FAQs: IRS

See also IRS web pages.

Q:   I want to create a cluster target, but I don't see this as an option in the MIPS/IRS/IRAC AOT window.

A:   Cluster targets are a TARGET definition, not an AOT definition. First, create the target in Spot, then create the AOT for that target.

Be careful when using map center offset or cluster array coordinate offsets for an asymmetric pattern about the map or cluster center, because it may yield differing results in different visibility windows. (see how this carefully-planned map changes 6 months later -- the small map on the far right has a separate target, but the others are controlled by map center offset. The AORs are exactly the same in each figure.

Q:   I want to do a spectral map - how come the slits are not exactly on the same place?

A:   The spectral map positions are in array coordinates, which are created by moving parallel and perpendicular to the slit in each case. Because the slits are not exactly parallel (or perpendicular) to each other, the map positions are not exactly in the same places either. The best way to illustrate this is to overlay SL and SH -- these modules are nearly 45 degrees with respect to each other, so the map positions are not at all the same.

Q:   What criteria should I follow to improve the probability that my peak-up will be successful?

A:   Detailed information and procedures for configuring peak-ups can be found in the Spitzer Observer's Manual and the Spitzer Observation Planning Cookbook.

For all targets:

• The peak-up target must be within 30 arcmin of the science target.
• Point sources are recommended for use as peak-up targets. If using an extended source for a peak-up target, it must have a centrally peaked flux distribution and FWHM smaller than 20 arcsec.
• Blind pointing of Spitzer is much better than anticipated before launch. We routinely achieve intrinsic pointing accuracies of 1-2 arcsec or better. Therefore, all observers should carefully assess their photometric accuracy requirements and peak-up needs. For example, if you have accurate target coordinates, and are using a wide slit and/or do not require the highest level of photometric flux calibration accuracy, then you may not need to use a peak-up at all.
• The peak-up target must be the brightest object in the field-of-view of the selected peak-up array (software-limited to 43*43 arcsec for IRS peak-up, ~40x40 arcsec for PCRS peak-up), in the wavelength band of the selected peak-up array (13.3-18.7 microns for IRS Blue, 18.5-26 microns for IRS Red, 5050-5950 angstroms for PCRS peak-up). Observers must manually inspect the fields around ALL peak-up targets (e.g., using Spot) to ensure that this condition is met. NOTE: since the optical, near-IR, and mid-IR colors of sources can be very different, ensuring that the peak-up field is "clean" at 2 microns (i.e., 2MASS K-band) is a necessary, but not sufficient, criterion for a successful peak-up. Whenever possible, inspect an image of the peak-up field obtained in a wavelength band as close as possible to that of the selected peak-up mode.
• Ensure that the best estimate of the peak-up target's IR flux density in the selected wavelength band (IRS peak-up) or V magnitude (PCRS peak-up) has been entered into the AOR. In addition, the brightness of the (point source) peak-up target should fall within the recommended brightness range for the selected peak-up mode: 5-150 mJy (IRS Blue), 15-340 mJy (IRS Red), or 7 < V < 10 (PCRS). In general, we suggest that IRS peak-up targets have a minimum flux density of 10 mJy in the selected wavelength band (Blue or Red). The minimum allowed target brightness for a successful IRS peak-up depends on the local background surface brightness - see the SOM for more information.

• Use the best known J2000 coordinates for the peak-up target (including cumulative corrections for proper motion since the epoch of measurement), or the best known offsets from the science target.
• Ensure that the best known proper motions for the peak-up target have been entered into the AOR. This is especially important for peak-ups on offset stars, but could also affect peak-ups on science targets with large proper motions.

• Stationary targets can be used for peak up of moving targets. See Spot's User Guide, section 11.2.2.
• If you are peaking up on the moving science target (or a co-moving target), the position of the target must be known to better than the dimensions of the selected peak-up array. Spitzer-centric positional error ellipses for the current ephemerides for near-Earth objects and comets are available through JPL's Horizons database -- see http://ssc.spitzer.caltech.edu/documents/horizons.pdf New in 2005, you can also peak up on a fixed target and shift to a moving target.
• Objects with large positional errors at the time of proposal submission can still be requested, but should be identified as such in the proposal. Note, however, that objects with large positional errors may not be scheduled if the ephemeris is not subsequently improved prior to the Spitzer observations. The SSC will endeavor to use the best ephemeris available from the Horizons database at the time of scheduling, typically 5 weeks before execution of the observation.
• Some targets have positional uncertainties that are initially relatively small, but are likely to increase in the 5-week interval between generation of the spacecraft schedule and execution of the observation. This type of behavior may be exhibited by objects that experience non-gravitational forces, such as comets nearing perihelion. In such cases, the observer may request a late ephemeris update.
• Typically, a moving target observer will not know the scheduled date of observation for their target at the time that the target is proposed. Consequently, he/she cannot check the corresponding peak-up array field-of-view for bright neighbors that could confuse the peak-up algorithm (see above). However, we strongly encourage moving target observers to use the Spot visualization tool to check for bright objects along the orbital track corresponding to the two Spitzer visibility windows for their target during the proposal Cycle. This can be done by choosing a date in the middle of each visibility window, and overlaying the moving target orbital track on a background image closest in wavelength to the peak-up array selected for the Spitzer observations. Timing constraint(s) will have to be applied to the AOR to avoid scheduling the observation at a time when the the peak-up could fail.

