The HST project has identified six types of calibration target essential to the good calibration of HST results:
1. UV spectrophotometric calibrations stars
2. Optical photometric and spectrophotometric calibration stars
3. Wavelength calibration targets
4. Astrometric calibration fields
5. Polarimetric calibration targets
6. Spatial flat field targets
Turnshek et al. (AJ, 99, 1243, 1990) list the calibration targets and provide finding charts. Bohlin et al. (ApJS, 73, 413, 1990) discuss the UV calibration of HST standards from IUE data. The basic data for the UV and optical spectrophotometric standards are held in the database CALOBS at STScI. Bohlin and Lindler (1992) provide details of the calibration data that are available.

The conversion of the IUE, HST FOS and optical spectra into standard star spectra on the white dwarf primary spectrophotometric scale (Bohlin, Colina & Finlay, AJ, 110, 1316, 1995) is described by Bohlin (Proceedings of STScI HST Calibration Workshop, eds. A. Koratkar & C. Leitherer, p. 49, 1994) and Bohlin (AJ, 111, 1743, 1996). There are spectra available for 23 stars in this white dwarf flux system and the data is held in the CALSPEC data base at STScI.

The data for the 23 stars in the white dwarf spectrophotometric system and a further 7 stars with IUE and model fluxes (indicated by an M in the last column of the table below) have been converted to ABMAG and to flux (ergs/cm/cm/s/A) using the formula
ABMAG = -2.5 alog10(Fnu) - 48.59
(Hamuy et al., PASP, 104, 533, 1992), where Fnu is in ergs/cm/cm/s/Hz.

  Star Name          alpha  (2000)  delta   Spec.    STMAG  Oke (1990)  Model opt.


                                             Type    (5460A)  fluxes      spectra


------------------------------------------------------------------------------------


 Zeta Cas (HR153)    00 36 58.30 +53 53 48.9  B2 IV    3.63                  M


 BPM 16274           00 50 03.18 -52 08 17.4  DA2     14.36                  M


 GD 50               03 48 50.06 -00 58 30.4  DA2     14.05      * 


 Hz4                 03 55 21.70 +09 47 18.7  DA4     14.45      *     


 LB227               04 09 28.76 +17 07 54.4  DA4     15.24      


 Hz2                 04 12 43.51 +11 51 50.4  DA3     14.02        


 Mu Col (HR1996)     05 45 59.92 -32 18 23.4  O9 V     5.15                  M


 HD 49798            06 48 04.64 -44 18 59.3  O6       8.31                  M


 HD 60753            07 33 27.26 -50 35 03.7  B3 IV    6.60                  M


 BD+75d325           08 10 49.31 +74 57 57.5  O5p      9.51      * 


 AGK+81d266          09 21 19.06 +81 43 28.6  sdO     11.88    


 GD 108              10 00 47.33 -07 33 31.2  sdB     13.57      * 


 Feige 34            10 39 36.71 +43 06 10.1  DO      11.12      *  


 HD 93521            10 48 23.51 +37 34 12.8  O9 Vp    6.95      * 


 Gamma UMa (HR4554)  11 53 49.83 +53 41 41.1  A0 V     2.59                  M


 Hz21                12 13 56.42 +32 56 30.8  DO2     14.67      * 


 Hz44                13 23 35.37 +36 08 00.0  sdO     11.68      *    


 GRW+70d5824         13 38 51.77 +70 17 08.5  DA3     12.80      * 


 Eta UMa (HR5191)    13 47 32.44 +49 18 48.0  B3 V     1.83                  M


 BD+33d2642          15 51 59.86 +32 56 54.8  B2 IV   10.73      * 


 Alpha Lyr (HR7001)  18 36 56.33 +38 47 01.1  A0 V    -0.01  


 LDS749B             21 32 15.75 +00 15 13.6  DB4     14.71  


 BD+28d4211          21 51 11.07 +28 51 51.8  Op      10.47      * 


 G93-48              21 52 25.33 +02 23 24.3  DA3     12.74  


 NGC 7293 Star       22 29 38.46 -20 50 13.3  Hot     13.48      * 


 Feige 110           23 19 58.39 -05 09 56.1  DOp     11.80      * 



STMAG = -2.5 log10(F(lam)) - 21.10 where F(lam) is in ergs/cm/cm/s/A.