is the wavelength-dependent integrated system throughput divided by the physical area of the HST mirror, which is given in the PHOTTAB reference file table.
, where to keep from degrading the spectral purity Npix * m must be less than W) is given by:
where:
) is the average value of the surface brightness, I, over the Npixs * Npix pixels in the rectangle being integrated.
where:
is as given above (i.e., the values in the _x2d.fits or _sx2.fits image).
is the plate scale in the dispersion direction as given in the CD1_1 header keyword.
* m * Npixs * ms over a Npix * m by Npixs * ms region on the sky, where Npix * m equals the width of the emission feature in the spectral direction.The factor W*d/m
which converts between diffuse continuum source surface brightness and diffuse emission line surface brightness is given in the CONT2EML header keyword and is simply the slit width expressed in Angstroms. The surface area of a pixel in arcseconds (ms * m) is given in the header keyword OMEGAPIX.Finally, the DIFF2PT keyword in the data header gives the conversion factor to flux units in erg cm-2 sec-1 Å-1 for a point source. The DIFF2PT keyword is calculated as:
DIFF2PT = W * ms / A
,where W and ms are as above and A
is the wavelength averaged point source aperture throughput for the science aperture, determined from the APTHRUTAB reference table. That is if you integrate the _x2d or _sx2 file over the full wings for a point source (which can be quite extensive) and multiply by the DIFF2PT parameter you will recover the flux of the point source. Of course, point source observers are better advised to use x1d to extract a one-dimensional aperture extracted spectrum from their long-slit first-order data, which will then use the wavelength dependent aperture throughput and the defined point source extraction aperture and calibration, as well as perform background subtraction.In general we note that the cross dispersion profiles can be quite extended (particularly in the far-UV and in the near-infrared); fluxes derived for extended sources from the _x2d.fits files as above assume that the sources are extended on scales which contain the bulk of the cross dispersion flux from a point source. As we make further progress analyzing the cross dispersion profiles and their effect on the accuracies of point source and diffuse source fluxes we will provide updates on the WWW and through the STANs.
See also Chapter 6 of the STIS Instrument Handbook for a more detailed discussion of units and conversions for different source types.