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14 Spectrography
14.4 Practical 10: Methane in the Atmosphere of Neptune
The solar system bodies Uranus and Neptune contain a significant amount of methane
in their atmosphere. This methane is sufficient to be detected by a small instrument
with a spectrograph.
14.4.1 Aims
You will be taking spectra of either Uranus or Neptune to identify key components in
their atmospheres. You will learn how to operate a telescope fitted with a spectrograph
and how to take, reduce, and interpret spectra.
14.4.2 Preparation
Before arriving at the site, you should have selected a suitable target and calibration
star. You should have produced finder charts for the night and have sourced a standard
spectrum. Uranus and Neptune are large enough to have a visible disk that should
make identification straightforward.
14.4.3 Imaging
On arrival, open the dome and cool the camera(s). If needed, take flats or obtain
suitable library flats.
Once the camera has been cooled to operating temperature, slew the telescope to
a bright star and do a focusing check. If you have a flip mirror or off-axis instrument,
use this to point the telescope and confirm that it is aligned and pointed at the correct
piece of sky. When you focus the spectrograph, you may have to open the slit wide.
Once you are aligned and focused, move the telescope to the source. Align the target
in the slit, which should now be closed to a suitable width. Image the target, with
binning if needed, making sure that every pixel stays within the linear range of the
CCD.
Once you have suitable spectra, take dark and bias frames and then turn on the
reference source and take another spectrum. Ensure that you can see the lamp lines
in the image.
Once you have your reference, slew to your calibration source and align in the
slit. Take the spectra, again while remaining in the linear range, and follow this with
dark and bias frames. Then take spectra with the reference lamp on.
14 Spectrography
14.4 Practical 10: Methane in the Atmosphere of Neptune
The solar system bodies Uranus and Neptune contain a significant amount of methane
in their atmosphere. This methane is sufficient to be detected by a small instrument
with a spectrograph.
14.4.1 Aims
You will be taking spectra of either Uranus or Neptune to identify key components in
their atmospheres. You will learn how to operate a telescope fitted with a spectrograph
and how to take, reduce, and interpret spectra.
14.4.2 Preparation
Before arriving at the site, you should have selected a suitable target and calibration
star. You should have produced finder charts for the night and have sourced a standard
spectrum. Uranus and Neptune are large enough to have a visible disk that should
make identification straightforward.
14.4.3 Imaging
On arrival, open the dome and cool the camera(s). If needed, take flats or obtain
suitable library flats.
Once the camera has been cooled to operating temperature, slew the telescope to
a bright star and do a focusing check. If you have a flip mirror or off-axis instrument,
use this to point the telescope and confirm that it is aligned and pointed at the correct
piece of sky. When you focus the spectrograph, you may have to open the slit wide.
Once you are aligned and focused, move the telescope to the source. Align the target
in the slit, which should now be closed to a suitable width. Image the target, with
binning if needed, making sure that every pixel stays within the linear range of the
CCD.
Once you have suitable spectra, take dark and bias frames and then turn on the
reference source and take another spectrum. Ensure that you can see the lamp lines
in the image.
Once you have your reference, slew to your calibration source and align in the
slit. Take the spectra, again while remaining in the linear range, and follow this with
dark and bias frames. Then take spectra with the reference lamp on.