1.3 Basic Instrument Operations

1.3.1 Target Acquisitions

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. If high-accuracy coordinates were not available, a spiral search (ACQ/SEARCH) would 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 could also be performed with either detector.

1.3.2 Routine Wavecals

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.

For TIME-TAG exposures taken at LP6, a non-concurrent wavelength calibration exposure will be automatically performed for each set of external spectrographic science exposures using the same spectral element, central wavelength, and FP-POS. Unlike AUTO or GO wavecals, these SPLIT wavecals are taken after moving the aperture to a different location on the detector. As described more in Section 3.4.12, LP6 exposures of length > 960s receive an additional wavecal shift to compensate for the lack of concurrent wavecal flashes that occur for more typical TIME-TAG exposures at other LPs.

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.

1.3.3 Typical COS Observing Sequence

For most observations, the following sequence of events occurs:

  • Acquire the object using ACQ/IMAGE with the NUV detector. This may be preceded by an ACQ/SEARCH if needed to scan a larger area of sky. If the target is bright enough, the ACQ/PEAKXD, ACQ/PEAKD sequence can be used.
  • Obtain a spectrum in TIME-TAG mode using TAGFLASH mode so that the data can be corrected for any OSM drifts, and with different FP-POS positions to enhance the signal-to-noise.
  • Obtain more spectra during additional orbits as needed to achieve the desired signal-to-noise.  Alternatively, one could specify additional cenwaves that include the features of interest.

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.

1 The flight software minor revision number in the SPT file header is a hexadecimal number identified as a decimal. Therefore, to obtain the correct flight software number used for a given observation, use the 'FLTSWVER' header keyword instead.