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.
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:
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.
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:
https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/iracinstrumenthandbook/
.
User input: full array or subarray mode, frame time and number of repeats.
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:
https://irsa.ipac.caltech.edu/data/SPITZER/docs/irs/irsinstrumenthandbook/.
User input: red filter or blue filter mode, frame time and number of repeats.
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
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.
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:
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 https://irsa.ipac.caltech.edu/data/SPITZER/docs/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:
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:
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 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: 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.
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:
IRS Peak-Up imaging: the confusion limit has not been
estimated on-sky. The SENS-PET reports a confusion warning when the PUI
sensitivity is less that the 24 micron confusion limit
for MIPS, as an estimate of when confusion may need to be
considered for PUI.
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 (https://irsa.ipac.caltech.edu/data/SPITZER/docs/mips/mipsinstrumenthandbook/).
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).
For MIPS 24 micron Photo/Super Resolution mode: 30s frame
time not recommended for high background (saturation
warning).
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.