Command Line Equivalent: compute_uncertainties_internally
Default Output Directory: <output_dir>/Sigma-apex
Depends On: Initial Settings; APEX Settings
Relevant Pipelines: All
Important Notes: Only required if you do not have the uncertainty images that come with all Spitzer data.
PURPOSE
This module is used to estimate the uncertainty images when no independent uncertainty measurement is available. This module is usually not turned on since almost all Spitzer data have independent uncertainty images (*unc*.fits) that come downloaded with Spitzer data from the archive. If you decide not to use the provided uncertainty images, or if you are not using Spitzer data, this module needs to be switched on. Warning: if the wrong Gain or Read Noise parameters are specified, this module may produce uncertainties that are too large, and this will result in no outlier detection.
INPUT
Gain in e-/image unit: (float) This is the parameter used to translate the measured data
counts to physical units of MJy/sr (or mJy/sq.arcsec). All Spitzer BCDs are in units of MJy/sr. This parameter has the same name as stored in the Spitzer BCD FITS header, but they have different units. MOPEX Gain has the unit of e-/MJy/sr (or e-/mJy/sq.arcsec) and the FITS header GAIN has the unit of e-/DN. Gain used by MOPEX can be computed from the GAIN parameter in the FITS header by using formula:
Equation 6.1
Here FLUXCONV is the flux conversion factor between e-/DN and surface brightness unit of MJy/sr (or mJy/sq.arcsec). FLUXCONV can be found in the Spitzer BCD FITS image header.
Read Noise in e-: (float) This parameter characterizes the noise in e- when the data are being read out from detector. It is also stored in the BCD FITS image header.
Confusion Sigma in e-: (float) The 1-sigma confusion noise limit. The source of this noise is from the spatially unresolved background galaxies. This information can be found in the IRAC and MIPS Instrument Handbooks.
S/N estimator output subdirectory: The subdirectory of <output_dir> that you wish to use for the output files. Default is Sigma-apex.
COMMAND LINE INPUT
&SNESTIMATORIN
Gain = 66.8,
Read_Noise = 8.8,
Confusion_Sigma = 0,
&END
In Global Parameters:
SIGMA_DIR = Sigma-apex
OUTPUT
S/N FITS Files (*_sigma.fits): The estimated uncertainty image corresponding to each input BCD image.
DISCUSSION
This module allows users to estimate the noise from the BCD data, if no uncertainty images are provided. Here you need to input gain, readout noise and confusion limit appropriate for the dataset. We suggest that users look at the BCD image header to find the gain and read out noise. This module should not be used unless you cannot use the Spitzer archive-provided uncertainty images.
The module estimates the pixel uncertainty for each pixel in the image using the following model:
Equation 6.2
where the parameters Read Noiseσreadnoise, Gaing, and Confusion Sigmaσconfusion are specified in the module settings. The last term is the Poisson noise determined by the pixel value, I.
The units of Read Noise and Confusion Sigma here are electrons. The module is designed to work with the images in DN units. If you are working with the images in units of surface brightness (MJy/sr; i.e. all Spitzer BCDs) then the Gain should include the conversion factor from surface brightness units to electrons (see the INPUT for this module). The product of this step is the uncertainty images.
