While COS was designed as a spectrograph, the NUV channel can be used for imaging observations. The COS/NUV plate scale of 23.5 mas per pixel provides the highest spatial sampling of any instrument aboard HST
. The image is corrected for the telescope's spherical aberration, but is degraded by zonal (polishing) errors on its primary and secondary mirrors (see Chapter 3
). The NUV imaging count-rate limit of 50 counts per second per pixel (Table 10.1
) corresponds to a GALEX
NUV magnitude of 17.6.
COS provides two observing modes, TIME-TAG
. In TIME-TAG
mode the position, arrival time, and (for the FUV channel) pulse height of each detected photon are recorded in the memory buffer. In ACCUM
mode only the locations of arriving photons are recorded.
mode is preferred because it allows for more sophisticated data reduction. For example, an observer may compare data from the night and day sides of the orbit or compute the count rate of an object whose intensity varies on short time scales. TIME-TAG
observations through the primary science aperture (PSA) allow the taking of occasional wavelength-calibration spectra during an exposure. These spectra are used by the COS data-reduction pipeline, calcos,
to correct drifts in the spectrum due to small motions of the Optics Select Mechanism (OSM). ACCUM
mode is designed for observations of targets that are too bright for TIME-TAG
mode. Because the lower information content of ACCUM
data reduces their utility for archival researchers, its use must be justified for each target.
modes may be used with either the FUV or NUV channel. For more information comparing TIME-TAG
see Section 5.2