





SPECPR Users' Manual                                            Page 8-f50.1


_F_5_0:  _W_a_v_e_l_e_n_g_t_h _R_e_g_i_s_t_r_a_t_i_o_n                           _A_l_i_a_s = _w_a_v_r_e_g

     This function allows for calibration of the wavelengths of  a  spectrum
against  a  reference  spectrum.   It was originally designed to correct the
wavelengths of the Mariner 6 and 7 infrared spectrometer data sets, using  a
spectrum of a carbon dioxide atmosphere.  Four files are required to run the
registration.

,in 8n 1) The original data which requires calibration in wavelength.

2) An approximate wavelength file for (1)

3) Data for a reference spectrum, which contains absorptions
   at known wavelengths.

4) The wavelength file for the reference data (3).

The output of the function is a new wavelength set for  the  original  data.
(1)  and  (2)  should have the same number of channels as should (3) and (4)
however the original data and the reference do not need  to  have  the  same
number of channels.

     To begin F50,from math operations, input the file ID  and  file  number
for  the  original  data  and  f50.  Then enter the wavelength file for this
data.  If this wavelength file is the default wavelength file  no  input  is
required.   You  will be prompted to enter the wavelength file and data file
for the reference spectrum.  The function will then overlay a  plot  of  the
original  and reference spectra The reference spectrum will be in the dotted
line.  If the reference spectrum needs to be scaled vertically, that can  be
done at this point.

     It is useful to determine the  number  of  "plot  windows"  or  regions
required  to adequately cover all the absorption features which will be used
for calibration.  Both the reference and original  absorption  feature  must
appear  in the plot window.  To determine the number of plot windows use the
standard crt plot scale changing commands  (see  section  9.3).   Enter  the
number  of  plot windows and change scale to the plot window at the smallest
wavelength.

     Within a plot window determine the number of absorption  features  that
will  be  used for wavelength calibration.  The program currently allows for
20 features total to be used.  Then, using the graphics cursor, position the
cursor  on  the absorption in the reference spectrum.  The program uses only
the  horizontal  position  so  the  vertical  position  of  the  cursor   is
irrelevant.   Then  position  the  cursor  on  the  corresponding absorption
feature in the original spectrum.  Repeat this process for each  feature  in
the plot window and in subsequent plot windows.  Be sure to always move from
shorter to longer wavelengths.

     The accuracy of the wavelengths is limited by the graphics  resolution,
so be sure your plot windows magnify each set of features adequately for the





wavelength precision you desire.

     The program uses the wavelength value obtained from the reference spec-
trum  and  the  channel  number from the original spectrum. In this way, the
channel of a particular absorption feature in the original data  is  associ-
ated  with a new (correct) wavelength.  A linear interpolation in wavelength
between each absorption feature is then constructed and  used  to  determine
the  wavelengths  for  all  channels of the original data.  If the first and
last absorption feature are not the first and last channel then the interpo-
lation derived for the first and second (or last and second-to-last) absorp-
tion features is extrapolated to the end channels.

     It should be noted that an "absorption feature" can  be  a  peak  or  a
spike  or any feature of the original spectrum that can be associated with a
definitive wavelength.  Also these features need not necessarily be  present
in  the reference spectrum, if the wavelength value is accurately known from
other means. (In this case, just pick the wavelength value off  the  x-axis,
rather than from the reference spectrum).

     Finally, the program will re-display the  reference  spectrum  and  the
original spectrum, now using the newly derived wavelengths for the original.
If the calibration was reasonably successful this new wavelength file can be
saved.   If  not,  you  can  begin again with more, or different calibration
points.  In practice, this seems to work best if you exit the  function  and
then start completely over again.

     This routine and manual page was written by Wendy Calvin.






























