July 2020 STAN
In this STAN we announce a CALSPEC update, including a schedule for the implementation of the resulting flux calibration changes for HST instruments. We also provide advice contributed by Amy Jones of the STIS team on verifying coordinates in APT for targets that have proper motions.
Updated CALSPEC and Schedule for Implementation of Flux Calibration Changes for HST Instruments
Motivated by improvements in stellar atmosphere models, in March 2020 the CALSPEC models for the primary spectrophotometric standard stars were updated, and, in addition, a re-examination of Vega’s spectral flux resulted in a ~0.9% (grey) increase in its absolute flux. The net result of the March 2020 changes is an increase in flux of up to 3% in the UV at wavelengths shorter than 0.15 microns, 2% increase between 0.15 and 0.4 microns, and an increase of 2% to 4% longward of 8 microns. From 0.4 to 8 microns, the change is <1.5%. Details are described in Bohlin, Hubeny and Rauch 2020, AJ, 160, 21. This update is designated as CALSPECv11.
CALSPEC is the database of the composite spectra of stars used as flux standards for HST. Stellar atmosphere models are available for a large fraction of the stars that are recommended as standards. Spectral energy distributions (SEDs) are based on observations made with one or more space instruments. CALSPECv11 updated the standard star models and the SEDs based on STIS, WFC3 and/or NICMOS spectroscopy (i.e. identified as _stis_, _stisnic_, _stiswfcnic_ ). FOS and IUE spectra have not been updated. CALSPECv11 files are available in the current CALSPEC directory. All versions of CALSPEC models and SEDs are available in the full database at https://archive.stsci.edu/hlsps/reference-atlases/cdbs/calspec/. Further CALSPEC information is available at www.stsci.edu/hst/instrumentation/reference-data-for-calibration-and-tools/astronomical-catalogs.
Each of the HST Instrument Teams will be updating flux calibrations and zeropoints over the next year, along with the concomitant throughput, sensitivity tables and photometry tables, as indicated below.
The ACS Team is preparing updated, CALSPECv11-consistent, absolute flux calibration to be released in September 2020 for most of its filter complement. Solar Blind Channel filters with revised zeropoints will include F115LP, F122M, F125LP, F140LP, F150LP, and F165LP. High Resolution Channel filters with revised zeropoints will include F220W, F250W, F330W, and F344N. Combined-use Wide Field Channel and High Resolution Channel filters with revised zeropoints will include F435W, F475W, F502N, F555W, F550M, F606W, F625W, F658N, F660N, F775W, F814W, F892N, and F850LP.
The WFC3 team, to keep the flux calibration of its two channels consistent, will update the UVIS and IR channel absolute flux calibrations and zeropoints to CALSPECv11 simultaneously, and will provide time-dependent UVIS zeropoints. This work is expected to be completed by fall 2020.
The COS team will use the new CALSPEC 11 models to recalibrate its photometric throughput tables (FLUXTABs) and time dependent sensitivity tables (TDSTABs). The update will include regenerating the FLUXTABs and TDSTABs for both the FUV detector at all lifetime positions and the NUV detector. The COS team will be carrying out this work during 2021.
The STIS team will coordinate efforts to deliver imaging zeropoint updates in line with ACS and WFC3 using the new CALSPEC v11 models. Updates will be made to the imaging photometric conversion tables (PHOTTABs) and image photometry keyword table (IMPHTTAB) by Winter 2020/2021. In the coming year, work will be carried out to derive new sensitivity curves for the spectroscopic modes where changes to the CALSPEC models exceed relative or absolute flux accuracy requirements. Updates for the most impacted modes will be prioritized with new spectroscopic PHOTTAB deliveries in Summer 2021.
