The details of acquiring objects with COS are described in Chapter 8 of the COS Instrument Handbook
. In brief, the COS flight software1
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 so that the aperture is at 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 such that centering algorithms can be used. 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
) 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
science observations, or in separate wavelength calibration exposures that are either automatic or user-specified.
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
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)”
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.