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, ApJ, 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. No aspect of on-board data-taking is affected by this parameter.