1.2	Observing with COS

1.2.1 Target Acquisitions

1.2.2 Observing Modes: Spectroscopic and Imaging

1.2.3 Observing Modes: TIME-TAG and ACCUM

1.2.4 Typical Observing Sequences

1.2.1 Target Acquisitions

The COS entrance apertures are 2.5 arcsec in diameter. To ensure that the target is centered in the aperture a target-acquisition procedure must be performed at the beginning of each visit. For moving targets acquisition should be repeated at the beginning of each orbit.

The COS flight software provides two methods for acquiring and centering a target in the aperture. The first method obtains a direct image of the aperture with the NUV channel and moves the telescope to the center of light. The second method centers the target using its dispersed spectrum and can be performed with either the NUV or FUV channel. For both methods a target’s center of light can be computed from either a single exposure or from a series of exposures that map out a grid on the sky. Target acquisitions are described in Chapter 8.

1.2.2 Observing Modes: Spectroscopic and Imaging

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.

1.2.3 Observing Modes: TIME-TAG and ACCUM

COS provides two observing modes, TIME-TAG and ACCUM. In TIME-TAG mode the position, arrival time, and (for FUV) pulse height of each detected photon are recorded in the memory buffer. In ACCUM mode only the locations of arriving photons is recorded.

TIME-TAG 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.

Both TIME-TAG and ACCUM modes may be used with either the FUV or NUV channel. For more information comparing TIME-TAG and ACCUM see Section 5.2.

1.2.4 Typical Observing Sequences

In the majority of cases the following sequence of events will produce high-quality data.

•	Acquire the object using ACQ/IMAGE. This should take about three minutes. See the examples in Chapter 9.

•	Obtain spectra in TIME-TAG mode using multiple FP-POS settings and FLASH=YES so the spectra can be corrected for flat-field anomalies and OSM drifts. The COS Exposure Time Calculator Web page (ETC) provides a means of calculating essential parameters, such as the BUFFER-TIME.

•	Obtain additional spectra during subsequent orbits to achieve the desired signal-to-noise ratio or wavelength coverage.