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10 Photometry
tion of photons. Scintillation occurs on small scales of about 30 cm and over time
scales often less than a second. Hence, it can be reduced with larger-aperture telescopes or increased exposure time. It is interesting to note that for larger telescopes,
increased exposure time is often not an option, since it would lead to saturation.
Our last source of noise is sky noise. Random scattering, absorption, and emission
of photons cause sky noise. It should be the largest contributor to the uncertainties
and is the dominant factor limiting observable magnitudes. Fundamentally, without
moving to a very high site, preferably in space, sky noise is going to be the principal
limit on photometric precision.
It should be clear that most of these sources of error, besides interpolation errors,
form part of the background of the science frame. Hence, the best way to reduce
these errors’ impact is to improve the signal-to-noise ratio.
10.5 Practical: Constructing an HR Diagram of an Open
Cluster
The Hertzsprung–Russell (HR) diagram embodies one of the most important concepts in modern astrophysics. Its construction tells us about the evolution of stars
and can be used to determine the age of a star cluster by the identification of the main
sequence turn-off. Constructing and interpreting a Hertzsprung–Russell diagram is
a vital skill for any undergraduate astrophysicist.
10.5.1 Aims
The aim of this practical is to construct an HR diagram and identify the main sequence
turn-off. The construction of the HR diagram will require taking and reducing images
of an open cluster in two bands. In addition, you will need to undertake aperture
photometry on both images and plot the results. You should also prepare a backup
cluster.
10.5.2 Observing
Once at the observatory, you should follow the standard imaging procedures:
• Cool the camera down to temperature and open the dome.
• Once the camera is down to temperature, take flats if needed.
• Slew the telescope to a focusing and alignment star. Confirm alignment and focus
the instrument.
10 Photometry
tion of photons. Scintillation occurs on small scales of about 30 cm and over time
scales often less than a second. Hence, it can be reduced with larger-aperture telescopes or increased exposure time. It is interesting to note that for larger telescopes,
increased exposure time is often not an option, since it would lead to saturation.
Our last source of noise is sky noise. Random scattering, absorption, and emission
of photons cause sky noise. It should be the largest contributor to the uncertainties
and is the dominant factor limiting observable magnitudes. Fundamentally, without
moving to a very high site, preferably in space, sky noise is going to be the principal
limit on photometric precision.
It should be clear that most of these sources of error, besides interpolation errors,
form part of the background of the science frame. Hence, the best way to reduce
these errors’ impact is to improve the signal-to-noise ratio.
10.5 Practical: Constructing an HR Diagram of an Open
Cluster
The Hertzsprung–Russell (HR) diagram embodies one of the most important concepts in modern astrophysics. Its construction tells us about the evolution of stars
and can be used to determine the age of a star cluster by the identification of the main
sequence turn-off. Constructing and interpreting a Hertzsprung–Russell diagram is
a vital skill for any undergraduate astrophysicist.
10.5.1 Aims
The aim of this practical is to construct an HR diagram and identify the main sequence
turn-off. The construction of the HR diagram will require taking and reducing images
of an open cluster in two bands. In addition, you will need to undertake aperture
photometry on both images and plot the results. You should also prepare a backup
cluster.
10.5.2 Observing
Once at the observatory, you should follow the standard imaging procedures:
• Cool the camera down to temperature and open the dome.
• Once the camera is down to temperature, take flats if needed.
• Slew the telescope to a focusing and alignment star. Confirm alignment and focus
the instrument.