The SRCXPOS keyword gives the estimated position of a source within a NIRSpec MSA shutter along the x-axis direction of the image, which is parallel to the dispersion direction and also corresponds to the narrow dimension of a shutter. Source positions run from 0x0 at the top left corner of a shutter, as seen projected on the sky, to 1x1 at the lower right corner of a shutter. The shutter center is therefore at 0.5x0.5. This value is estimated by the NIRSpec MSA Planning Tool and stored in the .estimated_source_in_shutter_x. field of the PPSDB nirspec_msa_metadata table. It is copied from the PPSDB table into the MSA meta data file created by DMS, and subsequently copied from the MSA meta data file by the calibration pipeline during level-2b processing and stored in the SRCXPOS keyword for each extracted MSA slit.
| Attribute | Value |
|---|---|
| data type | float |
| enumerated Values | not defined |
| units | |
| example | |
| default value | |
| special processing | OPTIONAL |
| source | Calibration Software |
| software source | |
| calculation | |
| DB destination | ['ScienceSlit.srcxpos'] |
| data type in DB | float |
| instrument | NIRCAM, NIRISS, NIRSPEC |
| mode | NRS_MSASPEC, NRS_FIXEDSLIT, NIS_WFSS, NRC_WFSS |
| product level | 2b |
| FITS hdu | SCI |
| FITS header section | Slit parameters |