6.1 Introduction to COS Imaging

The COS NUV channel has a plate scale of 23.5 mas per pixel, which provides the highest spatial sampling of any instrument aboard HST. Figure 6.1 shows an image of Pluto and its moon Charon obtained with COS. COS images are fully corrected for the telescope's spherical aberration, though not for the zonal (polishing) errors on its primary and secondary mirrors (Section 3.3). Because the optics image the sky onto the detector, rather than the aperture, COS images extend to a radius of 2 arcsec, but suffer considerable vignetting at radii greater than 0.4 arcsec, as shown in Figure 6.2.

To request an imaging observation specify CONFIG=COS/NUV and MODE=TIME-TAG or ACCUM. In TIME-TAG mode the minimum BUFFER-TIME is 80 seconds, which may be longer than the expected exposure time. ACCUM mode is recommended for such short exposures. The minimum COS exposure duration is 0.1 s, as discussed in Section 5.3. MIRRORB and/or the BOA can be used to obtain images of bright objects, but at some cost in spatial resolution; see Section 8.4 for details.

For long exposures, drifting of the Optics Select Mechanisms (OSMs) can be significant, ~3.5 pixels in the x dimension with an e-folding time of ~50 minutes (COS ISR 2010-10). Observers taking images with exposure times longer than ~200 s are urged to use MODE=TIME-TAG and FLASH=YES. The resulting lamp flash will illuminate the WCA, allowing one to track the drift accurately. By default, FLASH=NO for all imaging modes.

COS imaging in TIME-TAG mode allows for high-speed NUV photometry with a temporal resolution of 32 ms. STIS is capable of much finer time resolution (125 μs), but at lower sensitivity.