Spitzer Documentation & Tools
IRS Instrument Handbook

Chapter 3.    Operating Modes

3.1  Staring-Mode Spectroscopy

Staring Mode was the basic "point and shoot" operating mode of the IRS. In this mode, science targets were placed on one or more of the IRS slits for a specified integration time.  The IRS Staring Mode AOT could be configured as either Standard (Section 3.1.1) or Peak-Up Only (Section 3.1.2).

The SPICE and SMART tools were designed to reduce data obtained in Staring Mode.

3.1.1             Standard Staring Mode

In Standard Staring Mode, the observer selected from one of four ramp durations, and also the number of cycles of that ramp duration to be performed. The ramp duration is the time between the first and last non-destructive reads of the array, and corresponds to the "effective exposure time." The shortest ramp duration for all of the slits was six seconds. The longest ramp duration for the slits varies from 120 seconds to 480 seconds, depending upon the module. The ramp durations were chosen to provide the highest dynamic range, while minimizing the number of routine calibrations required in orbit. The IRS SL and LL modules reach background-limited performance in about 40 seconds and about 10 seconds, respectively. Longer ramp durations have smaller overhead times but may have been affected more severely by cosmic ray hits. The "cycles" parameter is the number of times a given spectrum was repeated before moving on to the next slit position or the next target. Total "integration time" is determined by both ramp duration and number of cycles.

Standard Staring Mode successively placed the target at the two nominal slit locations for each requested slit, located 1/3 and 2/3 of the way along the length of the slit, to provide redundancy against cosmic rays and detector artifacts.  The automatic "nodding" that occurred in Standard Staring Mode means that, for example, the choice of a 480 second ramp for one cycle in SH would produce two spectra, with each having an exposure time of 480 seconds, yielding a total on-source integration time of 960 seconds. The same exposure time with two cycles would produce four 480-second exposures, for a total on-source integration time of 1920 seconds.

Since the IRS slits are relatively narrow compared to the nominal pointing accuracy of the Observatory, it was often necessary to perform a peak-up acquisition (PUA) before starting a spectroscopic integration in order to achieve a given photometric accuracy (or even ensure that the target was in the slit).  Furthermore, a peak-up allowed an observer to obtain a spectrum of a source even if its coordinates were not precisely known - as long as it is known to be bright enough for the peak-up to function properly.

There were three peak-up options available: IRS Peak-Up, PCRS Peak-Up, and No Peak-Up.       IRS Peak-Up

The IRS Peak-Up option used 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 was then used to offset the science target to one of the IRS slits. An IRS peak-up could be performed on either the spectroscopic target itself, or a nearby (< 30 arcmin) object whose offset from the science target is accurately known.

The onboard peak-up algorithm measures the centroid of the brightest source in the selected peak-up array field-of-view. It performs two measurements. First an “acquisition” centroid (ACQ) is measured at the “blind” pointing of Spitzer. After this, the brightest source is moved to the “sweet spot” (SS) of the selected peak-up array and the centroid (of the brightest source in the field) is measured again. Finally, the SS centroid position of the peak-up target is used to move the science target to the first commanded slit for spectroscopic observations. The exposure number is 0000 for ACQ and 0001 for SS. At each position, three images (DCE numbers 0, 1, 2) are taken and processed onboard to produce the frame that is used by the peak- up algorithm. This fourth, processed frame is always DCE number 3.

In the processed peak-up images, any source that is bright enough for centroiding will be clearly visible. In the ACQ frame, it should be at or near pixel (107,30) for the blue peak-up array and (105,92) for the red peak-up array; for SS, it will be at or near pixel (108,28) or (106,94) for blue or red, respectively. The onboard software considers the center of the lower left corner pixel of the array to be (0,0); that is, pixel centers are labeled with integers and pixel edges are labeled with half-integers. Beginning in IRS25 the PU window has been trimmed to 24x24 pixels.

