Important Notes: Only needed if you do not have uncertainty images (these are typically provided with all Spitzer data).
PURPOSE
This module is used to estimate the uncertainty images from instrument gain and read noise when no uncertainty image is available. Warning: these are for an idealized case, and real uncertainties should be used if available. 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. In the case of Spitzer data, the BCDs are in surface brightness 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-/microJy/arcsec^(2)) 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 4.1
FLUXCONV is the flux conversion factor between DN/s and surface brightness unit of MJy/sr (or microJy/arcsec^(2)) and EXPTIME is in seconds. 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. For Spitzer data, the read noise is given by the FITS header keyword RONOISE.
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-overlap.
COMMAND LINE INPUT
&SNESTIMATORIN
Gain = 66.8,
Read_Noise = 8.8,
Confusion_Sigma = 0,
&END
In Global Parameters:
SIGMA_DIR = Sigma-overlap
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 4.2
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.
