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10 Photometry
The following practical will guide you through the process needed to identify
the individual components of (10.4). Once this is completed, you will only need to
identify the zero point during subsequent observations while the optical system is
unchanged.
10.3 Practical: Find the Zero Point and Transformations
Although raw, instrumental magnitudes are useful if you wish to undertake observations that may span more than one night or that require colour information, you
will need to reduce your photometric data so that observations between nights or
objects of different colours or observations at differing air masses can be compared.
In order to transform instrumental magnitudes into standard magnitudes, you need
to find your system’s extinction coefficient, colour transformation, and zero point.
This practical draws on many of the skills you will have obtained in previous
practicals, especially the Messier marathon practical, the bias and dark frame practical, and the flat frame practical. If you have not done these, it is recommended that
you read them before attempting this practical.
10.3.1 Aims
In this practical you will find the nightly zero point of an image as well as the colour
transformation and extinction coefficient for your optical system. You will also need
to determine levels of uncertainty in standard magnitude when these parameters are
used to convert instrumental magnitudes.
10.3.2 Preparation
In order to achieve the aims of the practical you will need to image three fields in
at least two photometric bands, preferentially B, V, and R at a single binning value.
These fields are the first-order field, the second-order field, and the uncertainty field.
The fields you will need to observe should contain a number of photometric standard
stars. Ideally, these will be Henden, Hipparcos red–blue pairs, or SDSS blue–red pairs
(lists of these are carried in Light Curve Photometry and Analysis, Warner 2007).
Your second-order field will need to be imaged as it crosses the local meridian at least
three times, without overexposing the photometric standards. The first-order fields
will need to be imaged a number of times at differing air masses. Again I suggest a
minimum of three times per air mass and a minimum of three air masses. Again you
should ensure that you do not overexpose the photometric standards.
10 Photometry
The following practical will guide you through the process needed to identify
the individual components of (10.4). Once this is completed, you will only need to
identify the zero point during subsequent observations while the optical system is
unchanged.
10.3 Practical: Find the Zero Point and Transformations
Although raw, instrumental magnitudes are useful if you wish to undertake observations that may span more than one night or that require colour information, you
will need to reduce your photometric data so that observations between nights or
objects of different colours or observations at differing air masses can be compared.
In order to transform instrumental magnitudes into standard magnitudes, you need
to find your system’s extinction coefficient, colour transformation, and zero point.
This practical draws on many of the skills you will have obtained in previous
practicals, especially the Messier marathon practical, the bias and dark frame practical, and the flat frame practical. If you have not done these, it is recommended that
you read them before attempting this practical.
10.3.1 Aims
In this practical you will find the nightly zero point of an image as well as the colour
transformation and extinction coefficient for your optical system. You will also need
to determine levels of uncertainty in standard magnitude when these parameters are
used to convert instrumental magnitudes.
10.3.2 Preparation
In order to achieve the aims of the practical you will need to image three fields in
at least two photometric bands, preferentially B, V, and R at a single binning value.
These fields are the first-order field, the second-order field, and the uncertainty field.
The fields you will need to observe should contain a number of photometric standard
stars. Ideally, these will be Henden, Hipparcos red–blue pairs, or SDSS blue–red pairs
(lists of these are carried in Light Curve Photometry and Analysis, Warner 2007).
Your second-order field will need to be imaged as it crosses the local meridian at least
three times, without overexposing the photometric standards. The first-order fields
will need to be imaged a number of times at differing air masses. Again I suggest a
minimum of three times per air mass and a minimum of three air masses. Again you
should ensure that you do not overexpose the photometric standards.