14.4 Practical 10: Methane in the Atmosphere of Neptune
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Save all your files to a transportable medium and check that they are in an acceptable condition, for example, that they have no plane trails, and once you are happy
with your work, shut down the observatory.
14.4.4 Reduction
Once you have the spectra, you have to reduce them in order to get science line
spectra:
• Using your favourite reduction package, remove the bias and dark frames from
the spectra and divide by the flat.
• Rotate the frames so that the spectra are flat and not tilted. If the spectra are curved,
remove this.
• Crop the spectrum and save to file. Perform the same action for the sky area,
ensuring that the cropped size is the same as the spectra and that it starts at the
same pixel column.
• Subtract the sky from the source to produce a sky removed spectrum.
• Load the reference spectra into a spectral reduction package such as IRIS, Vspec,
or Rspec and bin them, resulting in a line plot of intensity versus pixel number.
• Locate the emission lines in your reference spectra, noting the pixel value for each
line and its corresponding wavelength. Use this to find the linear dispersal of the
spectrograph.
• Load the calibration spectrum and calibrate it using the pixel location of the wavelengths found in the reference spectra and the calculated linear dispersal. Applying
this should create a line plot of intensity versus wavelength.
• Load the standard spectrum and bin it down to a line spectrum if it is not already,
and divide the calibration spectra by the standard spectra.
• Fit a polynomial to the resulting spectra. It may be necessary to remove some
spectral lines to get a smooth fit. Save this plot.
• Load the source spectrum and bin it. Calibrate this spectrum using the data from
the reference spectra, as previously.
• Take the calibrated source spectrum and divide it by the polynomial you fitted to
the calibration spectrum.
• Normalise your spectrum by finding the value of the highest peak and dividing
through by that value.
14.4.5 Analysis
Using a spectral line catalogue, identify and label the spectral line within your spectrum. Pay particular attention to lines redder than 600 nm, as these may be molecular
lines. Additionally, compare the solar spectrum to your spectrum.
203
Save all your files to a transportable medium and check that they are in an acceptable condition, for example, that they have no plane trails, and once you are happy
with your work, shut down the observatory.
14.4.4 Reduction
Once you have the spectra, you have to reduce them in order to get science line
spectra:
• Using your favourite reduction package, remove the bias and dark frames from
the spectra and divide by the flat.
• Rotate the frames so that the spectra are flat and not tilted. If the spectra are curved,
remove this.
• Crop the spectrum and save to file. Perform the same action for the sky area,
ensuring that the cropped size is the same as the spectra and that it starts at the
same pixel column.
• Subtract the sky from the source to produce a sky removed spectrum.
• Load the reference spectra into a spectral reduction package such as IRIS, Vspec,
or Rspec and bin them, resulting in a line plot of intensity versus pixel number.
• Locate the emission lines in your reference spectra, noting the pixel value for each
line and its corresponding wavelength. Use this to find the linear dispersal of the
spectrograph.
• Load the calibration spectrum and calibrate it using the pixel location of the wavelengths found in the reference spectra and the calculated linear dispersal. Applying
this should create a line plot of intensity versus wavelength.
• Load the standard spectrum and bin it down to a line spectrum if it is not already,
and divide the calibration spectra by the standard spectra.
• Fit a polynomial to the resulting spectra. It may be necessary to remove some
spectral lines to get a smooth fit. Save this plot.
• Load the source spectrum and bin it. Calibrate this spectrum using the data from
the reference spectra, as previously.
• Take the calibrated source spectrum and divide it by the polynomial you fitted to
the calibration spectrum.
• Normalise your spectrum by finding the value of the highest peak and dividing
through by that value.
14.4.5 Analysis
Using a spectral line catalogue, identify and label the spectral line within your spectrum. Pay particular attention to lines redder than 600 nm, as these may be molecular
lines. Additionally, compare the solar spectrum to your spectrum.