10.1 Introduction
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to the overall luminosity of the system. Even in a system comprising a pair of stars
of equal mass, the colour is not overly affected, although any distance based on the
measured brightness of such a system would be less accurate than a single system.
There are two approaches to calibrating raw instrumental magnitudes. The first
of these is differential photometry, which uses reference stars, preferably of the same
spectral class and in the same field of view and hence imaged at the same air mass.
Differential photometry is often used to determine the magnitude of variable stars
and to identify exoplanet transits. The magnitude of the reference star does not need
to have been accurately determined (as long as it isn’t variable itself).
The measurement needed in differential photometry is the difference in magnitude
between two or more sources, δMag, rather than an apparent magnitude.
Results of very high accuracy and precision can be obtained using this method.
Determining an object’s apparent magnitude differentially to anything better than
a few tenths of a magnitude is challenging, mostly due to the dearth of stars with
very well determined magnitudes. Both the Landolt (UBVRI) and Henden (BV)
catalogues provide well-determined standard stars. However, these are often not
accessible for observers at high latitudes.
The second method, all-sky photometry, uses a well-calibrated optical system to
determine magnitude directly from images. All-sky photometry is used mostly for
large-scale surveys and also for regions well away from a suitable calibration star.
Photometric accuracies of a few hundredths of a magnitude are possible for a welldetermined system, which is a factor of 10 poorer than that achieved by differential
photometry.
In the rest of this chapter, I will concentrate on differential aperture photometry, as
this is the form most commonly encountered by undergraduates. Differential aperture
photometry is versatile, and many of its aspects form the foundations of other forms
of photometry.
10.2 Measuring
In this section, I will walk through how to perform standard aperture photometry that
will yield an uncalibrated instrumental magnitude using the Aperture Photometry
Tool (APT). APT is freeware, and as it is written in Java, it will run on almost any
platform. Later on, I will be using a similar programme, AstroImageJ, to perform
differential photometry. Like APT, AstroImageJ is freeware and multi-platform. APT
is more suited to field photometry, measuring all the stars in a frame, while AstroImageJ is more suitable for time series photometry, measuring just a few objects but
over many frames.
147
to the overall luminosity of the system. Even in a system comprising a pair of stars
of equal mass, the colour is not overly affected, although any distance based on the
measured brightness of such a system would be less accurate than a single system.
There are two approaches to calibrating raw instrumental magnitudes. The first
of these is differential photometry, which uses reference stars, preferably of the same
spectral class and in the same field of view and hence imaged at the same air mass.
Differential photometry is often used to determine the magnitude of variable stars
and to identify exoplanet transits. The magnitude of the reference star does not need
to have been accurately determined (as long as it isn’t variable itself).
The measurement needed in differential photometry is the difference in magnitude
between two or more sources, δMag, rather than an apparent magnitude.
Results of very high accuracy and precision can be obtained using this method.
Determining an object’s apparent magnitude differentially to anything better than
a few tenths of a magnitude is challenging, mostly due to the dearth of stars with
very well determined magnitudes. Both the Landolt (UBVRI) and Henden (BV)
catalogues provide well-determined standard stars. However, these are often not
accessible for observers at high latitudes.
The second method, all-sky photometry, uses a well-calibrated optical system to
determine magnitude directly from images. All-sky photometry is used mostly for
large-scale surveys and also for regions well away from a suitable calibration star.
Photometric accuracies of a few hundredths of a magnitude are possible for a welldetermined system, which is a factor of 10 poorer than that achieved by differential
photometry.
In the rest of this chapter, I will concentrate on differential aperture photometry, as
this is the form most commonly encountered by undergraduates. Differential aperture
photometry is versatile, and many of its aspects form the foundations of other forms
of photometry.
10.2 Measuring
In this section, I will walk through how to perform standard aperture photometry that
will yield an uncalibrated instrumental magnitude using the Aperture Photometry
Tool (APT). APT is freeware, and as it is written in Java, it will run on almost any
platform. Later on, I will be using a similar programme, AstroImageJ, to perform
differential photometry. Like APT, AstroImageJ is freeware and multi-platform. APT
is more suited to field photometry, measuring all the stars in a frame, while AstroImageJ is more suitable for time series photometry, measuring just a few objects but
over many frames.