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Sensitivity - Performance Estimation Tool
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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/obs/bg.html.

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

B. IRAC Observing Parameters

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 Mapping AOT webpage: http://ssc.spitzer.caltech.edu/irac/aotintro.html

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

C. 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 AOT webpage: http://ssc.spitzer.caltech.edu/irs/aotintro.html

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

D. 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 AOT description webpage: http://ssc.spitzer.caltech.edu/mips/aotintro.html

    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 AOT description webpage: http://ssc.spitzer.caltech.edu/mips/aotintro.html

    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: 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:

  3. EXPOSURE TIME PER PIXEL IN IRAC/IRS PUI/MIPS bands:

  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 chapter of the Spitzer Observer's Manual (http://ssc.spitzer.caltech.edu/documents/som/) and the backgrounds page (http://ssc.spitzer.caltech.edu/documents/compendium/resolution/background.html) in the The Infrared Compendium (http://ssc.spitzer.caltech.edu/documents/compendium/ ) for more information.

    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 needs to be modified. This has led to the development of an "enhanced" 160 micron small field photometric mode for cycle-4. This mode will not be available in SPOT until after the proposals have been accepted in 2007, but you may propose to use it in cycle-4.

      The new 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. Preliminary tests indicate 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. If you plan to use the enhanced mode you should submit your proposal with small-field 160 micron photometry AORs and add 40% to the requested 160 micron time. If your proposal is accepted you will then modify the AORs to utilize the enhanced mode, which will be available in SPOT in time for cycle-4 (accepted) program modifications.

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