The FITS headers of the peak-up images contain information about the operation of the telescope and the peak-up algorithm (see below). The data in each FITS file shows the combination of the three individual exposures. The onboard processing includes cosmic ray rejection, flat-fielding, and background subtraction. The data are in units of DN (Data Number).

The world coordinate system (WCS) for the peak-up images is provided in the FITS header. The red and blue peak-up arrays share a common WCS. The WCS is described in the system CTYPE1 = ‘RA---TAN-SIP’ and CTYPE2 = DEC—TAN-SIP’, not ‘RA—TAN’ and ‘DEC—TAN’. This system is the same as that used for IRAC and MIPS images, and includes non-linear distortion terms. In particular, image viewers like DS9, GAIA, and ATV used to display the full SL and peak-up images will give coordinates valid for the peak-up imaging fields of view.

If the peak-up algorithm does not find a valid centroid, then it will report a failure. In the peak-up images, particularly the processed DCE (see above), there will be no visible source. You can also identify these failures from the FITS header by looking at the value of the “PU centroid quality code,” which is 0 for a failure and 1 for a success.

The peak-up algorithm can also result in a false positive. In this case, the peak-up software centroids on something other than the intended target and reports a success. You can look at the peak-up images to see if your intended source is at the centroid position reported in the header. The value of the centroid is given in the AXCNTRD1 and AYCNTRD1 keywords; note that these values are given in centipixels, so must be divided by 100.0 in order to compare with pixel coordinates in the peak-up images. In addition, the value of the PTGDIFF keyword in the header gives the difference in arcseconds between the requested and reconstructed pointing. This value is almost always less than 1 arcsecond, which indicates that the intended coordinates were placed on either the acquisition or sweet spot.       PCRS Peak-Up

The second peak-up target acquisition option was PCRS Peak-Up. This choice selected the Pointing Calibration and Reference Sensor (PCRS) as the peak-up instrument. The PCRS operated in the visual part of the spectrum (from 5050-5950 Angstroms) and its main function was to calibrate and remove the drift between the star trackers and the telescope. The field-of-view for the PCRS is 40x40 arcsec2, comprised of a 4x4 array of 10 arcsec pixels. Consequently, the PCRS functioned more like a "quad cell" centroiding device than a standard CCD that adequately samples the PSF. The PCRS could measure the centroid of stars in the 7.0 mag < V < 10 mag range to an accuracy of 0.14 pixels (1 sigma radial).  The observer had the choice of peaking up on either the science target itself or an offset star.

The primary reference catalog for the PCRS was the PCRS Guide Star Catalog, which contains a carefully-selected subset of stars from the Tycho catalog.       No Peak-Up

The third peak-up target acquisition option is No Peak-Up. In this case, the telescope slewed to the observer-specified science target position without refining the pointing accuracy, waited for the nominal settling time, and began the spectroscopic observations. Positional uncertainty in this case is the nominal pointing accuracy of Spitzer, 1.0 arcsec.

3.1.2             Peak-Up Only Staring Mode

The Peak-Up Only observation type was intended to provide "early target acquisition" functionality to the IRS Standard Staring mode. In a Peak-Up Only observation, the peak-up algorithm was run in its normal fashion to acquire the science target but no spectroscopic data were taken.  Peak-Up Only was intended to test the peak-up algorithm, not to obtain science images, which could be accomplished with the Peak-Up Imaging AOT (Section 3.3). Testing the algorithm was useful in cases where long spectroscopic observations were needed but no obvious peak-up target presented itself from other ground-based or space-based data, so it was desirable to confirm the success of the peak-up before exposing the spectrum. The peak-up images and the position of the target selected by the software were sent to the ground in this mode. Since the peak-up software ran its normal course, the overheads (including the initial slew overhead and the settling times within the peak-up process) were significant. In addition, the peak-up itself would have had to be repeated in conjunction with the actual spectroscopic exposure. The DCS images obtained during a Peak-Up Only observation were dark-current-subtracted, cleaned of cosmic rays, and flat-fielded on-board and these images are available in the Spitzer Heritage Archive (all of which was done for all Peak-Up data).