Spitzer Documentation & Tools
High Precision IRAC Photometry Using PCRS Peak-up

Updated 10 April 2013

PCRS peak-up has been added to the Spot observation planning tool for post-cryogenic IRAC observations. This option, used frequently in the cryogenic mission with IRS, provides enhanced accuracy in positioning a target on a science instrument FOV, and can improve greatly the photometric precision in staring mode observations.

Because of intrapixel sensitivity variations (check the link for how to obtain the best possible correction using a pixel phase map constructed at the subarray sweet spot) the measured flux of a point source varies by about 8% in the 3.6 μm array (Channel 1) and 4% in the 4.5 μm array (Channel 2), depending on where the target centroid falls on a pixel. This "pixel phase effect" is the most significant source of correlated noise in staring mode observations. PCRS peak-up allows the positioning (and repositioning for multiple observations) of a target to an accuracy of about 0.1 pixel, reducing the range of the pixel phase effect. In parallel, we are building high precision maps of the intrapixel photometric variation in a region approximately 1/4 pixels on a side, centered on the intrapixel gain "sweet spots" (positions of maximum sensitivity) for the Channel 1 and Channel 2 subarray FOVs. These maps will enable the intra-pixel gain correction of staring mode observations of point sources without using the science observations themselves. The latest versions of the "sweet spot" maps are now available on the Spitzer website, along with an IDL program to use the maps to correct aperture photometry.

The PCRS peak-up mode is designed to facilitate high precision (<1000 parts per million) relative photometry and will not generally benefit other observations. The peak-up takes 2-5 minutes of observing overhead per AOR for the acquistion on the PCRS, offset to IRAC, and associated telescope motion settling. For existing programs the additional time required to obtain the PCRS peak-up will not be charged to original program time allocation.

Using PCRS Peak-up

The Pointing Calibration and Reference Sensor (PCRS) operates in the visual part of the spectrum (505-595 nanometers), and its main function is to calibrate and remove the optical offset between the star trackers and the telescope. The PCRS can measure the centroids of stars in the 7.0 mag < V < 12.5 mag range to an accuracy of better than 0.14" (1 sigma radial). For PCRS peak-up to succeed, the following must be true:

  1. The peak-up target must be in the magnitude range 7.0 mag < V < 12.5 mag.
  2. There should be no stars brighter than about 13.0 V mag within 40 arcseconds of the peak-up target.

When adding PCRS peak-up to your observation, you have the choice of peaking up on either the science target itself or an offset star. If the visual magnitude of your target is between 7 and 12.5, then we recommend you use the target itself. Otherwise you can either use the "PCRS selection" button in Spot to choose a nearby star from the PCRS Guide Star Catalog, which contains a carefully selected subset of stars from the Tycho catalog, or specify your own offset star. Keep in mind the requirements above in choosing your peak-up star (both of which are automatically met by the PCRS Guide Star Catalog).

If you are using an offset target, we recommend that you choose the closest possible star to your science target, to optimize the slew from the offset star on PCRS to the science target on IRAC. Please make sure that the relative astrometric positions of your peak-up star and your science target are as precisely known as possible, as the efficacy of the peak-up using the guide star method depends directly on the knowledge of the angular offset between the guide star and the target.

See the Spot Version 19 Release Notes for the mechanics on how to use Spot to add PCRS peak-up to an AOR.

Putting Your Science Target on the "Sweet Spot"

After peak-up, you can ensure that your target is placed in the "sweet spot" region by specifying your target as a Fixed Cluster target in Spot, with Array (Row,Col) offsets given in the table below. (MODE = readout mode)

Ch Mode FOV Array Row (") Array Col (")
1 Full Full -0.344 0.171
1 Full Sub 130.932 127.429
1 Sub Sub -0.352 0.064
2 Full Full -0.113 0.398
2 Full Sub 126.649 124.529
2 Sub Sub -0.511 0.039
Last updated January 23, 2012.