Update Schedule by HST Instrument:
|Instrument||Modes||CALSPEC Version||Expected Update||Comment|
|WFC3||UVIS||V11||Fall 2020||Includes time-dependence|
|COS||FUV/NUV||V11||End of 2021||FUV at all lifetime positions|
ACS: Advanced Camera for Surveys
COS: Cosmic Origins Spectrograph
FOS: Faint Object Spectrograph (retired HST instrument)
IUE: International Ultraviolet Explorer
NICMOS: Near Infrared Camera and Multi object Spectrograph (inactive HST instrument)
STIS: Space Telescope Imaging Spectrograph
WFC3: Wide Field Camera 3
SED: spectral energy distribution
Verifying Gaia Coordinates for Targets with Proper Motions in APT
The Astronomer’s Proposal Tool (APT) allows you to retrieve coordinates of your proposed target directly from SIMBAD, which has ingested Gaia Data Release 2 (DR2). Starting in 2018, APT began retrieving coordinates with proper motion, specifically requesting an epoch of 2015.5, the native epoch of Gaia DR2. There has been some confusion when comparing the coordinates APT fetches from SIMBAD with those listed on the SIMBAD website. Both use Gaia references when applicable. The Gaia DR2 positions and proper motions are given with equinox J2000 and epoch 2015.5. The SIMBAD database then retroactively modified the positions and motions back to an epoch of 2000. The coordinates retrieved from SIMBAD with APT use the original Gaia values for the 2015.5 epoch, and hence can be slightly different from those given on the SIMBAD website. We recommend that you either (1) use the positions and motions (verifying them with an external source) retrieved from the APT without changing the values, units, or the epoch, or (2) enter all new values for the positions, motions, units, and epoch from the SIMBAD website. It is important to double check that the coordinates are correct and consistent, by confirming that your target is in the cross hairs on the target confirmation chart. For the small target acquisition apertures used by both the STIS and COS instruments, being slightly misaligned in the target confirmation chart could mean that your science target will not be in the field of view for the target acquisition. On the zoomed-in chart the diameter of the open space within the cross hairs (i.e. the separation between the cross hairs) is about 4 to 5 arcsecs, about twice the 2.5 arcsec diameter of the COS aperture. Per the policies of the telescope time review board (TTRB), observations that fail because of incorrectly specified target coordinates are generally not approved for reobservation. Contact hsthelp.stsci.edu with any additional questions or concerns.
Here is an example using the APT with a star with high proper motions, VB 8. In Phase I of the APT, under “Fixed Targets”, “Fixed Target Resolver”, the APT found and uploaded the information for VB 8. After converting the Phase I to Phase II in the APT and agreeing to copy the coordinates for the target, here is a screen shot of the APT window with the positions and proper motions with epoch of 2015.5.
Note that it gives SIMBAD as the reference frame, but on the SIMBAD website the positions and motions have been precessed to an epoch of 2000. Also, the units for the proper motion are in sec of time/yr and arcsec/yr for the Right Ascension (RA) and Declination (dec) in the APT, whereas on the website they are in mas/yr for both directions. Here is a screen shot of the SIMBAD website for VB 8.
The two working options are to either use the coordinates as retrieved by the APT, or enter from scratch the positions, proper motions, and epoch to what is shown in SIMBAD (or another reliable source). We can double check the coordinates and proper motions by using the APT target confirmation charts. These charts demonstrate the effects of proper motion when it is supplied for the target. You will see both the coordinates at the epoch you specified (orange circle) as well as the modified coordinates at the epoch of the DSS image used (pink cross hairs). In this example the DSS Image Epoch is 1988.31. This allows you to double check that your supplied combination of coordinates and proper motion is correct and consistent because the object will appear in the cross hairs. The target confirmation chart for using the coordinates as retrieved by APT shows the target in the cross hairs. The Specified Epoch is 2015.5 and the RA, dec, and proper motions shown on the chart match those shown in the Phase II APT screen shot above.
By changing only the proper motions and epoch retrieved by the APT to SIMBAD’s website values, the target is no longer within the cross hairs. The Specified Epoch is now 2000 and the proper motions (including units) match what is provided on the SIMBAD website. The RA and dec are still the values that were retrieved by APT.
Using the SIMBAD’s website coordinates, motions, and epoch, the target confirmation chart again shows the target within the cross hairs.