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March 28, 2022

March 2022 STAN

In this STAN we discuss improved flux calibration of most FUV settings and improved wavelength calibration of Stripe B of the NUV/G230L/2950 setting. We also provide updates on the release of CalCOS 3.4 and how to install or upgrade to it. Finally, we invite COS users to contribute slides for the STScI booth display at the Summer 2022 AAS meeting.

Improved FUV Flux Calibration

New non-local thermodynamic equilibrium (NLTE) models were recently calculated for several primary HST flux standard white dwarf (WD) stars, which were normalized to match the measured absolute flux of Vega in the visible with Midcourse Space Experiment (MSX) values for Sirius in the infrared (Bohlin, Hubeny, & Rauch 2020, AJ, 160, 21). Scaling to these newly derived fluxes for the primary calibrators resulted in changes to all the standard WD models in CALSPEC at the level of about 2%. Additionally, an entirely new model for WD 0308–565 was calculated to match recently obtained STIS data. The revised model required a higher effective temperature than the previous model and predicted fluxes that were higher than before by about 2% at 1950 Å, smoothly rising towards shorter wavelengths to about 5% at 1200 Å and up to 10% at 915 Å.

The increased model fluxes imply corresponding decreases in the predicted sensitivities. New Photometric Throughput Table (FLUXTAB) reference files have been delivered for all FUV cenwaves and lifetime positions from LP1 to LP4. (The initial LP5 calibration in 2021 was already done with the new CALSPEC models.) WD 0308–565 has been used as the flux calibrator for almost all COS FUV spectroscopic modes at all lifetime positions, with the only notable exception being the use of GD 71 for the G160M modes on detector segment FUVA (~ 1530 – 1800 Å). Users with FUV spectra obtained at LP1 through LP4 who are interested in better absolute and relative flux calibration are encouraged to re-retrieve their data from MAST. The LP at which a spectrum was obtained can be found in the LIFE_ADJ keyword of the primary FITS header.


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Improved NUV Wavelength Calibration

Consecutively obtained spectra of emission-line targets with NUV cenwaves G230L/2635 and G230L/2950 revealed a relative offset of the Mg II λ2800 line. An analysis of radial velocity standard spectra obtained over the lifetime of COS revealed that the zero point of Stripe B of the G230L/2950 setting, which extends from 2750 to 3150 Å, required an update. A new NUV Dispersion Coefficient Table (DISPTAB) was created, tested, and delivered in which this zero point was changed from 2761.13 Å to 2761.61 Å. This wavelength shift corresponds to a velocity shift of 49 km s-1 at 2950 Å. Users with G230L/2950 spectra whose analysis would benefit from the improved zero point are encouraged to re-retrieve their data from MAST.


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Release of CalCOS Version 3.4

On March 3, 2022, version 3.4 of CalCOS was released. This version includes code changes for implementing the "SPLIT wavecal" wavelength calibration procedure at Lifetime Position 6 (LP6). Due to a light leak through the flat-field calibration aperture, wavecal lamps cannot be flashed on during a science exposure at LP6. Therefore, science exposures ending sufficiently long after an initial wavecal exposure is taken will have a simulated wavecal shift applied at a fixed time interval. This shift will effectively simulate the wavecal shift measured during the middle lamp flash that is taken during typical TAGFLASH science exposures. The magnitude of this simulated shift has been derived from empirically modeling COS data and will result in an estimated increase in wavecal uncertainty of < 0.5 pixels. Additional fixes for this build include the addition of unit tests, warnings for when users try to calibrate files without wavecal data, and the functionality of transferring the wavecal solution from one FUV segment to another when the science segment does not contain any wavecal lines (this primarily affects G130M/1096 observations currently at LP2). The vast majority of data currently in the archive should be unaffected by these changes, so PIs do not need to retrieve their data again unless they are interested in the improved FUV flux or NUV wavelength calibrations mentioned above.


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How to Install or Upgrade CalCOS

The CalCOS 3.4 is no longer compatible with the current version of AstroConda as it requires Python 3.8;  therefore some users may encounter difficulty with installing or upgrading CalCOS using Conda/AstroConda. For example, the command "conda update calcos" may not successfully update to the latest version. Here we provide reliable instructions on how to install or upgrade to the latest version of CalCOS. The latest version and associated release notes for CalCOS are located on GitHub.

For an installation, first install a minimal Conda or Miniconda environment, and then install CalCOS:

  1. conda create -n my_new_env python=3.8
  2. conda activate my_new_env
  3. pip install calcos

Typing calcos --version on the command line while in a Conda environment should now return the latest version.

For an upgrade, do the following within a Python 3.8 Conda environment:

  1. pip install numpy --upgrade
  2. pip install calcos --upgrade

Again, one can verify that the latest version is installed with calcos --version. If the upgrade option does not work, we recommend that users create a new Conda environment and follow the installation instructions above.

Users who have further questions about installing or upgrading CalCOS may contact the HST help desk.


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Call for AAS Slides about COS Science

At each AAS meeting's STScI booth, slides are shown to illustrate the capabilities of Hubble's science instruments. Users are invited to submit slides for the Summer 2022 AAS meeting that demonstrate their scientific findings with COS. Each finding should be contained in a single slide. Slides should convey their message mainly through graphics, with a small amount of descriptive text. They should be designed to be understood by astronomers and students from the broad range of specialties present at AAS meetings. See here for examples from previous meetings.

Users interested in submitting slides for the Summer 2022 AAS meeting should submit them using this form by April 12, 2022. The preferred format is PowerPoint. The COS team may edit slides for clarity and visual appeal; submitting a slide does not guarantee that it will be displayed. Depending on the response, some slides submitted for the Summer 2022 meeting may instead be shown at a later meeting.


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Please Contact the HST Help Desk with any Questions