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Next: 8.3.3 Time and THDA Up: 8.3 High-Dispersion Wavelength Calibration Previous: 8.3.1 Parameterization of Dispersion

8.3.2 Calculation of the Dispersion Coefficients

  As is the case in low dispersion, each set of high-dispersion WAVECAL images is processed to provide the relation between wavelength and pixel position. The analytic method is also the same multi-step process as used in low dispersion; it simply has been expanded to include a group of echelle orders rather than one single order. The high-dispersion line libraries are based on an updated set of Pt-Ne line positions measured by Sansonetti et al., (1992) at the NIST. The line positions for all cameras, and therefore the dispersion solutions, are expressed in vacuum wavelengths.

The linear mapping of high-dispersion images from pixel space to Å ngstroms was carried out with the IRAF routine identify. This task identifies the emission lines for a single order in a reference WAVECAL spectrum and generates a dispersion solution which is a one-dimensional fitted function (Chebyshev polynomial) of wavelength versus pixel number. The next step involves the use of the IRAF task reidentify which maps the reference-image Chebyshev solution derived from the identify step to an ensemble of images. The final dispersion solution for a given order is averaged from several hundred individual solutions output from reidentify and consists of a starting wavelength and wavelength increment per pixel. This process is repeated for every order to yield a set of order-by-order solutions.

Some orders, particularly those at the shorter wavelengths, have too few Pt-Ne lines in the WAVECAL spectra for valid individual dispersion solutions. In these cases the IRAF tasks ecidentify and ecreidentify were used to determine two-dimensional dispersion solutions (as a function of wavelength and order number versus pixel) for a specified block of orders; thus the Chebyshev solutions for these orders are coupled. The types of IRAF solutions used for the wavelength linearization and the time and temperature correlation steps are listed in Tables 8.5 and 8.6. The block solution simultaneously solves for three contiguous orders and is applied only to the central order of the block. The global solution solves for all orders and usually is utilized only (with the exception of the LWR) for the higher orders.


 
 
Table 8.5:  IRAF Solutions Used for the Wavelength Linearization Step
Method LWP LWR SWP
Order by Order 69,70,72, 67,72,73,75-82, 66-101
  73,75-102 85-96,98  
Block 71,74 71,97,99  
Global 103-127 68-70,74,83, 102-125
    84,100-127  


 
 
Table 8.6:  IRAF Solutions Used for Time and Temperature Correlation Step
Method LWP LWR SWP
Order by Order 69,70,72 67,68,71,73, 66-101
  73,75-109 75,76,78-82  
Block 71,74 72,77,93-99  
Global 110-127 69,70,74,83, 102-105
    84,90,100-127  


next up previous contents
Next: 8.3.3 Time and THDA Up: 8.3 High-Dispersion Wavelength Calibration Previous: 8.3.1 Parameterization of Dispersion
Karen Levay
12/4/1997