Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

6.6 Extended Objects

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As noted in Chapter 7 on page 83, for a centered point source approximately 95% of the light passes through the aperture. An extended source will have a changing level of transmission as a function of radius from the center of the aperture, out to a radius of about 2 arcsec. Standard pipeline processing will not be able to reconstruct the image with high accuracy for the case of spatially variable extended objects, or for multiple point sources within a radius of 2 arcsec.

The use of MIRRORB presents another complication with respect to imaging multiple sources. MIRRORB actually produces two images: a primary image from the first surface of the order-sorter filter, and a secondary image from the second surface of the filter. Thus, the image will contain "double sources." Ground testing shows that the secondary image will contain ~1/2 the flux of the primary image. Due to the slight wedge-shape of the order-sorter filter, the secondary image projects ~20 pixels in the -y (dispersion) direction on the MAMA detector from the primary image. This is easily separable for an isolated point source but may present difficulties for extended sources or crowded fields.