DISPERSER Reference File (NIRSpec only)

REFTYPE

DISPERSER

Data model

DisperserModel

Reference Selection Keywords for DISPERSER

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

Instrument	Keywords
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

Reference File Format

The disperser reference file contains reference data about the NIRSpec dispersers (gratings or the prism).

Files applicable to gratings have a field:

groovedensity

Number of grooves per meter in a grating

The following fields are common for all gratings and the prism:

grating

Name of grating

gwa_tiltx

temperatures

Temperatures measured where the GWA sensor is

zeroreadings

Value of GWA sensor reading which corresponds to disperser model parameters

tilt_model

Model of the relation between THETA_Y vs GWA_X sensor reading

gwa_tilty

temperatures

Temperatures measured where the GWA sensor is

zeroreadings

Value of GWA sensor reading which corresponds to disperser model parameters

tilt_model

Model of the relation between THETA_X vs GWA_Y sensor reading

tilt_x

Angle (in degrees) between the grating surface and the reference surface (the mirror)

tilt_y

Angle (in degrees) between the grating surface and the reference surface (the mirror)

theta_x

Element alignment angle in x-axis (in degrees)

theta_y

Element alignment angle in y-axis (in degrees)

theta_z

Element alignment angle in z-axis (in degrees)

The prism reference file has in addition the following fields:

angle

Angle between the front and back surface of the prosm (in degrees)

kcoef

K coefficients of Selmeir equation, describing the material

lcoef

L coeffficients describing the material

tcoef

Six constants, describing the thermal behavior of the glass

tref

Temperature (in K), used to compute the change in temperature relative to the reference temperature of the glass

pref

Reference pressure (in ATM)

wbound

Min and Max wavelength (in meters) for which the model is valid