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CRDS File Structure Explanation

HST Reference Data

Like most other HST data, the reference data are FITS files. In the case of legacy instruments and WFPC2, reference files and science data are in "waiver" FITS format. In both cases, this reference data can contain information in the form of tables or images

For a detailed discussion of understanding and working with these file types, see the HST Data Handbook.

Each operational reference file past or present has a unique name. The file name root has the format YMDHHMMSI_suffix.fits where each of the first eight values are known as the root name and selected according to the rules provided in the following table:

Value Description Rule
Y denotes the year Values A to Z or 0 to 9; where 1990=A and 2015=0
M denotes the month Values 1 to C, where January = 1
D denotes the day of the month Values 1 to V
HH denotes the hour Values 00 to 23
MM denotes the minutes Values 00 to 59
S denotes seconds Values 0 to T
I denotes the instrument j=ACS, n=NICMOS, u=WFPC2, o=STIS, f=FGS, l=COS, i=WFC3, v=HSP, w=WFPC, x=FOC, y=FOS, z=GHRS, m=multiple


The reference file type defines the file suffix appended to the unique root name. For example, p9e1544dj_drk.fits is an ACS file (the last character in the rootname is j); it is a dark file (has suffix drk); it was named on 14 September 2005 at 15 hours 44 minutes 26 seconds.

JWST Reference Data

JWST reference files can be in either FITS, ASDF, or JSON format; with extension fits, asdf and json, respectively. Their names are sequential and generated during the file delivery process by the CRDS delivery tools. These have the form: jwst_instrument_type_####.ext. Here a few examples:


Pysynphot Instrument Reference Data

Synphot throughput and emissivity reference files are all FITS tables which have two of more colums. These columns should have the following type of data

  • column 1: wavelength
  • column 2: throughput (default)
  • column 3 to n: throughput for different parameterized values (e.g. MJDs)
  • wavelengths must be in increasing order
  • there must be no duplicate wavelengths
  • there must be no wavelengths with negative or zero values

These have a sequential naming convention too. HST instrument related files have the format instrument_component_[wavelenght]_####_suffix.fits; where #### increases with each new file and suffix can be syn for throughput files or th for emissivity files.


HST Help Desk 

Please contact the HST Help Desk with any questions.