Chapter 12
High-Precision Differential Photometry
Abstract It is possible, but challenging, to achieve the millimagnitude uncertainties
needed for research-level time series photometry with small telescopes. In order to
do so, the telescope–instrument uncertainties need to be evaluated and addressed, the
check stars must be carefully selected, and nonstandard methods such as defocused
photometry should be considered.
12.1 Introduction
In recent years, with improving noise levels and the development of high-sensitivity
low-cost CCD, and now CMOS, astronomical cameras, the new science of exoplanet
detection has given small telescopes, such as those found in teaching observatories,
a greatly extended utility. Further, the capacity of small telescopes to perform millimagnitude photometry, in far from ideal environments, has enabled their repurposing
to a new research life.
This chapter is largely derived from excellent observational work undertaken by
the late Peter Beck at the University of Hertfordshire. If you wish to read more
details, his thesis can be found online. The chapter focuses on achieving millimagnitude differential photometry in the context of exoplanet transit observations through
the improved understanding that comes through the calibration of the telescope and
instrument package. However, there are lessons here for anyone wishing to undertake astronomical photometry. Many of these processes should be undertaken by
observatory staff if your observatory is staffed.
There are four fundamental sources of noise in photometric measurements:
• Poisson noise in the target data and the check stars.
• Calibration noise within the bias, dark, and flat-field frames.
• Scintillation noise caused by the effects of the atmosphere.
• Read noise caused by the electronics and cables, with electromagnetic interference
in the observatory.
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_12
169
High-Precision Differential Photometry
Abstract It is possible, but challenging, to achieve the millimagnitude uncertainties
needed for research-level time series photometry with small telescopes. In order to
do so, the telescope–instrument uncertainties need to be evaluated and addressed, the
check stars must be carefully selected, and nonstandard methods such as defocused
photometry should be considered.
12.1 Introduction
In recent years, with improving noise levels and the development of high-sensitivity
low-cost CCD, and now CMOS, astronomical cameras, the new science of exoplanet
detection has given small telescopes, such as those found in teaching observatories,
a greatly extended utility. Further, the capacity of small telescopes to perform millimagnitude photometry, in far from ideal environments, has enabled their repurposing
to a new research life.
This chapter is largely derived from excellent observational work undertaken by
the late Peter Beck at the University of Hertfordshire. If you wish to read more
details, his thesis can be found online. The chapter focuses on achieving millimagnitude differential photometry in the context of exoplanet transit observations through
the improved understanding that comes through the calibration of the telescope and
instrument package. However, there are lessons here for anyone wishing to undertake astronomical photometry. Many of these processes should be undertaken by
observatory staff if your observatory is staffed.
There are four fundamental sources of noise in photometric measurements:
• Poisson noise in the target data and the check stars.
• Calibration noise within the bias, dark, and flat-field frames.
• Scintillation noise caused by the effects of the atmosphere.
• Read noise caused by the electronics and cables, with electromagnetic interference
in the observatory.
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_12
169