10.3 Practical: Find the Zero Point and Transformations
153
You will need an uncertainty field, again containing photometric standards, that
will be used to determine the uncertainty in your photometric calibrations. The calibrations should be imaged close to the meridian but not at the same air mass as either
of your other two fields.
Once you have identified your fields, you will need to produce finder charts with
the photometric standard marked with their magnitudes and the time that you should
start and stop imaging with the exposure time for each band.
Unlike most of the other practicals in this book, this practical requires very good
photometric conditions. There can be no Moon at the time of imaging, and the weather
must be very good, with no cirrus. Good seeing is desirable but not as important as
the Moon status or weather.
10.3.3 Imaging
When you arrive at the observatory, get the camera down to its operational temperature as usual and ensure that it is ready for use. You will need to take multiple flat,
bias, and dark calibration frames in order to make master calibration frames. This
practical will suffer if you are not careful with your calibration frames, especially
the flat frames.
1. As usual, before you start science imaging, make sure the telescope is focused,
that the binning is set correctly, the autoguider is operating correctly, and that the
camera is not subframing.
2. Slew the telescope to the first field some time before you intend to image and take
a point image to ensure that the telescope is pointing at the correct field and is
centred.
3. As the target reaches the required altitude, start imaging. Take the images as a
filter sequence, i.e., VRBVRBVRB rather than VVVRRRBBB.
4. You will have some time between image sets. Use this time to check that the
images are of acceptable quality, with no trailing, overexposed stars, or plane
trails.
5. Once you have imaged all your first- and second-order fields together with the
uncertainty field, shut the telescope down (if required). Ensure that you have all
the data on a removable medium, including the calibration files.
10.3.4 Photometry
1. In order to perform photometry, you will need to turn your light frames into
science frames, as outlined in Chap. 9.
2. Once you have your science frames, you will need to find the seeing of each
image. If you are using APT, this can be done by dropping an aperture of a star
153
You will need an uncertainty field, again containing photometric standards, that
will be used to determine the uncertainty in your photometric calibrations. The calibrations should be imaged close to the meridian but not at the same air mass as either
of your other two fields.
Once you have identified your fields, you will need to produce finder charts with
the photometric standard marked with their magnitudes and the time that you should
start and stop imaging with the exposure time for each band.
Unlike most of the other practicals in this book, this practical requires very good
photometric conditions. There can be no Moon at the time of imaging, and the weather
must be very good, with no cirrus. Good seeing is desirable but not as important as
the Moon status or weather.
10.3.3 Imaging
When you arrive at the observatory, get the camera down to its operational temperature as usual and ensure that it is ready for use. You will need to take multiple flat,
bias, and dark calibration frames in order to make master calibration frames. This
practical will suffer if you are not careful with your calibration frames, especially
the flat frames.
1. As usual, before you start science imaging, make sure the telescope is focused,
that the binning is set correctly, the autoguider is operating correctly, and that the
camera is not subframing.
2. Slew the telescope to the first field some time before you intend to image and take
a point image to ensure that the telescope is pointing at the correct field and is
centred.
3. As the target reaches the required altitude, start imaging. Take the images as a
filter sequence, i.e., VRBVRBVRB rather than VVVRRRBBB.
4. You will have some time between image sets. Use this time to check that the
images are of acceptable quality, with no trailing, overexposed stars, or plane
trails.
5. Once you have imaged all your first- and second-order fields together with the
uncertainty field, shut the telescope down (if required). Ensure that you have all
the data on a removable medium, including the calibration files.
10.3.4 Photometry
1. In order to perform photometry, you will need to turn your light frames into
science frames, as outlined in Chap. 9.
2. Once you have your science frames, you will need to find the seeing of each
image. If you are using APT, this can be done by dropping an aperture of a star