# Reference File¶

The pathloss correction step uses a PATHLOSS reference file.

## PATHLOSS Reference File¶

REFTYPE

PATHLOSS

Data model

PathlossModel

The PATHLOSS reference file contains correction factors as functions of source position in the aperture and wavelength.

### Reference Selection Keywords for PATHLOSS¶

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

Instrument

Keywords

NIRISS

INSTRUME, EXP_TYPE, DATE-OBS, TIME-OBS

NIRSpec

INSTRUME, EXP_TYPE, 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 PATHLOSS¶

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

Keyword

Data Model Name

EXP_TYPE

model.meta.exposure.type

### Reference File Format¶

The PATHLOSS reference files are FITS files with extensions for each of the aperture types. The FITS primary HDU does not contain a data array.

#### NIRSpec IFU¶

The NIRSpec IFU PATHLOSS reference file just four extensions: one pair for point sources and one pair for uniform sources. In each pair, there are either 3-D arrays for point sources, because the pathloss correction depends on the position of the source in the aperture, or 1-D arrays for uniform sources. The pair of arrays are the pathloss correction itself as a function of decenter in the aperture (pointsource only) and wavelength, and the variance on this measurement (currently estimated). The data apply equally to all IFU slices. The structure of the FITS file is as follows:

HDU

EXTNAME

XTENSION

Dimensions

Data type

1

PS

ImageHDU

21 x 21 x 21

float64

2

PSVAR

ImageHDU

21 x 21 x 21

float64

3

UNI

ImageHDU

21

float64

4

UNIVAR

ImageHDU

21

float64

#### NIRSpec Fixed Slit¶

The NIRSpec Fixed Slit reference file has the following FITS structure:

HDU

EXTNAME

EXTVER

XTENSION

Dimensions

Data type

1

PS

1

ImageHDU

21 x 21 x 21

float64

2

PSVAR

1

ImageHDU

21 x 21 x 21

float64

3

UNI

1

ImageHDU

21

float64

4

UNIVAR

1

ImageHDU

21

float64

5

PS

2

ImageHDU

21 x 21 x 21

float64

6

PSVAR

2

ImageHDU

21 x 21 x 21

float64

7

UNI

2

ImageHDU

21

float64

8

UNIVAR

2

ImageHDU

21

float64

9

PS

3

ImageHDU

21 x 21 x 21

float64

10

PSVAR

3

ImageHDU

21 x 21 x 21

float64

11

UNI

3

ImageHDU

21

float64

12

UNIVAR

3

ImageHDU

21

float64

13

PS

4

ImageHDU

21 x 21 x 21

float64

14

PSVAR

4

ImageHDU

21 x 21 x 21

float64

15

UNI

4

ImageHDU

21

float64

16

UNIVAR

4

ImageHDU

21

float64

HDU’s 1–4 are for the S200A1 aperture, 5–8 are for S200A2, 9–12 are for S200B1, and 13–16 are for S1600A1. Currently there is no reference data for the S400A1 aperture.

#### NIRSpec MSASPEC¶

The NIRSpec MSASPEC reference file has 2 sets of 4 extensions: one set for the 1x1 aperture (slitlet) size and one set for the 1x3 aperture (slitlet) size. Currently there is not any reference data for other aperture sizes. The FITS file has the following structure:

HDU

EXTNAME

EXTVER

XTENSION

Dimensions

Data type

1

PS

1

ImageHDU

21 x 63 x 21

float64

2

PSVAR

1

ImageHDU

21 x 63 x 21

float64

3

UNI

1

ImageHDU

21

float64

4

UNIVAR

1

ImageHDU

21

float64

5

PS

2

ImageHDU

21 x 63 x 21

float64

6

PSVAR

2

ImageHDU

21 x 63 x 21

float64

7

UNI

2

ImageHDU

21

float64

8

UNIVAR

2

ImageHDU

21

float64

#### NIRISS SOSS¶

The NIRISS SOSS reference file has just 1 extension HDU. The structure of the FITS file is as follows:

HDU

EXTNAME

XTENSION

Dimensions

Data type

1

PS

ImageHDU

17 x 2040 x 1

float32

The PS extension contains a 3-D array of correction values. The third dimension (length = 1) is a dummy to force the array dimensionality to be the same as the NIRSpec reference file arrays. The other 2 dimensions refer to the number of columns in the correction (the same as the number of columns in the science data) and the range of values for the Pupil Wheel position (PWCPOS).

The headers of the PS extensions in all of the above reference files should contain the WCS information that describes what variables the correction depends on and how they relate to the dimensions of the correction array.

For the NIRSpec reference files (IFU, Fixed Slit, and MSASPEC), the WCS keywords should have the values shown in the tables below. Dimension 1 expresses the decenter along the dispersion direction for a point source:

Keyword

Value

Comment

CRPIX1

1.0

Reference pixel in fastest dimension

CRVAL1

-0.5

Coordinate value at this reference pixel

CDELT1

0.05

Change in coordinate value for unit change in index

CTYPE1

‘UNITLESS’

Type of physical coordinate in this dimension

Dimension 2 expresses the decenter along the direction perpendicular to the dispersion for a point source:

 CRPIX2 1.0 Reference pixel in fastest dimension CRVAL2 -0.5 Coordinate value at this reference pixel CDELT2 0.05 Change in coordinate value for unit change in index CTYPE2 ‘UNITLESS’ Type of physical coordinate in this dimension

Dimension 3 expresses the change of correction as a function of wavelength:

 CRPIX3 1.0 Reference pixel in fastest dimension CRVAL3 6.0E-7 Coordinate value at this reference pixel CDELT3 2.35E-7 Change in coordinate value for unit change in index CTYPE3 ‘METER’ Type of physical coordinate in this dimension

The NIRISS SOSS reference file should also have WCS components, but their interpretation is different from those in the NIRSpec reference file. Dimension 1 expresses the column number in the science data:

Keyword

Value

Comment

CRPIX1

5.0

Reference pixel in fastest dimension

CRVAL1

5.0

Coordinate value at this reference pixel

CDELT1

1.0

Change in coordinate value for unit change in index

CTYPE1

‘PIXEL’

Type of physical coordinate in this dimension

Dimension 2 expresses the value of the PWCPOS keyword:

 CRPIX2 9.0 Reference pixel in fastest dimension CRVAL2 245.304 Coordinate value at this reference pixel CDELT2 0.1 Change in coordinate value for unit change in index CTYPE2 ‘Pupil Wheel Setting’ Type of physical coordinate in this dimension

Dimension 3 is a dummy axis for the NIRISS SOSS reference file:

 CRPIX3 1.0 Reference pixel in fastest dimension CRVAL3 1.0 Coordinate value at this reference pixel CDELT3 1.0 Change in coordinate value for unit change in index CTYPE3 ‘Dummy’ Type of physical coordinate in this dimension