Current Cosmic Ray Monitor: 2026 STIS CR Table
CCD exposures taken with the Space Telescope Imaging Spectrograph (STIS) are split into multiple subexposures (CR-SPLITS) which are later combined for the purpose of rejecting cosmic rays. This monitor runs daily on STIS CCD spectroscopic datasets that have both a calibrated cosmic ray rejected image (_crj.fits) and a 1d extracted spectrum (_sx1.fits or _x1d.fits) available on MAST. The monitor provides basic information about the exposures from the public Phase II and MAST, as well as the pixels identified as cosmic rays by the pipeline. The pixels identified as cosmic rays are also presented visually in two interactive plots (STACK_PLOT and SPLIT_PLOT). The first is an image of the detector or subarray with each pixel color coded to the number of times across all subexposures it was flagged as a cosmic ray. The second is a zoomed image of each CR-SPLIT showing which pixels are flagged as cosmic rays in and around the extraction box. These are useful for quickly identifying cases of cosmic ray over-flagging, discussed below. For a full list of available columns, see the Column Definitions section.
Cosmic Ray Over-Flagging
There is a known issue when a target is not spatially well aligned in the aperture between CR-SPLITS. In these cases, the default cosmic ray rejection algorithm can identify valid spectral data in one or more of the cr-splits as cosmic rays, leading to an underestimation of the counts in the spectrum after combining. This is discussed in STIS ISR 2019-02, section 2 of STIS ISR 2026-0N, and the caveats section of STIS ISR 1998-22
Following the methodology discussed in more detail in STIS ISR 2026-0N, the Cosmic Ray Monitor provides tools to help identify cases of cosmic ray over-flagging. In short, the fraction of pixels identified as cosmic rays inside and outside the default spectral extraction box (AVG_EXTR_FRAC and AVG_OUTSIDE_FRAC respectively) should be similar. The ratio of pixels inside and outside the extraction box flagged as cosmic rays over the combined exposure is given as COMBINED_RATIO, and the monitor runs a statistical test to compute COMBINED_RATIO_THRESHOLD: the largest COMBINED_RATIO under the null hypothesis that cosmic rays are randomly distributed accross the detector to a 99.73% confidence level. Datasets where the observed COMBINED_RATIO is ≥ COMBINED_RATIO_THRESHOLD likely have valid data being flagged in one or more subexposures and are highlighted in orange. Section 4 of STIS ISR 2019-02 and section 6 of STIS ISR 2026-0N provide suggestions for custom cosmic ray rejection that can help mitigate this overflagging.
- 2025 STIS CR table
- 2024 STIS CR table
- 2023 STIS CR table
- 2022 STIS CR table
- 2021 STIS CR table
- 2020 STIS CR table
- 2019 STIS CR table
- 2018 STIS CR table
- 2017 STIS CR table
- 2016 STIS CR table
- 2015 STIS CR table
- 2014 STIS CR table
- 2013 STIS CR table
- 2012 STIS CR table
- 2011 STIS CR table
- 2010 STIS CR table
- 2009 STIS CR table
- 2004 STIS CR table
- 2003 STIS CR table
- 2002 STIS CR table
- 2001 STIS CR table
- 2000 STIS CR table
- 1999 STIS CR table
- 1998 STIS CR table
- 1997 STIS CR table
Column Definitions
| Column Name | Description |
|---|---|
| OBSID | Dataset name |
| PROPOSID | ID number assigned to the proposal |
| LINENUM | Proposal logsheet line number. First two values usually indicate visit ID, remaining values indicate the line of the exposure in the Phase II file |
| TARGNAME | Target name designated by the observer |
| TARDESCR | Target description designated by the observer |
| APERTURE | Aperture configuration |
| OBS_TIME | UT start time of first exposure |
| CRSPLIT | Number of cosmic ray split exposures |
| NRPTEXP | Number of specified identical repeat exposures in set (default is 1 if CRSPLIT is specified, usually used to obtain time-resolved images) |
| COMBINED_EXPOSURE_TIME | Total exposure time across all subexposures |
| N_TOTAL_CRPIX | Total number of cosmic ray flagged pixels across all subexposures |
| AVG_EXTR_FRAC | Average cosmic ray rejection rate (n total cosmic ray flags/ n total pixels) in the extraction box across all subexposures. Equivalent to the average of the extr_fracs result from stistools.ocrreject_exam.ocrreject_exam() |
| AVG_OUTSIDE_FRAC | Average cosmic ray rejection rate (n total cosmic ray flags/ n total pixels) outside the extraction box across all subexposures. Equivalent to the average of the outside_fracs result from stistools.ocrreject_exam.ocrreject_exam() |
| COMBINED_RATIO | Cosmic ray rejection rate inside the extraction box divided by the rate outside it, combined across all splits (AVG_EXTR_FRAC/AVG_OUTSIDE_FRAC) |
| COMBINED_RATIO_THRESHOLD | The maximum COMBINED_RATIO under the null hypothesis of randomly distributed cosmic rays to a 99.73% confidence level |
| OVERFLAGGED_STAT | Boolean indicating if the observation is likely overflagged. True if the observed COMBINED_RATIO ≥ COMBINED_RATIO_THRESHOLD. |
| QUALITY | Procedural Data Quality comments from MAST (e.g., LOCKLOST, RECENTER, TDF-DOWN, SLEWING), as well as custom quality comments in cases where a dataset could not be processed by the monitor. |
| STACK_PLOT | Image of the detector or subarray with each pixel color-coded based on the number of times it was flagged as a cosmic ray across all subexposures |
| SPLIT_PLOT | Zoomed image of each subexposure around the extraction box showing which pixels are flagged as cosmic rays |