This page makes a plot of either the atmospheric transmission around a specified wavelength or an approximate plot of normalized atmospheric emission around the specified wavelength (assuming a constant temperature atmosphere), or produces a list of the strongest atmospheric line transitions near that wavelength.
Given a central wavelength in microns this page can produce a plot of the theoretical atmospheric transmission over a range of +/-0.025 microns from the specified wavelength. The values are calculated for Mauna Kea and for two water columns: 1.0 PMM of water (black curve) and 4.0 PMM of water (red curve). If the option for marking lines is selected above then the stronger lines are marked with a dash above the plot and the molecule name is given. In such cases the [vacuum] wavelength in microns is listed at the right edge of the plot. Internal to the page the line positions are given in wavenumbers, taken from the HITRAN04 compilation. Up to 50 lines are marked. If one happens to select a region with more than 50 strong lines in the 0.05 micron wavelength range then one only gets the first 50 lines marked on the plot.
The plots are intended as an aid to identifying atmospheric lines when observing with the echelle mode of Michelle. On the plot the wavelengths of the lines are rounded off somewhat. The line wavenumbers (and associated vacumm wavelengths) can be obtained by choosing the "line list" option. These values are given at the origional HITRAN04 accuracy.
As what is observed with Michelle are the atmospheric lines in emission there is an option not to plot the transmission values [which is what are originally calculated by the ATRAN program] but rather to convert from transmission to emission assuming that the atmosphere has a constant temperature of 270 K. The transmission is assumed to reflect the atmospheric optical depth and wavelength*wavelength flux density values are calculated on this basis. This produces something similar to what is observed at the telescope. One cannot depend on the detailed line strengths since different lines form at different temperatures, but it does produce something that can be compared with an observed atmospheric spectrum. The values shown are normalized to peak in this option, since there is no ready way to simulate the real atmospheric conditions of any particular observation.
The plot output options are to get the plot in postscript form, in gif form, in jpeg form, or as an xmgr 4.1.2 save file. The latter is possible because xmgr is what is used to plot the graph internally. Note that if you want to use xmgr to display the plot you may need to use xmgr 4.1.2 rather than its current descendent xmgrace. Sometimes the files work with xmgrace, but at times it fails and it is not clear why. The plots may come up in landscape orientation in the browser window.
If you want only the list of transitions choose the "line list" option. In that case the page returns the line list used to mark the plots. The lines are selected from the HITRAN04 database on the basis of the Einstein A value:
water lines with Aij > 1.e-27
CO2 lines with Aij > 1.e-24
O3 lines wityh Aij > 1.e-22
other lines with Aij > 4.e-20
The line list is not limited to 50 lines as is the case when making a plot, so depending on the wavelength chosen a large number of line transitions may be listed.
This page was last modified 20 February 2008.Kevin Volk (email@example.com)