1.2 Observing with COS

1.2.1  Target Acquisitions

The two 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 (but not at the beginning of subsequent orbits within the visit).

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, the 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 also 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 HST's primary and secondary mirrors (see Chapter 3). Because COS' detectors are photon-counting, there are limits to the brightness of sources for a given configuration.  For the PSA, the NUV imaging count-rate screening 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 the FUV channel) pulse height of each detected photon are recorded in the memory buffer. With regard to the accuracy of the arrival time, the HST spacecraft computer (HST486) clock has a precision of 125 milliseconds (1/8th second per tick). Our goal is to maintain the clock's accuracy compared to UTC to within 10 milliseconds. In ACCUM mode, only the locations of arriving photons are 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 COS' 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 COS/NUV ACQ/IMAGE. This typically takes about three minutes total time. See examples in Chapter 9.

• Obtain spectra in TIME-TAG mode using the FP-POS=ALL setting and FLASH=YES so the spectra can be corrected for flat-field anomalies and OSM drifts. (For dual-segment spectroscopy at LP5 with cenwave 1291, FP-POS=ALL is not available) The COS Exposure Time Calculator webpage (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.

1.2.5  Observing Bright Objects with COS

Observing bright objects with exposure times longer than necessary for a maximum achievable signal-to-noise ratio (SNR) have recently been found to significantly impact the lifetime of COS. This SNR value for an observation is limited by the fixed pattern noise on the detector, which is a function of grating and the number of FP-POS obtained. ETC SNR values above this limit derived from longer exposure times do not generally yield improvements in usable signal. This extra use of the detector also unnecessarily reduces its lifetime, which should be avoided.

Exposure times of bright objects should therefore be limited within an orbit to not exceed the maximum achievable SNR for a given combination of grating and number of FP-POS used.  Users should consult the 50th percentile measurements listed in Table 1 of COS ISR 2023-11. Exceptions to this guideline should be justified explicitly in the Phase I proposal.  See also Section 5.8 for more details.