104
8 Imaging
8.2 Planning
As stated previously, telescope time is a precious commodity, and you should plan
your evening’s observing in advance to get the maximum benefit out of the telescope.
If you are observing as part of a practical, you should read the practical in advance
and understand its aims and processes. Reading or rereading the practical in the dome
is a waste of time. You should know what telescope you will be using and its optical
characteristics, especially the pointing limitations, the limiting magnitude, and the
field of view, as well as the range of filters available.
You will have to choose your target from a supplied list of options or select a target
that meets a set of criteria, or perhaps you will have been explicitly assigned a target.
You will need to ensure that the target will be visible at the time and date you will be
observing and that it is within the limiting magnitude of the telescope. Remember
that for extended objects, the quoted magnitude is an integrated magnitude. If you
know the approximate area of the object, you can get an idea of its point source
magnitude, which you will need to know to determine whether the target is bright
enough to be observed as well as to gauge the exposure time. You should write down
the target’s name, the RA and Dec in J2000, and its magnitude in the band you are
observing. If you are not asked to target a specific object, then it is always advisable
to have a backup object. If the Moon is going to be up, it is best to avoid the region
of the sky it is occupying. Some telescope control applications have a Lorentzian
Moon avoidance function built in, which warns you if the telescope is pointing too
close to the Moon, the acceptable distance depending on the phase. If the Moon is
full and you are doing photometry or deep sky imaging, you may wish to observe at
a later date, advice that also applies if there is a bright solar system object nearby.
For each target, you should produce a finder chart, which will help you confirm
that the target you are looking at is the target you want to be looking at. Most star
fields are indistinguishable from one another. Even if your target is obvious, such as
a galaxy, having a finder chart that encompasses a larger area than the telescope’s
field of view helps. If the telescope moves close to the target but not close enough
to put it in the field of view, a finder chart can help in finding where the telescope is
pointing and how to move it so that your target is in the centre of the field.
There are several finder chart tools to help you find your target. Perhaps the
most popular in the professional community is Aladdin, which can be accessed via
SIMBAD. Additionally, DS9 has an image server option that allows the downloading
of survey images. The DS9 image servers can be found in Analysis Image Server,
where a number of image servers are listed; we suggest using one of the DSS (Digital
Sky Survey) servers, as they offer mostly optical images. You can search by object
name or coordinates and can also determine the image dimensions. Before you print
out the finder image, I suggest inverting it so that the stars appear black against a
white background, as this makes annotation easier. Mark the target on the finder chart
and mark the orientation of the axis. An example of a DS9 finder chart is shown in
Fig. 8.1.
8 Imaging
8.2 Planning
As stated previously, telescope time is a precious commodity, and you should plan
your evening’s observing in advance to get the maximum benefit out of the telescope.
If you are observing as part of a practical, you should read the practical in advance
and understand its aims and processes. Reading or rereading the practical in the dome
is a waste of time. You should know what telescope you will be using and its optical
characteristics, especially the pointing limitations, the limiting magnitude, and the
field of view, as well as the range of filters available.
You will have to choose your target from a supplied list of options or select a target
that meets a set of criteria, or perhaps you will have been explicitly assigned a target.
You will need to ensure that the target will be visible at the time and date you will be
observing and that it is within the limiting magnitude of the telescope. Remember
that for extended objects, the quoted magnitude is an integrated magnitude. If you
know the approximate area of the object, you can get an idea of its point source
magnitude, which you will need to know to determine whether the target is bright
enough to be observed as well as to gauge the exposure time. You should write down
the target’s name, the RA and Dec in J2000, and its magnitude in the band you are
observing. If you are not asked to target a specific object, then it is always advisable
to have a backup object. If the Moon is going to be up, it is best to avoid the region
of the sky it is occupying. Some telescope control applications have a Lorentzian
Moon avoidance function built in, which warns you if the telescope is pointing too
close to the Moon, the acceptable distance depending on the phase. If the Moon is
full and you are doing photometry or deep sky imaging, you may wish to observe at
a later date, advice that also applies if there is a bright solar system object nearby.
For each target, you should produce a finder chart, which will help you confirm
that the target you are looking at is the target you want to be looking at. Most star
fields are indistinguishable from one another. Even if your target is obvious, such as
a galaxy, having a finder chart that encompasses a larger area than the telescope’s
field of view helps. If the telescope moves close to the target but not close enough
to put it in the field of view, a finder chart can help in finding where the telescope is
pointing and how to move it so that your target is in the centre of the field.
There are several finder chart tools to help you find your target. Perhaps the
most popular in the professional community is Aladdin, which can be accessed via
SIMBAD. Additionally, DS9 has an image server option that allows the downloading
of survey images. The DS9 image servers can be found in Analysis Image Server,
where a number of image servers are listed; we suggest using one of the DSS (Digital
Sky Survey) servers, as they offer mostly optical images. You can search by object
name or coordinates and can also determine the image dimensions. Before you print
out the finder image, I suggest inverting it so that the stars appear black against a
white background, as this makes annotation easier. Mark the target on the finder chart
and mark the orientation of the axis. An example of a DS9 finder chart is shown in
Fig. 8.1.