110
8 Imaging
encoders that tell the controller the position of the telescope in Alt-Az could be
offset. It should be easy to check that the time and position are correct, make sure
that you are using the time without daylight saving, which is often the cause of errors.
If you can, synchronise the telescope to an RA and Dec; in addition to syncing
with a known star, a process known as plate solving may help. Plate solving involves
automatically identifying the stars in the image and using their relative positions
matched against stellar catalogues to determine the centre of the image, which can
then be used to synchronise the telescope. A number of telescope control packages,
including CCDsoft and MaximDL, have this feature. It is also available as online
services from astrometry.net. Note that if the telescope is a long way out of focus,
most plate solving programs will struggle, as they will be unable to identify stars
in the frame, in which case you may wish to jump to the focusing section of this
practical and then return to aligning the telescope.
If the time and location are correct and plate solving fails to correct the pointing
problem, try to home the telescope if it has this feature. Homing moves the telescope
to the absolute zero position of the encoders. If you have a minor problem with the
encoders, this may resolve it. If this fails, try parking the telescope. This will put
the telescope into a known position. If there is a pointing problem, you might find
that the telescope does not return to the park position. If that is the case, unlock the
telescope and move it to its park position by hand, although you should check with
the observatory staff beforehand whether that is an approved procedure.
If none of these processes clears the pointing problem, then you may wish to
escalate the problem to the observatory staff.
8.3.3 Focusing
There is a tendency for users new to telescopes to struggle with focusing. However, once you know the correct technique, focusing is straightforward. Focusing
is achieved by moving the focal plane of the telescope so that it is coplanar with
the CCD or retina. Focusing may be accomplished by either moving the principal
optical component, i.e., the mirror, or moving the detector using a focuser, which is a
mechanical or electromechanical device that moves the position of the sensor. If you
have the ability to move the mirror and have use of a focuser, use only the focuser.
The only reason you should need the mirror focus is for changing the detector, and
in most cases, this should be done only by observatory staff. It is likely that you will
have to check and adjust the focus several times during the night. This will be partly
due to the contraction or expansion of the telescope due to temperature changes, but
also to changes in air mass and the slight movement of the camera as the telescope
is moved between horizon and zenith. The air mass is the path length that the light
takes through the atmosphere, with an object at the zenith having an air mass of 1.0.
Air mass can be calculated from the angular distance that an object lies from the
zenith, called the zenith distance and denoted by z. The air mass X can be found
from the sec of z. Before I take a science image, I always like to take a short focusing
8 Imaging
encoders that tell the controller the position of the telescope in Alt-Az could be
offset. It should be easy to check that the time and position are correct, make sure
that you are using the time without daylight saving, which is often the cause of errors.
If you can, synchronise the telescope to an RA and Dec; in addition to syncing
with a known star, a process known as plate solving may help. Plate solving involves
automatically identifying the stars in the image and using their relative positions
matched against stellar catalogues to determine the centre of the image, which can
then be used to synchronise the telescope. A number of telescope control packages,
including CCDsoft and MaximDL, have this feature. It is also available as online
services from astrometry.net. Note that if the telescope is a long way out of focus,
most plate solving programs will struggle, as they will be unable to identify stars
in the frame, in which case you may wish to jump to the focusing section of this
practical and then return to aligning the telescope.
If the time and location are correct and plate solving fails to correct the pointing
problem, try to home the telescope if it has this feature. Homing moves the telescope
to the absolute zero position of the encoders. If you have a minor problem with the
encoders, this may resolve it. If this fails, try parking the telescope. This will put
the telescope into a known position. If there is a pointing problem, you might find
that the telescope does not return to the park position. If that is the case, unlock the
telescope and move it to its park position by hand, although you should check with
the observatory staff beforehand whether that is an approved procedure.
If none of these processes clears the pointing problem, then you may wish to
escalate the problem to the observatory staff.
8.3.3 Focusing
There is a tendency for users new to telescopes to struggle with focusing. However, once you know the correct technique, focusing is straightforward. Focusing
is achieved by moving the focal plane of the telescope so that it is coplanar with
the CCD or retina. Focusing may be accomplished by either moving the principal
optical component, i.e., the mirror, or moving the detector using a focuser, which is a
mechanical or electromechanical device that moves the position of the sensor. If you
have the ability to move the mirror and have use of a focuser, use only the focuser.
The only reason you should need the mirror focus is for changing the detector, and
in most cases, this should be done only by observatory staff. It is likely that you will
have to check and adjust the focus several times during the night. This will be partly
due to the contraction or expansion of the telescope due to temperature changes, but
also to changes in air mass and the slight movement of the camera as the telescope
is moved between horizon and zenith. The air mass is the path length that the light
takes through the atmosphere, with an object at the zenith having an air mass of 1.0.
Air mass can be calculated from the angular distance that an object lies from the
zenith, called the zenith distance and denoted by z. The air mass X can be found
from the sec of z. Before I take a science image, I always like to take a short focusing