The details of acquiring objects with COS are described in Chapter 7 of the COS Instrument Handbook. In brief, the COS flight software provides several methods for acquiring and centering a target in the aperture in both imaging and dispersed light modes. The simplest and fastest method uses the
ACQ/IMAGE command to obtain a direct NUV image of the target field and then moves the telescope to the centroid of the measured light. This is the preferred method, but the target coordinates must be accurate enough to ensure that it falls within the aperture after the initial pointing of the telescope. With less accurate coordinates, a spiral search (
ACQ/SEARCH) should be performed with either detector prior to other acquisition methods to ensure the target will fall within the aperture. The other COS acquisition methods (
ACQ/PEAKXD and
ACQ/PEAKD) use dispersed light from the target, and can also be performed with either detector.
Routine wavelength calibration exposures, or wavecals, are needed by the COS calibration pipeline, calcos, to compensate for the effects of OSM drifts. All wavelength calibration exposures are taken in
TIME-TAG mode. They may be obtained in either the
TAGFLASH mode, where
FLASH=YES for
TIME-TAG science observations, or in separate wavelength calibration exposures that are either automatic or user-specified.
For TAGFLASH exposures, the wavecal lamp is turned on briefly at the start of an externally targeted exposure, and again at predefined intervals throughout the exposure. In this mode, photons from the external science target and the internal wavelength calibration source are recorded simultaneously on different portions of the detector; see
Figure 1.8 and
Figure 1.10.
For TIME-TAG exposures not done in
TAGFLASh mode, a separate wavecal exposure will be automatically performed (
AUTO wavecal) for each set of external spectrographic science exposures using the same spectral element, central wavelength, and
FP-POS value. These automatic wavecals are performed after the first such science exposure and after each subsequent science exposure if more than 40 minutes of visibility time has elapsed since the previous wavecal and the same spectrograph set-up has been in use over that time.
Observers also have the ability to insert additional wavecals by specifying TARGET=WAVE (GO wavecal). These exposures will use the same calibration lamp configurations and exposure times as the automatic wavecals. The only way to tell the difference between GO and automatic wavecal data is to look at the
memtype header keyword, which will be discussed later in
Table 2.6 of the
“Association Tables (ASN)” Section.
The typical COS observing sequence depends greatly on the type of observation specified. Typical COS observations use TIME-TAG mode and the PSA, with simultaneous wavelength calibrations taken via
TAGFLASH. Multiple exposures are often used to cover the FUV detector gap, or to produce full wavelength coverage from the NUV wavelength stripes.
