April 2022 STAN

In this STAN we announce the delivery of updated reference files for the first STIS flux recalibration release and a new Jupyter Notebook for aligning and combining STIS images with DrizzlePac.

Updated STIS Flux Calibration for Prioritized Modes

A new set of reference files, including STIS throughputs and blaze functions (relative sensitivities across echelle orders), has been released. These new calibration files incorporate the recent improvements to the stellar atmospheric models (Bohlin et al. 2020, CALSPECv11) as well as the re-examination of the Vega spectral flux. The STIS team prioritized the most widely used modes and derived sensitivity curves and other calibration reference files. The first release includes the following high priority modes (each of these gratings have only a single central wavelength option): low-resolution gratings, FUV/G140L, NUV/G230L, CCD/G230LB, CCD/G430L, and medium-resolution echelle, FUV/E140M. 

Low-resolution Modes

The sensitivity curves for the low-resolution modes were derived using observations of the standard DA white dwarfs G 191-B2B, GD 71, and GD 153, along with their updated CALSPECv11 models. These standards have been observed regularly during the lifetime of STIS. Only the latter two stars were used to calibrate G140L and G230L. The improvements to the models of these primary standard stars increased their fluxes by ~2% between the wavelengths of 1500 and 4000 A, and ~<1% for wavelengths higher than 4000A compared to the CALSPECv4 and v5 models, which were used previously to derive the STIS sensitivities for these modes.

We highlight that the sensitivity curves delivered in the new photometric throughput files (PHOTTAB) result in updated absolute flux calibration for the STIS low resolution observations, including pre- and post-Servicing Mission 4 (SM4; in 2009). In Table 1 we list the names of the reference files being replaced along with the recently released files for the different modes and USEAFTER dates. We note that the throughputs for the G750L mode will be included in the next release later this year. 

Echelle Mode

The E140M grating provides medium resolution echelle spectra from wavelengths ~1140 to ~1700 A. Similar to the latest E140M update (February 2020 STAN), the sensitivity curves and blaze shift coefficients were derived using observations of the standard DA white dwarf G 191-B2B. The sensitivity curves, as well as the characterization of the blaze shape for this particular mode was last updated in early 2020 using the CALSPECv7 atmospheric models (February 2020 STAN). To best characterize the evolution of this mode the team delivered three PHOTTABs and two ripple tables (RIPTAB), to be used in the calibration of post-SM4 data. Our analysis indicated that the new CALSPECv11 models increase the FUV fluxes by ~1% at 1150 A, and ~<3% at 1700 A. We note that in spite of the flux increase of ~<3% on the red end of the E140M wavelength coverage, in the overlapping regions between the new E140M calibration and the current E230M calibration the flux differences remain within the promised absolute flux accuracy of ~8% (see STIS Instrument Handbook Section 16.1). The STIS team is currently working to update the reference files for the E230M mode, which will be included in the next release.

The recently delivered reference files are replacing the previous version, and are meant to be used for post-SM4 observations only (see Table 1). We highlight that the STIS Team is preparing an Instrument Science Report (ISR) describing in detail the creation of the sensitivities and changes to the blaze functions for the E140M mode. As a final note, we remind users that the team has created a Python module, stisblazefix, to help optimize the alignment of the blaze function shifts for individual echelle exposures with the potential to further improve the CALSTIS calibrated products.

The new reference files are available on the HST Calibration Reference Data System (CRDS). All of the STIS observations impacted by these new reference files have now been reprocessed and are available from the HST Archive. As usual, we advise users who require the best flux calibration to re-retrieve their observations and verify that their observations have been calibrated with the new reference files (see Table 1). 

Due to the impact these calibration changes have on the quality of the STIS observations, the STIS Team has created and continues to maintain the STIS Flux Recalibration webpage to keep the HST community informed on the status and progress of this effort. Lastly, for any questions regarding these updates, please contact the HST Help Desk.

New DrizzlePac Notebook for STIS Imaging

We announce the release of a new Jupyter Notebook with examples of aligning and combining STIS images using tools from DrizzlePac (available on the STIS Software Tools webpage and new STIS-Notebooks GitHub respository). The Notebook includes examples for all three STIS detectors: the CCD, NUV MAMA and FUV MAMA. We also include an example of applying the new pixel-based charge transfer inefficiency (CTI) correction to STIS CCD data with the stis_cti code.

STIS data go through different calibration procedures relative to WFC3 and ACS, for which DrizzlePac is commonly used. DrizzlePac has been previously unsupported for STIS data due to these calibration differences. Tools from DrizzlePac were designed to work on bias- and dark-subtracted, and flat-fielded images (FLT files) as is used for WFC3 and ACS. AstroDrizzle is then typically used to further calibrate (cosmic-ray reject, sky subtract, distortion correct) and combine images. The STIS pipeline already includes specialized calibration steps to cosmic-ray reject (CCD only), sky subtract and distortion correct data. The examples in the new Notebook go over methods to use DrizzlePac appropriately on the already calibrated STIS data without applying these additional steps.

The Notebook goes through some workarounds for implementing DrizzlePac codes on STIS data. We include examples of aligning images for all three detectors to sub-pixel accuracy with the tweakreg routine. We then show examples of using AstroDrizzle as a basic mosaicking tool for combining fully-calibrated STIS products. We also include some tips and tricks for using the DrizzlePac codes and additional information that may be useful for applying these methods to a wider range of STIS imaging.

Questions about using DrizzlePac codes on STIS imaging can be directed to the HST Help Desk.