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Return to the SENS-PET

Overview: An imaging "Sensitivity Performance Estimation Tool" (SENS-PET). For user configured Spitzer observing parameters, the SENS-PET returns an estimate of the point source and diffuse emission instrument sensitivities, and total integration depth per pixel.

Input: choose expected background level, and configure IRAC/IRS Peak-Up Imaging/MIPS instrument settings.

Output: sensitivity and integration depth per pixel for the selected observing mode.


Input parameters:

A. Background

In general, the IR background contribution is a combination of the zodiacal light, interstellar medium, and cosmic background radiation. At IRAC wavelengths, the background is dominated by zodiacal light, so a general rule-of-thumb is that the background is ``low'' near the ecliptic poles (absolute ecliptic latitude greater than about 60 degrees), ``high'' if it is in the ecliptic plane (absolute ecliptic latitude less than 30 degrees, say), and ``medium'' for all other ecliptic latitudes.

For all of the Spitzer instruments, the background estimates were generated using a prototype version of the Spitzer background estimator. Three lines of sight were chosen to represent low, medium, and high background observations, depending upon galactic/ecliptic latitude of the target. The lines of sight were as follows:

                  |  GALACTIC   |  ECLIPTIC
                  | glon | glat | elon | elat 
             ---- | ----- ----- | ----- -----
             LOW  |   96 | +30  |  239 | 89   
             MED  |  105 | -20  |   10 | 40   
             HIGH |  187 |  +1  |   91 |  0   
  

The background model includes the zodiacal light, interstellar medium (cirrus), and the cosmic infrared background (at wavelengths greater than 100 microns). For further details, see the SSC background webpage: http://ssc.spitzer.caltech.edu/warmmission/propkit/som/bg/.

User input: one of the three background levels, low, medium, or high, depending on the coordinates of the target.

B. Warm IRAC Observing Parameters

Use this section to estimate IRAC sensitivities in channels 1 and 2 after the depletion of cryogen.

Choice of either full array or subarray modes, allowed frame times and number of repeats.

Note: In the Warm IRAC Mapping AOT, the user can select mapping/dither strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly.

User input: full array or subarray mode, frame time and number of repeats.

C. IRAC Observing Parameters

Use this section to estimate IRAC sensitivities in channels 1-4 during the cryogenic cold mission.

Choice of either full array or subarray modes, allowed frame times and number of repeats.

Note: In the IRAC Mapping AOT, the user can select mapping/dither strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly. For more information on configuring IRAC mapping observations, see the IRAC handbook: http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook/ .

User input: full array or subarray mode, frame time and number of repeats.

D. IRS Peak-Up Imaging (PUI) Observing Parameters

Choice of either red filter or blue filter modes, allowed frame times and number of repeats.

Note: In the IRS PUI AOT, the user can select mapping/dither strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly. For more information on configuring IRS peak-up imaging observations, see the IRS handbook: http://ssc.spitzer.caltech.edu/irs/irsinstrumenthandbook/.

User input: red filter or blue filter mode, frame time and number of repeats.

E. MIPS Observing Parameters

Choose one of the two MIPS observing modes:
  1. PHOTOMETRY AND SUPER RESOLUTION MODE: For each of the three MIPS passbands, the user selects the instrument configuration, determining the exposure time, number of repeats, as well as pixel scale (70 micron array only), and field size.

    Note: In the MIPS Photometry and Super Resolution AOT, the user can select field size/sky offset strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly.

    For further information, see the SSC MIPS handbook: http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/.

    User input: pixel scale (70 micron array only), field size, exposure time, and number of repeats.

  2. SCAN MAP MODE:

    For each of the three MIPS passbands, the user selects the instrument configuration, determining the scan rate, and number of scan passes.

    Note: In the MIPS Scan Map AOT, the user can select cross scan steps and number of map cycles that may increase the depth of coverage per pixel, for a given number of scan passes. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of scan passes accordingly.

    For further information, see the SSC MIPS handbook: http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/.

    User input: scan rate (70 micron array only), and number of map passes.

Output:

  1. SENSITIVITY IN IRAC/IRS PUI/MIPS bands (1-sigma): For point sources, the instrument sensitivities have been pre-calculated as a function of exposure time, number of repeats, and background level. The output is for each Spitzer passband: Warm IRAC = 3.6, 4.5 microns; IRAC = 3.6, 4.5, 5.8, 8.0 microns; IRS PUI = 16 or 22 microns; MIPS = 24, 70, 160 microns.

    Note: For IRAC in subarray mode, 64 exposures are taken. The sensitivity estimates returned by the PET assumes that the sensitivity of the combined image has scaled as 1/sqrt(64) times the sensitivity of the individual images.

    Note: IRS PUI point source sensitivities are provided for very small apertures (13 and 29 square arcseconds in blue, red respectively. This is a much smaller aperture than used to estimate the MIPS 24 micron sensitivities (approximately 740 square arcseconds). Accounting for apertures, sensitivities for PUI and MIPS 24 micron imaging are similar. Note that the definition of Frame Time is different for IRS and MIPS (see SOM).

    Further details on sensitivities are available online, and in the SOM. Please see:

  2. Diffuse emission sensitivity: the conversion from point source sensitivities to extended source sensitivities are applied follows:

    • IRAC:
      • Extended Source Sensitivity [MJy/ster] = 0.030 * Throughput Correction for Point Sources * Point Source Sensitivity [micro-Jy] / Throughput Correction for Background / sqrt(Npix).
        See Section 6.2.1 of the Spitzer Observer's Manual (SOM v8.0), or the IRAC handbook at http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook.
      • Npix characterizes the IRAC PSF in each passband, and is (7.0, 7.2, 10.8, 13.4) for the IRAC 3.6, 4.5, 5.8, and 8.0 micron bands, respectively (see the SOM v8.0, Table 6.1). The throughput corrections are given in Table 6.6 of the SOM v8.0.
    • For IRS, extended source sensitivities are estimated from observations of resolved galaxies, in small ( <= 50 sq. arcsecond) regions.

