EX-PET Help

Return to the EX-PET

Overview: An "Extragalactic Performance Estimation Tool" (EX-PET) to estimate the flux density in the Spitzer IRAC + MIPS imaging passbands for various point source SEDs. For user configured Spitzer observing parameters, the PET also returns an estimate of the instrument sensitivity, total integration depth per pixel, and expected S/N.

Input: choose an SED model, set normalization parameters, expected background level, and configure IRAC/MIPS instrument settings.

Output: summary of SED properties, intrinsic flux , color corrections and observed flux in the Spitzer passbands and a signal-to-noise estimate for the observing parameters chosen.

Input parameters:

A. SED model

Note: these are provided as a guideline only. Observers are responsible for verifying the expected source flux densities for their observations.

Choose a SED model, and set normalization parameters. For the normalization, specify the flux density (in mJy) at a specified wavelength. If the flux density is unknown, but a magnitude is, the online "Magnitude-to-Flux-Density Converter" can do the conversion.

Choices for the SED model are:

  1. COMPOSITE SED: The composite SEDs were obtained by averaging multiwavelength observations of 59 galaxies, classified into the following categories:

    See: Schmitt, Kinney, Calzetti and Bergmann, 1997, AJ, 114, 592 for further details.

    User input: object class from the list above, redshift, normalization, and wavelength of the normalization, (both in the observer's frame), to specify [default is ].

  2. NAMED OBJECT SED: Similar to the composite SEDs in the class above, these are a compilation of multiwavelength observations of a single selected, representative galaxy in the classes listed under composite SEDs above (e.g., LINER, Seyfert 2, etc.)

    User input: object SED, redshift, normalization, and wavelength of the normalization, (both in the observer's frame), to specify [default is ].

    SED choices:

    • Arp 220: LINER. SED plot. References:
      • Carico et al. 1988, AJ, 95, 356
      • Duc et al. 1997, A&AS, 124, 533
      • Klaas et al. 2001, A&A, 379, 823
      • Lisenfeld, Isaak, & Hills 2000, MNRAS, 312, 433
      • Rigopoulou, Lawrence, & Rowan-Robinson 1996, MNRAS, 278, 1049
      • Rigopoulou et al. 1999, ApJ, 118, 2625
      • Sanders et al. 1988, ApJ, 325, 74
      • Silva & Danese 1998, ApJ, 509, 103
    • M82: Starburst. SED plot. References:
      • Code & Welch 1982, ApJ, 256, 1
      • Genzel et al., 1976, AJ, 81, 1084
      • Jaffe et al., 1984
      • Klein, Wielebinski, & Morsi 1988, A&A, 190, 41
      • Rieke & Low 1972, ApJ, 176, L95
      • Rigopoulou et al., 1999, AJ, 118, 2625
      • Schmitt et al., AJ, 1997, 114, 592
      • Soifer et al 1987, ApJ, 320, 238
      • Telesco & Harper 1980, ApJ, 235, 392

  3. MODIFIED BLACKBODY: For low opacity, the modified blackbody is approximated as

    User input: temperature, T, emissivity index, , normalization, and wavelength of the normalization, to specify [default is ].

  4. BLACKBODY: Standard blackbody SED, given by

    User input: temperature, T, normalization, and wavelength of the normalization, to specify [default is ].

  5. POWER LAW:

    User input: power law index, , normalization, and wavelength of the normalization, to specify [default is ].

B. 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). See the SSC background webpage for further details:

https://irsa.ipac.caltech.edu/data/SPITZER/docs/files/spitzer/BackgroundEstimates.pdf

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

C. 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.

User input: full array or subarray 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.

    See the MIPS handbook for further information:

    https://irsa.ipac.caltech.edu/data/SPITZER/docs/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.

