Observatory & Science Instruments: More on the PCS
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Pointing Accuracy and Stability
The blind pointing accuracy is the same as the on-board attitude knowledge, <0.5 arcsec (1 sigma radial rms) with a stability of <0.1 arcsec (1 sigma radial rms over 200 sec). In the incremental pointing mode, the PCS performs controlled repositioning of the boresight with an offset accuracy usually no worse than 0.55 arcsec (1 sigma radial rms) across angular distances of up to 30 arcsec and usually better than 0.4 arcsec. This accuracy is sufficient to move a source from the IRS Peak-Up array to one of the spectrometer slits.IRS observers can use the PCRS for acquisition of their science target; see the IRS chapter (chapter 7) of the SOM for more information.
Scanning Stability and Performance
Spitzer can execute linear scans at selected rates from 0.01 arcsec/s to 20 arcsec/s. The MIPS Scan Map mode uses rates from 2 to 20 arcsec/s.
Tracking Capabilities
Spitzer does not have a true tracking capability for Solar System Objects (SSOs). However, Spitzer simulates tracking by scanning in linear track segments at rates up to 1 arcsec/s. The linear track segments are linear in equatorial coordinate space; they are commanded as a vector rate in J2000 coordinates, passing through a specified RA and Dec at a specified time.The SSO ephemerides are maintained on the ground rather than on-board Spitzer, but an observer may specify flexible scheduling constraints and the linear pseudo-track specification (start point, rate and direction, in equatorial J2000 coordinates) is calculated at the time of scheduling. The observation is executed at the scheduled time (within a window of +3, -0 s) and the PCS follows the track as specified, assuming the given start point corresponds to the time given in the track command. All other PCS movements can be superposed on a specified track, including dithers and scans.
PCS measurements indicate that the blind track acquisition accuracy is ~< 0.5 arcsec independent of the track rate, which is consistent with expected performance on fixed targets. The track stability is better than scan mode requirements: ~0.5 arcsec in 1000s. The offset accuracy during tracking is better than ~0.55 arcseconds, also consistent with the required performance on fixed targets.
Pointing Reconstruction
Pointing reconstruction refers to the post-facto determination of where the telescope was pointed with a greater accuracy than was known by the flight system. Spitzer has a requirement to perform pointing reconstruction to 1.4 arcsec. Based on in-orbit results, Spitzer meets this requirement, except for observations scheduled immediately after an instrument changeover (in which case the inaccuracy may be as much as ~2 arcsec). The nominal pointing is reported in the data products. Based on post-BCD processing and comparison of extracted sources with 2MASS sources, users will receive data with IRAC data pointing reconstruction generally <1 arcsec, and MIPS scan pointing reconstruction generally <1.5 arcsec, with respect to the 2MASS coordinate system. Relative pointing (relative separation of two objects within an AOR) is good to <0.5 arcsec.