    • MIPS: The estimate used in the PET assumes equal sensitivity to point and diffuse sources. Under this assumption, the relation between point and diffuse source sensitivities is:
      Extended Source Sensitivity [MJy/ster] = W(lambda) * Point Source Sensitivity [micro-Jy],
      where the conversion factors are given in Table 8.12 of the SOM v8.0:

      Passband: W(lambda):
      24 microns 1.04e-3
      70 microns (default scale) 8.81e-5
      70 microns (fine scale) 1.77e-4
      160 microns 2.52e-5
  3. EXPOSURE TIME PER PIXEL IN IRAC/IRS PUI/MIPS bands:

    • For IRAC, the total exposure time per pixel is as follows. In full-array mode, it is approximately the frame time multiplied by the number of repeats. In subarray mode, exposures are taken in sets of 64, hence the exposure time = 64 * frame time. These estimates returned by the PET do not subtract the time taken in the endpoint readouts; for more information, see the IRAC handbook: http://ssc.spitzer.caltech.edu/irac/iracinstrumenthandbook/.

      Note: In the IRAC Mapping AOT, the user can select mapping/dither strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly.

    • For IRS, the ramp durations are given in the SOM v8.0, Table 7.6.

      Note: In the IRS Peak Up Imaging AOT, the user can select mapping/dither strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly.

    • For MIPS, the total exposure time per pixel is somewhat more complex. For example, Photometry/Super Resolution compact source photometry mode yields 14 exposures per cycle in the 24 micron band, and hence a 3 second frame time gives a 42 second integration time per pixel per cycle.

      The MIPS Photometry and Super-Resolution approximate integration times per pixel are listed in SOM v8.0, Table 8.11. The single-pass MIPS scan map integration times per pixel are summarized the the SOM v8.0, Table 8.6. See also the MIPS handbook: http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/.

      Note: In the MIPS Scan Map AOT, the user can select cross scan steps and number of map cycles that may increase the depth of coverage per pixel, for a given number of scan passes. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of scan passes accordingly.

      Similarly, in the MIPS Photometry and Super Resolution AOT, the user can select field size/sky offset strategies that may increase the depth of coverage per pixel, for a given number of repeats. In the Performance Estimation Tool, the user mimics this by adjusting the "effective" number of repeats accordingly.

  4. Notes/Warnings:

    1. Confusion limits: warnings are given if the predicted sensitivity falls below the prediction for the 1-sigma confusion limit. Details on the confusion limit predictions are given online at:

      Also note that the confusion limits are lower limits to the actual position dependent on-sky confusion. The lower limits shown on the low background sensitivity charts are for regions of lowest expected confusion at high Galactic latitudes and on clean sky. The observer should consider the local confusion caused by background sources when planning observations. Confusion will likely be more important in higher background regions, and can limit the sensitivity that can be achieved. Local sources of confusion, such as cirrus and the stellar background, are highly variable and can be very localized.

      Also note that the accuracy of photometry at 70 and 160 microns will often be confusion-limited. Because MIPS provides much smaller effective beams and higher sensitivity than any previous mission, determining the confusion limit set by such sources is difficult. Current estimates of the 1-sigma confusion limits range from about 0.5 to 1.3 mJy at 70 microns, and from about 7 to 19 mJy at 160 microns (Xu et al, 2001, ApJ, 562, 179; Franceschini et al., 2002, astro-ph/0202463; Dole et al., 2002, ApJ, 585, 617). The above values should serve as a guide for determining if a particular observing program is feasible. Other factors may influence the effective confusion limit for a particular observation. In some instances, it may be reasonable to integrate somewhat below the level of the confusion, for example when the observer has a priori knowledge of a source position. On the other hand, the presence of a nearby bright source with its diffraction artifacts will increase the effective confusion limit. Moving targets offer the possibility of taking a second "shadow" observation, allowing the suppression of confusing source by subtracting them away. Observers are warned that they need to specify AORs with enough cycles to provide adequate rejection of cosmic rays and other artifacts, even if a very short integration would nominally be adequate to reach the confusion limit. See the MIPS handbook for more information (http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/).

    2. IRS PUI point source sensitivities are provided for very small apertures which contain half the light. This is a much smaller aperture than used to estimate the MIPS 24 micron sensitivities. Note that the definition of Frame Time is different for IRS and MIPS (see SOM).

    3. For MIPS 24 micron Photo/Super Resolution mode: 30s frame time not recommended for high background (saturation warning).
    4. 160 micron enhanced mode: One of the conclusions from the analysis of nearly three years of 160 micron calibration data, plus analyis of some technically challenging science programs (e.g., new planets in the Solar System or Kuiper Belt objects) is that to improve both the photometric accuracy and repeatability of the 160 micron small field observations, the AOT needed to be modified. This led to the development of an "enhanced" 160 micron small field photometric mode for cycle-4.

      The 160 micron enhanced photometric mode relies on the same principles of small field photometry, but provides a larger field of view and a more uniform coverage at a given single nodding position. This is accomplished by increasing the number of DCEs, modifying the stim-cycle and optimizing scan mirror dither pattern. Tests indicated that both the repeatability and the photometric accuracy are improved by 15-20%, with a decrease in sensitivity of approximately 22%.

      The 160-micron enhanced photometric mode will take 30 DCEs per cycle, in comparison with the 20 DCEs obtained by small field photometry. We estimate a 40% time increase per cycle to operate in this mode.

      Note: Sensitivity for enhanced mode has only been estimated in the case of low background.