Note the following:

  • You should select Offset Coordinates "Array" (Row/Perp, Col/Para) when specifying the cluster target.
  • Make sure to check "Observe the offsets only" when specifying the cluster target.
  • The IRAC Instrument Support team is only characterizing the Channel 1 and Channel 2 Subarray sweet spots, so to use the available gain maps to correct your photometry, you should use the offsets given for FOV=Sub for your given readout mode (Full or Sub). If you choose to place your target on the Full Array sweet spot (FOV=full), you will have to obtain your own intra-pixel sensitivity maps.
  • If you choose Mode=Full and FOV=Sub, you will receive one full array image per exposure ("Number of Frames"), with the target placed on the Subarray sweet spot (located on pixel [24,232] on the full array). (Note that we use the convention that the center of the bottom left BCD pixel is labeled [1.0,1.0]).
  • To get the best results, you should use staring mode ("Mapping Mode" = No and "Dither Pattern" = No) and use only one FOV per AOR. Offsetting to another FOV or to multiple positions within a map will be less accurate than the initial PCRS to Science Instrument offset and will eliminate the benefit of using PCRS peak-up.

Breaking Up Long Stares Into Multiple AORs

One important scientific benefit of the PCRS peak-up is the ability to reposition a target onto the same spot on a pixel during a long stare, thus enhancing the precision of time-domain measurements. Typically, the Spitzer pointing system drifts systematically by approximately 0.35" per day. This means that in about 12 hours a target placed halfway between the center and the edge of the well-characterized 0.5-pixel "sweet spot" of the Channel 1 or Channel 2 subarray has a good chance of drifting out of the calibrated region. To eliminate this problem, we recommend breaking up all staring observations that are longer than 12 hrs into separate AORs lasting 12 hrs or less, all of which should have PCRS peak-up. The AORs should be identical copies of each other, with the exception being that the Number of Frames should be changed to make each AOR shorter than 12 hrs and obtain the total duration as desired. They should also be linked together using Chain constraints, to ensure that they are scheduled as a single unit. We do not recommend peak-ups more often than about 10 hours, however, as the reacquisition error using peak-up is of order 0.1 arcsecond, which is comparable to or in excess of the expected pointing drift for observations shorter than about 10 hours. These minimum and maximum duration recommendations were changed to be more conservative regarding the error in re-acquiring the source.

Other Considerations for Long Staring Mode Observations

In addition to using the PCRS peak-up in long staring observations, we recommend adding a 30 minute long PCRS peak-up AOR chained in front of the long scientific staring mode AOR with PCRS peak-up to eliminate an initial drift. Please read the Memo on high precision relative photometry observations for more information.

An Example 40-hour Observation with PCRS Peak-up

For a 40-hour staring mode observation you need to specify in Spot:

  • One 30 min PCRS peak-up AOR with target on sweet spot
  • a 12-hour PCRS peak-up AOR with target on sweet spot
  • a second 12-hour PCRS peak-up AOR with target on sweet spot
  • a third 12-hour PCRS peak-up AOR with target on sweet spot
  • a 4-hour PCRS peak-up AOR with target on sweet spot
  • Chain all the AORs together so that they will be observed back to back (you can safely ignore any warnings that Spot may give about a chain being longer than 24 hours)

Checklist

In summary, you should make sure your AORs fulfill the following conditions:

  • Long stares are broken up into chained 10-12 hr AORs, each containing a PCRS Peak-Up
  • A 1/2 hour long AOR with PCRS Peak-Up should be added to the beginning of each chain, at the same target and channel as the subsequent AOR in the chain.
  • All AORs in a chain have the same target position and proper motion, and their Peak-Up target coordinates are the same.
  • The target should be specified as a "Fixed Cluster", and the offsets should be specified as given in the table above for the current channel, readout mode, and FOV. (The SSC is only building an intrapixel gain map for the subarray sweet spot, so we strongly recommend using the offsets corresponding to the Subarray FOV.)
  • "Array Row/Perp, Col/Para" should be selected for the "Offset Coordinates", and "Observe the offsets only" should be selected.
  • The Peak-Up V magnitude is within the acceptable range (7 to 12.5).
  • All AORs with the same Peak-Up target have the same Peak-Up V Magnitude.
  • All AORs in a chain have the same readout modes and frametimes, per channel.
  • Only one channel is selected under "Field of View" and "Data Collection".
  • If you have specified a proper motion for your main or Peak-Up target, the given position should be consistent with the given Epoch (i.e., the current equinox J2000 position of the target may be derived from the given position by multiplying the proper motion by the time between the given Epoch and the current epoch).
  • If your target is within 20 pc of the Sun, please contact the SSC at help@spitzer.caltech.edu so that we can adjust the coordinates to account for annual parallax.

Perl Script to Check AORs

We have made available a perl script, check_peakup_aors.pl, that may help you review your AORs before submitting to the SSC.

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