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IRS: Peak-Up Algorithm |
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An important step in the preparation of Staring mode and Spectral Mapping mode AORs is consideration of the peak-up target acquisition settings, which are found in the middle of the AOT entry forms. Peak-Up Imaging mode does not use a peak-up target acquisition. The motivation for having an on-board peak-up capability is essentially two-fold. First, since the IRS slits are relatively narrow compared to the nominal pointing accuracy of the Observatory, it will often be necessary to perform a peak-up before starting a spectroscopic integration in order to achieve a given photometric accuracy (or even ensure that the target is in the slit). Second, a peak-up allows an observer to obtain a spectrum of a source even if its coordinates are not precisely known -- as long as it is known to be bright enough for the peak-up to function properly. There are three peak-up options available: IRS Peak-Up, PCRS Peak-Up, and No Peak-Up.
IRS Peak-UpThe IRS Peak-Up option uses software resident in the Combined Electronics to accurately find a target on one of the two IRS peak-up fields-of-view (blue or red) that share the SL detector array. The position determined for the peak-up target is then used to offset the science target to one of the IRS slits. An IRS peak-up can be performed on either the spectroscopic target itself, or a nearby (delta R < 30 arcmin) object whose offset from the science target is accurately known. If an offset peak-up target is desired, the observer can enter this directly into the AOT.
PCRS Peak-UpThe second peak-up target acquisition option is PCRS Peak-Up. This choice selects the Pointing Calibration and Reference Sensor (PCRS) as the peak-up instrument. The PCRS operates in the visual part of the spectrum (from 5050- 5950 Angstroms) and its main function is to calibrate and remove the drift between the star trackers and the telescope. The field-of-view for the PCRS is 40x40 arcsec, comprised of a 4x4 array of 10 arcsec pixels. Consequently, the PCRS functions more like a "quad cell" centroiding device than a standard CCD that adequately samples the PSF. The PCRS can measure the centroid of stars in the 7.0 mag < V < 10 mag range to an accuracy of 0.14 (1 sigma radial). As in the case of the IRS Peak-Up for a stationary science target, the observer has the choice of peaking up on either the science target itself or an offset star.The primary reference catalog for the PCRS is the PCRS Guide Star Catalog, which contains a carefully selected subset of stars from the Tycho catalog. In the future, it will be possible to select PCRS Guide Star Catalog stars from within Spot; meanwhile, the PCRS Guide Star Catalog and an IDL-based search routine are available on the SSC website.
No Peak-UpThe third peak-up target acquisition option is No Peak-Up. In this case, the telescope will slew to the observer-specified science target position without refining the pointing accuracy, wait for the nominal settling time, and begin the spectroscopic observations. Positional uncertainty in this case is the nominal pointing accuracy of Spitzer, 1.0 arcsec.Really, check the IRS chapter in the Spitzer Observer's Manual. We mean it. There are many useful pages there on peak-up.
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/irs/peakup.html
