Calspec

CALSPEC contains the composite stellar spectra that are the fundamental flux standards for HST calibrations. All files are in machine independent binary FITS table format. Information about the pedigree of a given spectrum is in the header of the FITS table file, which can be read with the IRAF hedit task or by mrdfits.pro in IDL.

The Table below summarizes the set of recommended standard star spectra. Columns 2-4 give the spectral type, V, and B magnitudes of the stars. More documentation on some of these stars, eg. coordinates, finding charts and spectral types are in Turnshek et al. (1990), Bohlin, Colina & Finley (1995), and Colina & Bohlin (1997). A set of A-stars for JWST calibration were developed by Bohlin & Cohen (2008) from NICMOS observations, while all 8 of these stars (HD165459 and 1*_nic_002.fits) have Spitzer IRAC observations and coordinates documented in Reach et al. (2005). Column 5 is the computer compatible file name with the plus and minus signs converted to underscores. Thus, the actual CALSPEC file name is the prefix in column 5, plus one of the suffixes in columns 6-9, plus ".fits". For example, a standard that has STIS data is "bd_28d4211_stis_001.fits".

High fidelity models are available for a few stars, as listed in column 6. Models for the three fundamental primary standards GD71, GD153, and G191B2B (Bohlin 2003) provide the basis for the secondary flux standards and are calculated with the Hubeny NLTE code for pure hydrogen. The observational spectra from columns 7-9 can be compared with the models; and in the case of G191B2B, there is ultraviolet line blanketing at the ~1 percent level. The model calculations extend to 30 microns and cover the long wavelength limits of 2.7 microns for NICMOS, 1.1 microns for STIS and ACS, and 27 microns for JWST. Vega was observed by STIS (Bohlin & Gilliland 2004a) and revised by Bohlin(2007). The new composite flux standard alpha_lyr_stis_004.fits consists of IUE data from 1256-1675A, STIS CCD fluxes from 1675-5300A, and a specially tailored Kurucz 9400K model longward of 5300A (Kurucz 2005). From 900-1256A, the hotter Kurucz (2003) 9550K model is more continuous with the IUE flux. The STIS and ACS observations of the SDSS standard BD+17 4708 are documented in Bohlin & Gilliland (2004b) and are now extended to 2.5microns with NICMOS data.

Column 7 lists the CDBS suffix names for the second choice standard stars with STIS data from Bohlin, Dickinson, & Calzetti (2001) and NICMOS fluxes per Bohlin (2007). The naming convention in column 7 is _STIS_ for STIS only, _NIC_ for NICMOS only, and _STISNIC_ for stars with both types of spectrophotometry. Tabulated in column 8 are the next best standard star flux distributions, which are composed of FOS spectra in the UV and Oke spectra at the longer wavelengths. Also appearing in column 8 are the three solar analogs that are comprised of FOS observation but do not have "_FOS" in the CALSPEC file name.

The names for the last set of standard stars appear in column 9 of the Table. The application of corrections to the original IUE and optical fluxes produces a consistent set of spectrophotometric standards from 1150 to 9200A (Bohlin 1996 and references therein). This set of standards is composed of IUE+Oke data only.

CALSPEC also contains the ultraviolet to near-infrared absolute flux distribution of the Sun (filename: sun_reference_stis_001.fits) to 2.7 microns. The solar reference spectrum combines absolute flux measurements from space and from the ground with a model spectrum for the near-infrared (Colina, Bohlin, & Castelli 1996). The newer absolute solar flux measurements of Thuillier (2003) may be a better choice for a solar reference spectrum from 1990-23975A.

2009 January Update

The 2009 January CALSPEC delivery encompasses six new models and 22 standard star flux distributions. The model flux distributions are for pure hydrogen in NLTE and are computed with the TLUSTY code, version 203, while the previous models used the outdated version 200. The calculated SEDs for the three primary standards G191B2B, GD153, and GD71 are placed on an absolute flux scale using a synthetic calculation of the V magnitudes relative to Vega and provide the flux calibrations for both the STIS and the NICMOS spectrophotometry. The switch to TLUSTY203 caused a small change in the continuum fluxes with a 0.3% increase in the 1300A/10000A flux ratio. In a narrow region around 4000A between H-delta and H-epsilon, the new models are up to ~0.9% fainter. In addition, the synthetic V magnitude calculation was corrected to remove an extra factor of wavelength that had been included explicitly but was discovered to be already included in the Cohen et al. (2003) V filter transmissions. The correction of this error made all stars fainter by a constant 0.35%, independent of wavelength. A new model for the historical standard HZ43 is included for completeness. Pure hydrogen NLTE models are provided for the first time for the faint stars WD1057+719 and WD1657+343; and these models include a small amount of interstellar reddening (Bohlin 2007).

