Subject: Beginner Question -- Absolute Fluxes

From: gibson at crisium.geop.ubc.ca (Brad Gibson)
Date: Thu, 5 Dec 91 20:38:04 GMT

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I have been "playing" with some fits 1D spectra of late and have some very
novice questions (warning: I'm a first time user of "real" data -- I usually
live in the bizarre realm of theory).  First the source:  an atlas of synthetic
stellar spectra (Pickles, A. 1985, ApJS, 59, 33).  Note: As I haven't had any 
luck in hearing from Andrew Pickles himself, I figured I
would take a shot in the dark and see if anyone on the net had any ideas before
I contacted him again.

What I want?:  real fluxes (in ergs/sec/cm^2/Angstrom).

What do I have?: one-D spectra in which the ordinate is liners in F(lambda) and
the spectra have been normalized to 100 in the wavelength region 5450-5500A.

My problem:  I don't know explicitly what the normalization factor is for each
spectrum in the atlas (48 of different spectral type and luminosity class).

I have included below the fits header for one of the spectra (OV).  As my
familiarity with fits/iraf is less than minimal, perhaps someone can let me
know if there is a simple way to invert this normalized data to real fluxes.
Naively, I though that if I simply took the normalized data (values from about
0 to a few hundred, depending upon spectrum under consideration), multiplied by
BSCALE and then added BZERO, I would end up with the REAL (i.e., flux) result
... unfortunately this does not lead to the desired numbers.

Maybe there is no simple one step way for me to obtain the absolute fluxes, but
I figure it can't hurt to ask.  Thanks in advance for any help!

Cheers,
  Brad

SIMPLE  =                    T  /  FITS STANDARD
BITPIX  =                   32  /  FITS BITS/PIXEL
NAXIS   =                    3  /  NUMBER OF AXES
NAXIS1  =                 2134  /
NAXIS2  =                    1  /
NAXIS3  =                    1  /
BSCALE  =      1.0127740409E-7  /  REAL = TAPE*BSCALE + BZERO
BZERO   =       2.2793748140E2  /
OBJECT  = 'OV      '  /
ORIGIN  = 'KPNO-IRAF'  /
DATE    = '21-06-91'  /
IRAFNAME= 'slib001.imh'  /  NAME OF IRAF IMAGE FILE
IRAF-MAX=           4.454288E2  /  DATA MAX
IRAF-MIN=           1.044613E1  /  DATA MIN
IRAF-B/P=                   32  /  DATA BITS/PIXEL
IRAFTYPE= 'FLOATING'  /
COMMENT =     SADFILE:- DISK$USER:[PICKLES.STARLIB]SSS      MAP    1
CTYPE1  = 'LAMBDA  '           /     METRES
PIXOR   = 'CENTRE  '           /     VALUE REFERS TO CENTRE OF PIXEL
BUNIT   = 'FLAMBDA '           /     ERG/CM**2/SEC/A
CRVAL1  =  3600E-10            /     METRES
CDELT1  =  3E-10               /     METRES
HISTORY 'KAPTEYN '           /
END

-- 
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  Brad K. Gibson                           INTERNET: gibson at geop.ubc.ca
  Dept. of Geophysics & Astronomy          BITNET:   userbgib at ubcmtsg.bitnet
  #129-2219 Main Mall
  University of British Columbia      
  Vancouver, British Columbia, Canada
  V6T 1Z4
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From: cflatter at nrao.edu (Chris Flatters)
Date: Thu, 5 Dec 1991 22:21:55 GMT

In article 10584 at cs.ubc.ca, gibson at crisium.geop.ubc.ca (Brad Gibson) writes:
>.... unfortunately this does not lead to the desired numbers.

This header states that the data on the tape are already in
ergs cm^-2 sec^-1 angstrom^-1 (sigh - whatever happened to SI units)
and have a range of 10.45 to 445.43.  If the data are not really
in ergs per whatever the header is lying and does not provide the
necessary information to recover the true values.

(BSCALE and BZERO are applied to the 32-bit integer values in the
FITS file).

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Chris Flatters                  |  cflatter at nrao.edu
AIPS Scientific Programmer      |
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