Ratio-ing by a Standard Star

Any spectra observed at the Earth's surface will differ from true due to atmospheric effects, particularly wavelength dependent transmission, noticeable in the infra-red due to water vapour and carbon dioxide. The correction of these effects is to compare the source with another having a well-behaved and understood spectral shape i.e. viz. a standard source.

In general, the flux density from an astronomical source, , and the flux density of a standard, , will both suffer attenuation and diminution by atmospheric transmission, T(), and the instrumental response, I(). The measured signals for each type of source will be proportional to the products of the three terms (loosely, STI). Early type (A) stars are known to approximate very well to black body sources[3]Actually, infra-red astronomers consider all standard stars as black bodies irrespective of stellar type! and so, for a star of known spectral type and luminosity class, the spectrum can be re-created using:

where is the black body density flux of the standard at some effective temperature, . This term must be proportional to the product of and some term which is dependent only upon the arbitrary brightness, , of the `ratio-ing' source i.e. the spectral standard. Thus:

It can be seen that dividing by a standard star in this way removes atmospheric absorption features from the spectrum. This depends upon the path length through which both source and standard are measured. To remain true both must be observed at, or close to, the same airmass.

Equation contains the key. Standard stars are observed in exactly the same way as other astronomical objects. They are sky subtracted into reduced group files, although a reduced observation can be filed as a standard if desired. Converting an observation to a standard involves Portable--CGS4DR extracting a 1-D spectrum, generating a normalised (model) black body, dividing the spectrum by this model black body and growing the result along the slit. It is filed in the index file as a frame beginning with `st' and stored in $rgdir/.

When the time comes to ratio a source, Portable--CGS4DR will select a standard based upon pre-determined criteria and divide the observation to be calibrated by it and, hence, re-create the spectrum. The output file has the label _dbs appended to it and is simply the ratio of the DN/exp in the source divided by the DN/exp in the standard i.e. no account is taken of exposure times.