Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

3.3 Basic Instrument Operations

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3.3.1 Target Acquisitions

The entrance apertures to COS are 2.5 arcsec in diameter. In order to ensure that the target is present and centered, a target acquisition procedure must be carried out.

The details of acquiring objects with COS are described in Chapter 7, but, in brief, the COS flight software provides two very different methods for acquiring and centering a target in the aperture. The simplest and fastest method uses the ACQ/IMAGE or ACQ/SEARCH commands to obtain a direct image of the aperture in the NUV and to then move the telescope to the centroid of the measured light. ACQ/IMAGE is the preferred method in most cases, but the object's coordinates have to be accurate enough to ensure that it falls within the aperture after the initial pointing of the telescope (this should be the case for accurate and precise coordinates provided in the GSC2 system). With less accurate coordinates one can still use ACQ/IMAGE if a spiral search is first done with ACQ/SEARCH. The other COS acquisition method uses dispersed light from the object to be observed, and can be performed with either the NUV or FUV detector. Acquisitions are described in Chapter 7.

3.3.2 Data Taking: TIME-TAG and ACCUM

Two modes are available to acquire spectra with COS.

In TIME-TAG mode, both the location on the detector and the time of arrival of individual photon events are recorded in the memory buffer. The location is recorded in pixel units, and the time to within 32 msec intervals. Having TIME-TAG data allows for more sophisticated data reduction if there is evidence after the fact for spectrum drift, say, or noise events. An observer can choose after the fact to compare, for instance, data from the night- and day sides of the orbit, or to obtain a continuous stream of data on an object with short-time-scale variability. Also, data from a TIME-TAG observation has individual events corrected for doppler displacement after the fact in calcos.

On the other hand, in TIME-TAG mode the maximum permissible count rate is more restrictive than for ACCUM mode, and this can prevent the observation of some bright objects (see Section 11.5.2 for information on rate limits). Also, for TIME-TAG mode the observer must provide a fairly accurate estimate of the BUFFER-TIME so that the memory buffer both does not overflow with too many events, and does not need to be read out too often either.

TIME-TAG mode includes an option (FLASH=YES) in which brief wavelength calibration spectra are obtained several times during the course of a long exposure. Doing this allows any drifts in the spectrum to be corrected; small motions of the optics selection mechanism have been seen during ground tests of COS. TIME-TAG is the preferred mode of use of COS in almost all cases.

The other mode option is ACCUM, which simply places photon events in their proper pixel location and integrates for a specified period of time.

Both TIME-TAG and ACCUM modes may be used with either the FUV or NUV channel. For more information comparing TIME-TAG to ACCUM, see Section 5.5.

3.3.3 Wavelength Calibration

The recommended mode of use of COS is TIME-TAG with FLASH=YES ("TAGFLASH" mode) in which case spectra are obtained concurrently with wavelength calibration information. As noted, Pt-Ne lamps provide the wavelength calibration spectra, and the reduction to wavelength is done automatically in calcos. ACCUM mode is sometimes to be preferred for brighter objects, and when used it too automatically causes wavecals to be taken, but as separate images. It is possible to completely suppress the wavelength calibration spectra taken by COS but doing so significantly lessens the archival quality of data and must be justified on a case-by-case basis.

3.3.4 Typical Observing Sequences

For most observers in the majority of cases the following sequence of events will produce data of the desired high quality:

• Acquisition of the object using ACQ/IMAGE. This should take at most about ten minutes (see the examples in Chapter 9). This can be preceded by an ACQ/SEARCH if needed to scan a larger area of sky, but that should not ordinarily be necessary.
• Obtaining spectra in TIME-TAG mode with FLASH=YES so that the spectra can be corrected for any drifts. The COS Exposure Time Calculator (ETC) provides a means of calculating essential parameters such as BUFFER-TIME.
• Obtaining more spectra during additional orbits as needed for fainter targets to achieve a desired signal-to-noise.

3.3.5 Data Storage and Transfer

Effective use of COS requires awareness of the rate at which a given observation is acquiring data and the capacity of the data buffer and the manner in which those data are transferred within HST for downlink. This topic is covered in Section 5.5.1.