Reference Files

The rscd correction step uses an RSCD reference file.

RSCD Reference File

REFTYPE:

RSCD

Data model:

RSCDModel

The RSCD reference file contains coefficients used to compute the correction.

Reference Selection Keywords for RSCD

CRDS selects appropriate RSCD references based on the following keywords. RSCD is not applicable for instruments not in the table. All keywords used for file selection are required.

Instrument	Keywords
MIRI	INSTRUME, DETECTOR, DATE-OBS, TIME-OBS

Standard Keywords

The following table lists the keywords that are required to be present in all reference files. The first column gives the FITS keyword names. The second column gives the jwst data model name for each keyword, which is useful when using data models in creating and populating a new reference file. The third column gives the equivalent meta tag in ASDF reference file headers, which is the same as the name within the data model meta tree (second column).

FITS Keyword	Data Model Name	ASDF meta tag
AUTHOR	model.meta.author	author
DATAMODL	model.meta.model_type	model_type
DATE	model.meta.date	date
DESCRIP	model.meta.description	description
FILENAME	model.meta.filename	N/A
INSTRUME	model.meta.instrument.name	instrument: {name}
PEDIGREE	model.meta.pedigree	pedigree
REFTYPE	model.meta.reftype	reftype
TELESCOP	model.meta.telescope	telescope
USEAFTER	model.meta.useafter	useafter

NOTE: More information on standard required keywords can be found here: Standard Required Keywords

Type Specific Keywords for RSCD

In addition to the standard reference file keywords listed above, the following keywords are required in RSCD reference files, because they are used as CRDS selectors (see Reference Selection Keywords for RSCD):

Keyword	Data Model Name
DETECTOR	model.meta.instrument.detector

Reference File Format

RSCD reference files are FITS format, with 1 BINTABLE extension. The FITS primary HDU does not contain a data array. The BINTABLE extension uses the identifier EXTNAME = “RSCD” and the characteristics of the table columns are as follows:

Column name	Data type	Notes
subarray	char*13	FULL or subarray name
readpatt	char*4	SLOW or FAST
rows	char*4	EVEN or ODD
tau	float32	e-folding time scale, in units of frames
ascale	float32	\(b{1}\) in equation 2
pow	float32	\(b{2}\) in equation 2
illum_zp	float32	\(illum_{zpt}\) in equation 2.1
illum_slope	float32	\(illum_{slope}\) in equation 2.1
illum2	float32	\(illum2\) in equation 2.1
param3	float32	\(b{3}\) in equation 2
crossopt	float32	\(Crossover Point\) in equation 2.2
sat_zp	float32	\(sat_\text{zp}\) in equation 3.1
sat_slope	float32	\(sat_\text{slope}\) in equation 3.1
sat_2	float32	\(sat_2\) in equation 3.1
sat_mzp	float32	\(sat_\text{mzp}\) in equation 3
sat_rowterm	float32	\(evenrow_{corrections}\) in equation 3.1
sat_scale	float32	\(scale_\text{sat}\) in equation 3

The entries in the first 3 columns of the table are used as row selection criteria, matching the exposure properties and row type of the data. The remaining 14 columns contain the parameter values for the double-exponential correction function.

The general form of the correction to be added to the input data is:

corrected data = input data + dn_accumulated * scale * exp(-T / tau)  (Equation 1)

where:

• T is the time since the last group in the previous integration

• tau is the exponential time constant found in the RSCD table

• dn_accumulated is the DN level that was accumulated for the pixel from the previous integration.

In cases where the last integration does not saturate, the \(scale\) term in equation 1 is determined according to:

\(scale = b{1}* [Counts{2}^{b{2}} * [1/exp(Counts{2}/b{3}) -1 ]\; \; (Equation \; 2)\)

The following two additional equations are used in Equation 2:

\(b{1} = ascale * (illum_{zpt} + illum_{slope}*N + illum2* N^2) \; \; (Equation \; 2.1)\)

\(Counts{2} = Final \, DN \, in \, the \, last \, group \, in \; the \, previous \, integration \, - Crossover \, Point \; \; (Equation \; 2.2)\)

where:

• N in equation 2.1 is the number of groups per integration

• Crossover Point in equation 2.2 is column CROSSOPT in the RSCD table.

If the previous integration saturates, \(scale\) is no longer calculated using equations 2 - 2.2. Instead it is calculated using equations 3 and 3.1:

\(scale_\text{sat} = slope * Counts{3} + sat_\text{mzp} \; \; (Equation \; 3)\)

\(slope = sat_{zp} + sat_{slope} * N + sat_2*N^2 + evenrow_{corrections} \; \; (Equation \; 3.1)\)

where:

• \(Counts{3}\) is an estimate of what the last group in the previous integration would have been if saturation did not exist

• \(scale_\text{sat}\) is sat_scale in the RSCD table

• \(sat_\text{mzp}\) is sat_mzp in the RSCD table

• \(sat_\text{zp}\) is sat_zp in the RSCD table

• \(sat_\text{slope}\) is sat_slope in the RSCD table

• \(sat_2\) is sat2 in the RSCD table

• \(evenrow_{corrections}\) is sat_rowterm in the RSCD table

• N is the number of groups per integration