The above changes that make the three primary WD models fainter by a few tenths of a percent affect all 22 newly measured SEDs by the same amount. There is one new standard, HD209458, delivered to CALSPEC for the first time. The G230LB STIS spectra of G191B2B have replaced the FOS fluxes at 1700-3850A in g191b2b_stisnic_002. A lot of new NICMOS observations are included in the updates of the stisnic_001 to stisnic_002 measurements; and all stars in this delivery have NICMOS fluxes measured to 2.5 microns, except for Vega, which is too bright for any NICMOS mode, and except for WD1657+343, which is too faint for the NICMOS G206 mode that covers the 1.9-2.5 micron range. Two stars AKG+81 266 and GRW+70 5824 have more STIS observations included and now also have NICMOS spectrophotometry, which promotes these two SEDs into the STISNIC category. All of the NICMOS spectra have been reprocessed with the new CALNICA algorithm (Dahlen et al. 2008) and the new 2008 flat fields. A correction for the correlation of the NICMOS sensitivity with temperature is also applied.

The order of preference for the choice of a standard flux distribution is from left to right in the Table, i.e. from the best in column 6 to the last choice with the lowest quality in column 9.

CDBS Files of the Flux Standards with Columns in Order of Preference(1)
--------------------------------------------------------------------------------
Star name    Sp. T.    V     B-V    CDBS name	Model	   STIS        FOS+Oke  IUE+Oke
  (1)          (2)    (3)    (4)       (5)	 (6)	    (7) 	 (8)	  (9)

1732526       A4V    12.53   0.12  1732526               _NIC_002
1740346       A6V    12.48   0.20  1740346		 _NIC_002	 
1743045       A8III  13.52   0.28  1743045		 _NIC_002
1802271       A2V    11.98   0.08  1802271               _NIC_002
1805292       A4V    12.28   0.14  1805292		 _NIC_002
1812095       A5V    11.74   0.20  1812095		 _NIC_002
1812524       A4V    12.27   0.18  1812524		 _NIC_002
2M0036+18     L3.5   21.34   ...   2M003618		 _STISNIC_002
2M0559-14     T5     I=19.14	   2M055914		 _STISNIC_002
AGK+81 266    sdO    11.94  -0.34  AGK_81D266		 _STISNIC_002		 _005
ALPHA-LYR     A0V     0.026  0.00  ALPHA_LYR		 _STIS_004		 _004
BD+17 4708    sdF8    9.47   0.44  BD_17D4708		 _STISNIC_002
BD+25 4655     ?      9.69  -0.31  BD_25D4655					 _002
BD+28 4211(2) Op     10.51  -0.34  BD_28D4211		 _STIS_001     _FOS_003  _005
BD+33 2642    B2IV   10.83  -0.17  BD_33D2642			       _FOS_003  _004
BD+75 325     O5p     9.55  -0.33  BD_75D325		 _STIS_001     _FOS_003  _005
C26202        G      16.64   ...   C26202		 _STISNIC_002
FEIGE110      D0p    11.83  -0.30  FEIGE110		 _STISNIC_002		 _005
FEIGE34       DO     11.18  -0.34  FEIGE34		 _STIS_001		 _005
FEIGE66       sdO    10.51  -0.29  FEIGE66					 _002
FEIGE67       sdO    11.82  -0.34  FEIGE67					 _002
G191B2B       DA0    11.781 -0.33  G191B2B     _MOD_005  _STISNIC_002  _FOS_003  _005
G93-48        DA3    12.74  -0.01  G93_48					 _004
GD108         sdB?   13.56  -0.22  GD108					 _005
GD153         DA1    13.346 -0.29  GD153       _MOD_005  _STISNIC_002  _FOS_003
GD50          DA2    14.06  -0.28  GD50 					 _004
GD71          DA1    13.032 -0.25  GD71        _MOD_006  _STISNIC_002  _FOS_003
GRW+70 5824   DA3    12.77  -0.09  GRW_70D5824  	 _STISNIC_002		 _005
HD165459      A4V     6.86   0.13  HD165459              _NIC_002
HD209458      G0V     7.65   0.59  HD209458              _STISNIC_002
HD93521       O9Vp    6.99  -0.27  HD93521		 _STIS_001		 _005
HZ2           DA3    13.88  -0.09  HZ2  					 _005
HZ21          DO2    14.69  -0.33  HZ21 		 _STIS_001		 _005
HZ4           DA4    14.51  +0.09  HZ4  		 _STIS_002		 _004
HZ43 (3)      DA     12.91  -0.31  HZ43        _MOD_005  _STIS_001     _FOS_003
HZ43B (3)     M      14.30   ...   HZ43B		 _STIS_001
HZ44          sdO    11.67  -0.29  HZ44 		 _STIS_001     _FOS_003  _005
KF01T5        K1III  13.56   ...   KF01T5		 _NIC_001
KF06T1       K1.5III 13.52   ...   KF06T1		 _NIC_001
KF06T2       K1.5III 13.8    ...   KF06T2		 _NIC_001
KF08T3       K0.5III 13.5    ...   KF08T3		 _NIC_001
LB227         DA4    15.32  +0.06  LB227					 _004
LDS749B       DBQ4   14.674 -0.04  LDS749B     _MOD_001	 _STISNIC_002		 _005
LTT9491       DC     14.10  +0.03  LTT9491					 _002
NGC7293       ...    13.53  -0.37  NGC7293					 _005
P041C         GOV    12.00   0.62  P041C		 _STISNIC_002  _001
P177D         G0V    13.47   0.66  P177D		 _STISNIC_002  _001
P330E         G0V    13.00   0.64  P330E		 _STISNIC_002  _001
SF1615+001A   G      16.753 +0.66  SF1615001A		 _STISNIC_002
SNAP-1        WD     15.6    ...   SNAP1		 _STISNIC_002
SNAP-2        G      16.2    ...   SNAP2		 _STISNIC_002
VB8           M7     16.81   ...   VB8  		 _STISNIC_002
WD1057+719    DA1.2  14.8    ...   WD1057_719  _MOD_001  _STISNIC_002
WD1657+343    DA1    16.1    ...   WD1657_343  _MOD_001  _STISNIC_002
SUN           G2V   -26.75   0.63  SUN_REFERENCE	 _STIS_001

