MOPEX Online Manual

Uncertainty Estimation

MOPEX has a number of ways of estimating the uncertainty in photometry output by APEX. The extract table output by MOPEX contains the following columns: SNR (signal-to-noise ratio), Delta_Flux, and ap_unc (aperture uncertainties), which are calculated as described below.

SNR Computation

There is currently a rough estimate of the point-source flux signal-to-noise ratio in the column SNR. The signal is the flux. The noise is estimated by scaling the background noise in the pixel nearest to the source by the pixel's contribution to the total PRF. In order to compute the SNR of the extraced point sources, an image of the background fluctuations in the mosaic image is produced by the module Gaussnoise. This produces the images coadd_Tile_*_Image_noise.fits. We define the factor, NP, as follows:

Then the extracted flux F_ps for a point source is divided by the scaled noise σ(x_ps,y_ps), at the location (x_ps,y_ps) of the point source, to yield the SNR_ps for the point source:

The product is written to the extract table in the column SNR.

Note: This calculation assumes only one PRF, the central one. It becomes very poor when the pixels are large relative to the PRF width (e.g. with IRAC channel 1 data), and generally gives an uncertainty estimate that is too low. It does not take into account the positional uncertainty.

Delta_Flux

Delta_Flux is the uncertainty in the PRF_fitting parameters, determined by the Hessian matrix:

Provided the fit is successful, the covariance matrix C_ab of the fitting parameters is defined by the inverse Hessian matrix:

Often, Delta_Flux will underestimate the true flux uncertainty because of correlated errors. The flux uncertainty is correlated with the positional uncertainty, and the formal uncertainties don't reflect this.

Aperture Uncertainties

σ² = Sum (σ(i)²).

The sum takes into account fractional pixels. If any have bad uncertainties, ap_unc is set to -9.99. Uncertainty in the background subtraction is not included.

The results are written out in the aperture.tbl and extract_raw.tbl as "ap_unc1", "ap_unc2", etc. The Apex module Select can select on them when creating the final extract.tbl.

Uncertainties will only be calculated if the uncertainty file is found, and is the right size. If not found, or not the right size, Aperture will proceed but set the aperture uncertainties to -9.99.

It is possible to run Aperture by itself on an extract list, but if the extract list already contains aperture columns, then Aperture will append new columns.