Where to find manuals

The best place to look for manuals is the IRAF documentation site. The
manuals on this site are in zipped (.Z) postscript format (after you
download them, unpack them with the unix "uncompress" command). More
specifically, go to the spectroscopy manual page and get the first two
manuals describing general CCD reductions and reductions of
(long-)slit spectra.

Get the data

See specific instructions on this...

Types of Frames

Object frames
Bias frames
Flat-field frames
Arc lamp frames

Getting Organized

check out the data files and catalog them

IMHEAD <filename> to see the object name and the data type

check out the processing status

CCDLIS <filename> to see processing steps already carried out

2-D Processing Flow Chart

process bias frames

CCDPROC subtract overscan, trim
ZEROCOMBINE average => bias.imh

process flat-field frames

CCDPROC subtract overscan, trim, subtract bias
FLATCOMBINE average => flat.imh
RESPONSE normalize to unit mean => bf.imh (balance factor)

process object and Arc lamp frames

CCDPROC subtract overscan, trip, subtract bias, divide by b.f.

extract spectra

APALL extract 1-D spectra of objects (subtract sky on the
fly, if not already done)

APALL extract spectra of arc lamps from same region
as coresponding object; use the command below

apall <input> out=<output> ref=<reference> trace- recen- back- intera-

check spectra for cosmic rays, clean them mannually, and re-extract

IMEDIT edit 2-D spectra to clean cosmic rays that were
not rejected by the extraction or sky subtraction

IMCOMBINE Combine object spectra to reject cosmic rays and improve signal to noise.

1-D Processing Flow Chart

derive and apply wavelength calibration

Before even starting on the wavelength calibraiton, one should identify
the lines in the spectrum of the arc lamp. This is not always an easy task.
Observatories have line identification charts and line lists that are
needed fot this step. For the specific grating setting and arc lamp used for
your data, here are line identification charts for the red lines and blue lines.

IDENTIFY identify the emission lines in the first arc spectrum
and fit a polynomical to the wavelength-pixel relation
to get a "wavelength solution"

REIDENTIFY once you have made the line identifications in the
first arc spectrum, you can repeat the procedure
automatically for other arc spectra, using the first
one as a reference

HEDIT edit the headers of the data files and insert reference
information about the wavelength solution (although
the IRAF manual suggests a number of ways of doing this
the one described here is the most straightforward)

DISPCOR aply the wavelength solution to the spectra
see cautionary note in detailed instructions

derive and apply flux calibration

For all of the task described below, IRAF uses its extensive database of
calibration data. The database resides in the directory onedstds$. At the
top level of this directory one can find tables giving the atmospheric
extinction as a function of wavelength at various observatories. Examples
are ctioextinct.dat and kpnoextinct.dat, which are plotted here. Under
onedstds$ there are many subdirectories with tables of standard star fluxes
vs wavelength (sampled coarsely, at intervals of 40-80 A). Examples of
two such tabulated spectra are plotted here. The task standard will need
to make use of these tables.

SETAIRMASS find the airmass in the middle of the exposure and
write it into the header

STANDARD break up the observed spectrum of the standard star
into small windows (40-80 A wide) and tabulate the
observed counts and nominal flux (or AB magnitude);
extinction in the Earth's atmosphere is taken into

SENSFUNC use the table derived by standard, above, to construct
the coarsely-sampled sensitivity function (for each
of the windows) and fit it; an example of the resulting
fit to the sensitivity function (OB-AB) is shown here.

CALIBRATE use the fit to the sensitivity function to derive a value
for each of the pixels in an object spectrum and apply
it to calibrate the flux scale; in the process the
atmospeheric extinction is also corrected.

IMCOMBINE combine multiple spectra of the same object into one
total spectrum.