(1) The unit of flux in all files is erg s^-1 cm^-2 A^-1.

(2) BD+28 4211 has a companion at 2.8arcsec (Landolt & Uomoto (2007).

(3) HZ43 and HZ3B are separated by ~3arcsec (Bohlin, Dickinson, & Calzetti 2001).

SUPPLEMENTAL CALSPEC contains previous CALSPEC versions and spectrophotometry that may be useful for special purposes.

UPDATES: 96Feb22, 99Jul19, 01Jun5, 04Feb27, 06Sep1, 07Oct-LDS749B (Bohlin & Koester 2008), 08Jul-Bohlin & Cohen (2008), 09Jan

REFERENCES

Bohlin, R.C. 1996, AJ, 111, 1743
Bohlin, R.C. 2000, AJ, 120, 437
Bohlin, R. 2003, 2002 HST Calibration Workshop, ed. S. Arribas, A. Koekemoer,
   & B. Whitmore, (Baltimore:STScI), p. 115
Bohlin, R. C. 2007, in The Future of Photometric, Spectrophotometric, and
   Polarimetric Standardization, ASP Conf. Series, Vol. 364, p. 315 ed. C. Sterken;
   also Astro-Ph 0608715
Bohlin. R. C., & Cohen, M. 2008, AJ, 136, 1171
Bohlin, R.C., Colina, L., & Finley, D.S. 1995, AJ, 110, 1316
Bohlin, R. C., Dickinson, M. E., & Calzetti, D. 2001, AJ, 122, 2118
Bohlin, R. C., & Gilliland, R. L. 2004a, AJ, 127, 3508
Bohlin, R. C., & Gilliland, R. L. 2004b, AJ, 128, 3053
Bohlin, R. C., & Koester, D. 2008, AJ, 135, 1092
Cohen, M., Megeath, S.~T., Hammersley, P.~L., Martin-Luis, F., & Stauffer, J.
	2003, AJ, 125, 2645
Colina, L., & Bohlin, R. 1994, AJ, 108, 1931
Colina, L., & Bohlin, R. 1997, AJ, 113, 1138
Colina, L., Bohlin, R.C., & Castelli, F. 1996, AJ, 112, 307
Dahlen, T.,McLaughlin, H., V. Laidler, M. Regan, E. Bergeron, R. Jedrzejewski,
	R. Bohlin, R. de Jong, & A. Viana 2008, Instrument Science Report,
	NICMOS 2008-02, (Baltimore:STScI)
Kurucz, R. 2003, http://kurucz.harvard.edu/
Kurucz, R. 2005, Vega spectrum at T=9400K, log g=3.90, [M/H]=-0.5,
   and zero microturbulent velocity, personal communication
Landolt, A., & Uomoto, A. 2007, AJ, 133, 768
Oke, J.B. 1990, AJ, 99, 1621
Reach, W. T., et al. 2005, PASP, 117, 978
Thuillier, G., Herse, M., Labs, D., Foujols, T., Peetermans, W., Gillotay, D.,
   Simon, P., & Mandel, H. 2003, Sol. Phys., 214, 1
Turnshek, D.A., Bohlin, R.C., Williamson, R., Lupie, O., Koornneef, J., &
   Morgan D. 1990, AJ, 99, 1243

Last modified, January 2009