    See the MIPS handbook for further information:

    https://irsa.ipac.caltech.edu/data/SPITZER/docs/mips/mipsinstrumenthandbook/

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

Output:

  1. WAVEBANDS: The Spitzer imaging passbands are normally quoted in the Spitzer documentation as 3.6, 4.5, 5.8, 8.0 for IRAC, and 24, 70, 160 microns for MIPS. The calculations in the PET are referenced to more precise determinations of the "average wavelength" in each passband, calculated as follows:

  2. SOURCE INTRINSIC FLUX DENSITY at Spitzer wavelengths: Based on the SED model chosen (and the normalization value/wavelength), the source intrinsic flux density in each of the Spitzer passbands.

    For the analytic SED models (power law, blackbody, modified blackbody) these are calculated analytically. For the named objects and composite SEDs, a simple log-linear interpolation is performed between the measured/pre-determined flux densities. Assumes point sources.

  3. COLOR CORRECTION: For named object and composite SEDs, the color corrections are small, and are assumed to be unity by the EX-PET. However, the EX-PET calculates the color corrections explicitly for blackbody, modified-blackbody and power law SEDs as follows.

    Both IRAC and MIPS are broadband photometers. The color correction provides a prescription for interpreting the data for sources with spectral shapes other than the nominal one assumed in the calibration process. With Spitzer, source flux densities, , are determined at the nominal instrument passband wavelengths. For IRAC and MIPS, the reference calibrator varies, and we consider each case in turn below.

    • IRAC: calibration is set that, accounting for the instrument response, for sources with a flat spectrum:

      For other SEDs, we need to calculate the color correction as follows. Define the color correction, K, as , where is the frequency corresponding to the ``average'' wavelength in each passband, defined as

      and is the system response function, available online at:
      https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/calibrationfiles/spectralresponse/.

      The quoted flux density in the Spitzer bands, , for a source with intrinsic flux density, , will be:

    • MIPS: the calibration is referenced to a 10000 K blackbody. The color correction is defined as:

      Where , and , where R is the system response function given online at:
      https://irsa.ipac.caltech.edu/data/SPITZER/docs/mips/calibrationfiles/spectralresponse/.

      The quoted flux density in the Spitzer bands, , for a source with intrinsic flux density, , will be:

  4. QUOTED SOURCE FLUX DENSITY at Spitzer wavelengths: The flux density corrected for the Spitzer passbands. For a source with intrinsic flux density, , the flux density in the Spitzer passbands will be , where K is the color correction.

    The output is the passband-corrected flux densities in each of the Spitzer passbands.

  5. SENSITIVITY IN IRAC/MIPS bands:

    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 (3.6, 4.5, 5.8, 8.0, 24, 70, 160 microns for IRAC+MIPS).

    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.

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

  6. EXPOSURE TIME PER PIXEL IN IRAC/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; see the IRAC handbook for more information:

      https://irsa.ipac.caltech.edu/data/SPITZER/docs/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 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.

      For all of the MIPS bands, and the Scan Map and Photometry and Super Resolution AOTs, the conversion from frame time to integration time per pixel per cycle is summarized in the MIPS handbook:

      https://irsa.ipac.caltech.edu/data/SPITZER/docs/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.

  7. S/N in IRAC/MIPS bands: a very simple estimate, dividing the color-corrected source flux density by the (1-sigma) sensitivity in each of the Spitzer passbands. Assumes point sources.

  8. Notes/Warnings:

    1. For MIPS 24 micron Photo/Super Resolution mode: 30s frame time not recommended for high background (saturation warning).

    2. 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 in the IRAC and MIPS handbooks:

      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.

    3. Saturation limits: warnings are given if the predicted source flux density is greater than the prediction for saturation.

      The saturation limits used in the PET do not include any diffuse emission components. See the instrument handbooks for more details:

    4. 160 micron enhanced mode: One of the conclusions from the analysis of many 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. This mode was not available in SPOT until after the proposals had been accepted in 2007, but you could propose to use it in 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 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 takes 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.

    5. See snirac for Warm IRAC S/N when the observations are not background-noise-limited.