece, along with with a type and perhaps a manufacturer’s
name. We will ignore the manufacturer’s name, as there are many, and it is largely
unimportant.
The magnification of the telescope with the eyepiece is the focal length of the
instrument divided by the focal length of the eyepiece. Unless you are looking at
a planet or the Moon, it is likely, at least initially, that you will not want a high
magnification, as that limits the field of view.
The point at which you have to place your eye to get the full field of view when
using an eyepiece is known as the eye relief. The farther from the eyepiece, the better,
with at least 15 mm needed for those wearing glasses. Short eye relief eyepieces are
uncomfortable to use, especially if they are so short that the user’s eyelashes touch
the lens.
The exit pupil is the size of the beam of focused light coming out of the eyepiece.
The exit pupil is the focal length of the eyepiece divided by the telescope’s focal
ratio. Hence a 30 mm eyepiece used with a 2000 mm focal length telescope with an
aperture of 200 mm will have an exit pupil of 3 mm. You should try to get an exit
pupil size of between 5 and 7 mm. Human dilated pupil diameter shrinks with age,
so bear this in mind.
3 The Telescope
the translation from encoder position to actual sky position. Once a GoTo telescope
is set up for the night, finding a target is just a case of picking a target from a list
held on the computer or telescope handset and pressing a button, although you may
occasionally need to re-align the telescope during the night, as position errors can
creep in. Gone are the days of finding a bright star first and then using a star map to
hop between stars to find a faint target.
3.3 Eyepiece
It is likely that for most of your observation time, you will not be looking through
the telescope. This is especially true for remote and robotic observations. However,
sometimes you do need to use your eyes, for example in resolving a problem or for
outreach and public engagement events at your observatory.
You cannot just look through a telescope. You need additional optics to bring the
image to focus on your retina, namely an eyepiece.
Eyepieces come in a bewildering array of types, but all do the same thing. They
take the light from the telescope and bring it to focus for an observer, although
occasionally you might need an eyepiece for an instrument if it cannot operate near
the primary focus. In general, eyepieces come in a range of barrel sizes. The barrow
is the part that slides into your focuser and is typically chromed. Standard sizes are
1.24 and 2 in., although 1 in. and sizes greater than 2 in. exist. Not that the size listed
is the size of the barrel, since many top-end eyepieces are wider than this in their
main body (this is to fit in all the optics). Adapters are available if your eyepiece is
too large or too small for the focuser barrel.
On the eyepiece, there should be a number, normally in millimetres, noting the
focal length of the eyepiece, along with with a type and perhaps a manufacturer’s
name. We will ignore the manufacturer’s name, as there are many, and it is largely
unimportant.
The magnification of the telescope with the eyepiece is the focal length of the
instrument divided by the focal length of the eyepiece. Unless you are looking at
a planet or the Moon, it is likely, at least initially, that you will not want a high
magnification, as that limits the field of view.
The point at which you have to place your eye to get the full field of view when
using an eyepiece is known as the eye relief. The farther from the eyepiece, the better,
with at least 15 mm needed for those wearing glasses. Short eye relief eyepieces are
uncomfortable to use, especially if they are so short that the user’s eyelashes touch
the lens.
The exit pupil is the size of the beam of focused light coming out of the eyepiece.
The exit pupil is the focal length of the eyepiece divided by the telescope’s focal
ratio. Hence a 30 mm eyepiece used with a 2000 mm focal length telescope with an
aperture of 200 mm will have an exit pupil of 3 mm. You should try to get an exit
pupil size of between 5 and 7 mm. Human dilated pupil diameter shrinks with age,
so bear this in mind.