106
8 Imaging
8.3 In-Dome Pre-observation Preparation
Before you make your first observation proper, you will need to take calibration
frames, check the pointing of the telescope, and focus up. These are a vital part of
the observation process, and their importance should not be overlooked.
8.3.1 Calibration Frames
Before you leave the observatory, you will need to create your calibration files, which
were discussed in Sect. 7.5. You may do this either before or after imaging. If you
arrive and it is cloudy, you have an ideal opportunity to take calibrations frames, while
if it is clear, then take the opportunity go to straight to imaging and take calibration
frames at the end. A summary of the calibration process is shown in Fig. 8.2. As
discussed earlier, many observatories like to take sky flats, which use the twilight
sky as their light source. However, for a small observatory, the amount of time to
get sky flats for narrowband filters is long, and it might not be possible to take all
the required sky flats in one session. An alternative is dome flats, for which the
inside of the observatory dome, illuminated with the dome lights, is used as the light
source. Often, the telescope should point at a part of the dome that has been specially
prepared for flat fielding, for example, by the use of a flat white screen. A more
modern process is to use the electroluminescent screen designed for the instrument
you are using. These are designed to give a virtually constant level of illumination.
Although it may not be as flat a source as the twilight sky, it is very stable and yields
very flat master flats.
In order to take your calibration frames, you will need to cool the camera down
to working temperature and take a series of bias frames, at the same binning as you
expect to (or have done) during your observing run. You need multiple biases to
make a master bias; eight should be enough. Depending on what software you will
be using later, it is advisable to name the files such that they can be correctly identified
and are postfixed with a number. For example, Bias−4 × 4 − 1.fit, Bias−4 × 4 −
2, Bias−4 × 4 − 3 would be an acceptable naming sequence, where 4 × 4 is the
binning, and the number is the frame number. Most camera control softwares should
do this for you, but you might need to set it up first. You may also need to set the
frame type card in the header to “Bias” if the camera hasn’t done this already.
You will also need to do the same with your dark frames, although these will also
be exposure specific. You should already have some idea from your planning what
exposure times you are going to use, but you should have a reasonable range of times,
say 1, 5, 10, 15, 30, 60, 120, 240, and 300 s. As with the bias frames, you should
take a series of dark frames for each exposure time you will use. If your telescope is
small enough to have a lens cap, I suggest putting this on when taking darks, as well
as turning the lights off, which will reduce the chance of stray light. If you can set
the camera software to take a series of darks automatically, as you can in MaxImDL
8 Imaging
8.3 In-Dome Pre-observation Preparation
Before you make your first observation proper, you will need to take calibration
frames, check the pointing of the telescope, and focus up. These are a vital part of
the observation process, and their importance should not be overlooked.
8.3.1 Calibration Frames
Before you leave the observatory, you will need to create your calibration files, which
were discussed in Sect. 7.5. You may do this either before or after imaging. If you
arrive and it is cloudy, you have an ideal opportunity to take calibrations frames, while
if it is clear, then take the opportunity go to straight to imaging and take calibration
frames at the end. A summary of the calibration process is shown in Fig. 8.2. As
discussed earlier, many observatories like to take sky flats, which use the twilight
sky as their light source. However, for a small observatory, the amount of time to
get sky flats for narrowband filters is long, and it might not be possible to take all
the required sky flats in one session. An alternative is dome flats, for which the
inside of the observatory dome, illuminated with the dome lights, is used as the light
source. Often, the telescope should point at a part of the dome that has been specially
prepared for flat fielding, for example, by the use of a flat white screen. A more
modern process is to use the electroluminescent screen designed for the instrument
you are using. These are designed to give a virtually constant level of illumination.
Although it may not be as flat a source as the twilight sky, it is very stable and yields
very flat master flats.
In order to take your calibration frames, you will need to cool the camera down
to working temperature and take a series of bias frames, at the same binning as you
expect to (or have done) during your observing run. You need multiple biases to
make a master bias; eight should be enough. Depending on what software you will
be using later, it is advisable to name the files such that they can be correctly identified
and are postfixed with a number. For example, Bias−4 × 4 − 1.fit, Bias−4 × 4 −
2, Bias−4 × 4 − 3 would be an acceptable naming sequence, where 4 × 4 is the
binning, and the number is the frame number. Most camera control softwares should
do this for you, but you might need to set it up first. You may also need to set the
frame type card in the header to “Bias” if the camera hasn’t done this already.
You will also need to do the same with your dark frames, although these will also
be exposure specific. You should already have some idea from your planning what
exposure times you are going to use, but you should have a reasonable range of times,
say 1, 5, 10, 15, 30, 60, 120, 240, and 300 s. As with the bias frames, you should
take a series of dark frames for each exposure time you will use. If your telescope is
small enough to have a lens cap, I suggest putting this on when taking darks, as well
as turning the lights off, which will reduce the chance of stray light. If you can set
the camera software to take a series of darks automatically, as you can in MaxImDL