36
3 The Telescope
the parallel position, light travels straight through to the instrument or eyepiece in the
normal mounting position. When it is flipped to the 45
◦ position, the light is diverted
to another detector at a 90
◦ angle to the primary detector. Hence you can have two
instruments attached to the telescope at the same time. Typically, this would be a
spectrograph and eyepiece, but not always.
On occasion, you might find it impossible to focus on the image, either visually
or with an instrument. This is often caused by the focuser lacking the degree of travel
to put the image in focus. This is overcome by use of an extension tube. The tube
contains no optics and just allows the positioning of the instrument at a more distant
point from the focus.
Parfocal rings are small components that can be affixed to an eyepiece or a camera
to match the position of the focuser between two items, thereby removing the need to
refocus between swaps. Typically, the matching is between a camera and an eyepiece,
and parfocal rings are especially useful when one is using a flip mirror.
If you are using a digital single lens reflex camera (DLSR) for imaging or a
webcam, it might be necessary to have a particular adapter. There are known as T-rings
and are specific to a camera manufacturer and sometimes to a camera generation. It
is not obvious by sight which T-Ring is the right one, so check with your observatory
staff and make sure it fits before planning the night’s observing.
3.4 Problems
A number of aberrations exist that appear in telescope optics. Some are inherently
unavoidable due to the design of the optics, some by the incorrect calibration of the
optics. Still others are caused by external factors or by poor operation of the telescope
and instrument package.
One of the most common problems I find with students is that their images frequently come out blank. Often, it is just a case of the exposure time of the camera
being set too short, the camera having lost connection with the control computer, or
the camera shutter being set too close. Most commonly, though, the reason is that
the lens cap is still on the telescope! I have seen this happen dozens of times with
undergraduate and postgraduate students and even academic staff. If you are getting
black images, run through the following checklist before getting help.
1. Is the lens cap on the telescope?
2. Is the slit of the dome (if it has one) aligned with the telescope. If not and you
have a tracking dome, park both the dome and telescope and reactivate the startup
procedure. Sometimes, the control software gets confused as to where the telescope
is pointing and, on occasion, where the dome is. If you have a non-tracking dome,
just move the slit to the correct position.
3. Is the exposure time too short? In any part of the sky, a telescope of reasonable size
should see at least some stars, but not if the exposure time on the camera is very
short. If the previous user was observing something very bright, such as Jupiter, for
3 The Telescope
the parallel position, light travels straight through to the instrument or eyepiece in the
normal mounting position. When it is flipped to the 45
◦ position, the light is diverted
to another detector at a 90
◦ angle to the primary detector. Hence you can have two
instruments attached to the telescope at the same time. Typically, this would be a
spectrograph and eyepiece, but not always.
On occasion, you might find it impossible to focus on the image, either visually
or with an instrument. This is often caused by the focuser lacking the degree of travel
to put the image in focus. This is overcome by use of an extension tube. The tube
contains no optics and just allows the positioning of the instrument at a more distant
point from the focus.
Parfocal rings are small components that can be affixed to an eyepiece or a camera
to match the position of the focuser between two items, thereby removing the need to
refocus between swaps. Typically, the matching is between a camera and an eyepiece,
and parfocal rings are especially useful when one is using a flip mirror.
If you are using a digital single lens reflex camera (DLSR) for imaging or a
webcam, it might be necessary to have a particular adapter. There are known as T-rings
and are specific to a camera manufacturer and sometimes to a camera generation. It
is not obvious by sight which T-Ring is the right one, so check with your observatory
staff and make sure it fits before planning the night’s observing.
3.4 Problems
A number of aberrations exist that appear in telescope optics. Some are inherently
unavoidable due to the design of the optics, some by the incorrect calibration of the
optics. Still others are caused by external factors or by poor operation of the telescope
and instrument package.
One of the most common problems I find with students is that their images frequently come out blank. Often, it is just a case of the exposure time of the camera
being set too short, the camera having lost connection with the control computer, or
the camera shutter being set too close. Most commonly, though, the reason is that
the lens cap is still on the telescope! I have seen this happen dozens of times with
undergraduate and postgraduate students and even academic staff. If you are getting
black images, run through the following checklist before getting help.
1. Is the lens cap on the telescope?
2. Is the slit of the dome (if it has one) aligned with the telescope. If not and you
have a tracking dome, park both the dome and telescope and reactivate the startup
procedure. Sometimes, the control software gets confused as to where the telescope
is pointing and, on occasion, where the dome is. If you have a non-tracking dome,
just move the slit to the correct position.
3. Is the exposure time too short? In any part of the sky, a telescope of reasonable size
should see at least some stars, but not if the exposure time on the camera is very
short. If the previous user was observing something very bright, such as Jupiter, for