Optical and UV Spectrophotometric Standard Stars
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This page last updated: June 9, 2005 - JMD
This page last checked: November 7, 2004 - JMD
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 to
10,000A, 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 classiflied 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 DoubleSpectrograph 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 longward 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 to
8800A 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 0.0005
magnitudes, 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 model
data, 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 data
set 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. MODEL
data 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 with
IUE.
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 their
installation 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 range
1150-8500A. 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.
jwalsh@eso.org
