Description

Steps in the various pipeline modules calculate variances due to different sources of noise or modify variances that were computed by previous steps. For some cases these arrays are being propagated to subsequent steps in the pipeline. Anytime a step creates or updates variances, the total error (ERR) array values are recomputed as the square root of the quadratic sum of all variances available to the step. Note that the ERR array values are always expressed as a standard deviation (the square root of the variance).

The table below is a summary of which steps create or update variance and error arrays, as well as which steps make use of these data. Details of how each step computes or uses these data are given in the subsequent sections below.

Step

Stage

Creates arrays

Updates arrays

Step uses

ramp_fitting

ELPP

VAR_POISSON, VAR_RNOISE

ERR

None

flat_field

ELPP

VAR_FLAT

ERR, VAR_POISSON, VAR_RNOISE

None

outlier_detection

HLPP

None

None

ERR

ELPP Processing

ELPP processing pipelines perform detector-level corrections and ramp fitting for individual exposures, for nearly all imaging and spectroscopic modes. Details of the pipelines can be found at roman_elp.

The ELPP pipeline steps that alter the ERR, VAR_POISSON, or VAR_RNOISE arrays the science countrate data are discussed below. Any step not listed here does not alter or use the variance or error arrays in any way and simply propagates the information to the next step.

ramp_fitting

This step calculates and populates the VAR_POISSON and VAR_RNOISE arrays to pass to the next step or saved in the optional output ‘rate’ files. The ERR array is updated as the square root of the quadratic sum of the variances. VAR_POISSON and VAR_RNOISE represent the uncertainty in the computed slopes (per pixel) due to Poisson and read noise, respectively. The details of the calculations can be found at ramp_fitting.

flat_field

The SCI array of the exposure being processed is divided by the flat-field reference image, and the VAR_POISSON and VAR_RNOISE arrays are divided by the square of the flat. A VAR_FLAT array is created, computed from the science data and the flat-field reference file ERR array. The science data ERR array is then updated to be the square root of the quadratic sum of the three variance arrays. For more details see flat_field.

HLPP Processing

HLPP pipelines perform additional instrument-level and observing-mode corrections and calibrations to produce fully calibrated exposures. The various steps that apply corrections and calibrations apply those same corrections/calibrations to all variance arrays and update the total ERR array.

outlier_detection

The outlier_detection step is used in all Stage 3 pipelines.