Q:   I got my peak-up data and I want to double check that it was done right. What should I look for?

A:   An onboard peak-up algorithm measures the centroid of the brightest source in the peak-up field. It performs two measurements. First, the centroid of the brightest source in the peak-up array field-of-view is measured at an "acquisition" pointing (ACQ). Next, the brightest source is moved to the array "sweet spot" (SS) and its centroid is measured again. This centroid position is then used to move the science target into the slit. At each position (ACQ and SS), three images (DCEs) are taken and processed onboard to produce the image frame that is used by the peak-up algorithm. This fourth, processed frame is always DCE number 0003, and the exposure number is 0000 for ACQ and 0001 for SS.

Hence, to examine the peak-up data yourself, the first step is to look at the files with exposure.dce numbers of 0000.0003 and 0001.0003. The FITS headers will 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 processing includes cosmic ray rejection, flat fielding, and background subtraction. The data are in units of DN. To convert to electrons per second, multiply by 4.6 to get to electrons, and then divide by the exposure time given in the RAMPTIME header keyword.

In the processed images, any source that is bright enough for centroiding will be clearly visible. Assuming that the predicted flux of the peak-up target was correct, you will see the peak-up target located near the center of the image. In the ACQ frame, it will be at or near pixel (107, 30) for Blue or (105, 92) for Red; in the SS frame, it will be at pixel (108, 28) or (106, 94) for Blue or Red, respectively. The onboard software considers the lower left corner of the array to be (0,0) with the lower left corner of that pixel (-0.5,-0.5); that is, pixel centers are labeled with integers coordinates.

It is possible that you will see other sources in the field, as well. It is the responsibility of the observer to have vetted the peak-up field to ensure that the brightest source is the intended peak-up target (see above FAQ). The brightest source in each of the ACQ and SS frames will be selected as the peak-up target by the peak-up algorithm.

Starting with software version S10, the world coordinate system (WCS) for the peak-up image will be 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', CTYPE2 = 'DEC--TAN-SIP', not 'RA--TAN' and 'DEC--TAN'.

Note that with the current onboard software processing, you may see column-by-column variation in the image. This is a known effect (informally referred to as "jailbarring"). The peak-up algorithm has been demonstrated to work as advertised in the presence of this effect. Future revisions of the onboard software will mitigate against this effect.

Q:   I think my peak-up failed. How can I tell?

A:   There are two ways for the peak-up to "fail".

1. If the 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 "peak-up centroid quality code", which is 0 for a failure and 1 for a success.

2. 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 peak-up target is at the centroid reported in the header. The value of the centroid is given in the AXCNTRD1 and AYCNTRD1 keywords; note that these values are in centipixels, so need to be divided by 100.0 to compare with pixel positions in the image. The onboard software considers the lower left corner of the array to be (0,0) with the lower left corner of that pixel (-0.5,-0.5); that is, pixel centers are labeled with integer coordinates.

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.

Q:   I didn't get a spectrum of my science target because there was a false positive. Was that exceptionally bad luck?

A:   The integrated number counts for sources with flux density of 1 mJy at 24 microns at high latitudes (see Marleau et al. 2004, ApJS, 154, 66) give 335 sources (both stars and galaxies) per square degree in a high galactic latitude field. That is one source per 10 peak-up fields. At a 24-micron flux density of 5 mJy, the number counts are only 25 per square degree or one source per ~150 peak-up fields. So if you have a peak-up source with an estimated flux of 1 mJy, it is not that unusual to have a second source in the peak-up array field-of-view that is as bright or brighter. The source density can be much higher in many regions of the sky.

The SOM recommends that your peak-up source flux density be larger than 5-10 mJy for both Blue and Red peak-up. If your source is fainter than this, you should check the peak-up data carefully to ensure your peak-up was performed as intended and, therefore, that your IRS spectrum was obtained for the correct target.

Q:   Are there IRS 'no observing zones'?

A:   We have received two queries regarding a rumor that the SSC is going to designate certain areas of the sky as off-limits to IRS. According to the rumor we are working on this now but aren't going to tell anyone which areas are banned until after the Cycle-3 proposal deadline.

This rumor is patently false.

The IRS saturation limits are outlined in the Spitzer Observer's Manual and further details about specific bright objects to avoid are provided on the web (http://ssc.spitzer.caltech.edu/documents/brightobj/). We review bright objects on a case-by-case basis and will continue to do this for the foreseeable future. We have no plan to change this.

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