|Space Telescope Science Institute|
|COS Instrument Handbook Cycle 23|
COS spectra of extended objects could have significantly lower resolution than those of point sources, depending on the spatial distribution of the source. For example, measurements of Lyman-α airglow lines, which uniformly fill the COS aperture, show R = 1450 for G130M and R = 165 for G140L. Filled-aperture observations of SNR N132D confirm R ~ 1500 for both FUV M gratings (France et al. 2009, ApJL, 707, L27). In the NUV the situation is much worse, because a source that fills the aperture will lead to cross-contamination among the three spectral stripes on the MAMA detector.A similar situation arises when multiple point sources fall within the aperture. COS was designed to resolve two point sources separated by 1 arcsec in the cross-dispersion direction, but on-orbit measurements of the XD profile reveal that such observations are possible only for selected grating and central-wavelength settings (Section 5.1.9). Note that light from a point source falling more than 0.4 arcsec from the center of the PSA will be attenuated (Section 8.8.2).The optional parameter EXTENDED (default = NO) sets a science header keyword to inform the calcos pipeline that the target is an extended source. The keyword may be used in the future to activate special data-reduction procedures, although none are currently in the pipeline. At present no aspect of on-board data-taking is affected by this parameter.A new spectral extraction algorithm will be used by the COS pipeline for calibrating COS/FUV data once operations move to LP3. This new extraction method is optimized for observations of point sources using the PSA aperture only. FUV spectra of extended sources will be poorly calibrated. As a result, COS/FUV science observations of extended targets will be offered as available-but-unsupported starting in Cycle 23 (See Section 2.4).