Updates to HST Standard-Star Fluxes

Updates to HST Standard-Star Fluxes
R. Bohlin and D. Lindler
Space Telescope Science Institute Newsletter, Vol. 9, No. 2,
p.19, 1992.

As part of an ongoing effort to to develop a thoroughly checked and consistent set of reference spectrophotomeqy from 1050 to10,000A, we have updated the composite stellar spectra that are used for the fundamental flux standards for HST calibrations.See Appendix B of the HST Cycle I Calibration Plan, version 1.0,of May l990, for additional documentation. These data are in an STScI data base named CRCALSPEC and have been divided into two categories to help the user understand the origin and accuracy of these flux distributions. The category 1 HST standard-star flux tables listed below were redelivered On June 24, 1992. Standard stars are classified as Category 1, if both IUE (Bohlin et al. 1990, ApJS, 73, 413) and Oke (1990, AJ, 99, 1621) spectra are available. Oke (1990) used the Double Spectrograph on the 5-m Hale Telescope to obtain these spectra for HST calibration; and these data are identified in the HST data bases by the keyword OBSMODE=OKE DBSP. Also listed are the Category 2 flux tables, some of which were last updated in August 1991. Category 2 standard stars do not have Oke DBSP data and should not be used for HST calibrations long ward of the IUE cutoff. Three additional flux tables, installed into the STScI CRCALSPEC data base in August 1989, are 1OLAC_001.TAB, ETA-AUR 001.TAB.and LAM-LEP.TAB. These three stars are not HST flux standards and their pedigree is unknown.

Category 1 Standard-Star Tables

A summary of changes for the June 24, 1993 delivery is as follows:
  1. Replace any Oke leterature, Stone, or model data with the available Oke DBSP data between 3200 and 8800A.
  2. Oke DBSP flux values decreased by by 0.04 magnitude units.(Note: HZ 44 already had the adjustment since March 7, 1990.)
  3. The long wavelength cutoff for Oke has been increased to8800A in all cases. We chose 8800A to retain the data for the Paschen jump at 8200A and to avoid the second-order contamination reported by Oke above 8800A.
  4. Statistical errors less than 0.0005 magnitude units were rounded to 0.0 on the tape delivered by Oke and were propagated as NULL into CRCALSPEC. These 0.0 values are changed to 0.0005magnitudes, while interpolated data are distinguished as NULL statistical errors.
  5. In the following wavelength regions, Oke (1990) has supplied interpolated data.
         5150 -- 5210A
         6846 -- 6950A
         7550 -- 7690A
    The statistical errors for the interpolated regions are set to NULL. The interpolated regions can be distinguished from modeldata, which have both the statistical error and the FWHM set to NULL.
  6. All wavelengths above 2000A are converted from air to vacuum.
  7. All model data are extracted from the previous CALSPEC dataset for each star. Scale factors are applied to the model data above 8800A to make the average flux in the model equal to the average flux observed by Oke in the 8500-8800A wavelength region.
  8. Scale factors applied to the Voyager data are the same as in the previous delivery of August 1, 1991 and match the Voyager to the IUE flux levels.
  9. The new CALSPEC data sets have flux units of F(lambda) in erg/s/cm/cm/A instead of STMAG.
The table below gives the source of the data for each wavelength region of the new CALSPEC data sets. The wavelength ranges are for the source data set and are given in air above 2000A. MODELdata were computed by the model atmosphere program XCAL by Keith Horne.

Category 2 Standard-Star Tables

Category 2 flux tables are not recommended for HST calibration longward of the IUE limit. Some of these tables were delivered on August 1, 1991 to:
  1. Replace model data with VOYAGER2 data shortward of 1155A.
  2. . Scale the VOYAGER2 or model data to improve the match withIUE.
  3. Adjust the short wavelength IUE wavelength cutoff.
  4. Replace OAO-2 data in ZETA-CAS with IUE data,
Category 2 flux tables are listed below along with the irinstallation dates.

Future Work

Without the complications of an intervening atmosphere, more accurate relative spectrophotometry should be possible with FOS than has been previously achieved over the wavelength range1150-8500A. Therefore a possible Category 3 of reference spectrophotometry could be created from HST observations of standard stars, after the absolute calibration of FOS is fully characterized. Furthermore, the time may be ripe to include the adjustment of the IUE absolute scale to the white-dwarf flux standard, as proposed by D. Finley and others.