5.2 Reading High Dispersion Data

The procedure most commonly used at the IUEDAC for reducing high dispersion IUE data is IUESPEC. This program extracts and concatenates any wavelength range from the high dispersion merged spectrum file. The H, W, F, and Q names represent the output vectors. IUESPEC applies a mean ripple correction (similar to that used by IUESIPS) for data from all three operational IUE cameras and optionally provides absolute fluxes for the spectra. IUESPEC also allows the user to adjust the fluxes for changes in instrumental sensitivity as a function of temperature (THDA).

IUESPEC,'IMAGET',H,W,F,Q,start_wave,end_wave

``Start_wave'' and ``end_wave'' are optional parameters. If you do not specify them, you will be prompted for them while executing the procedure. (Please note that if you extract an entire high dispersion spectrum, the total number of points may exceed the terminal's ability to display them. You may need to plot the resulting vectors in more than one plot.) If only ``start_wave'' is specified, three orders will be extracted centered on the order containing the specified wavelength.

An alternative procedure is to use IUEHI3, which allows you to extract three orders, select the splice points with the cursor, and use a different ripple correction:

IUEHI3,'IMAGET',M0,H,W,F,Q 

The M0 parameter is the highest of the three orders to be extracted, or a wavelength in Å. Note that the order numbers decrease with increasing wavelength. For example,

iuehi3,'swp12345hlg',90,h,w,f,q will extract orders 88, 89, and 90.

iuehi3,'swp12345hlg',1550,h,w,f,q will extract the same three orders, since order 90 includes 1550 Å.

Note that IUEHI3 does not calibrate the data. To calibrate it, you may use CALIB

The echelle blaze ``ripple'' correction factor may be calculated with Barker's procedure (see Ap.J. 89, 899, 1984), or it may be calculated interactively, with the results displayed so that the user can alter the echelle constant (K) until the results are satisfactory. IUESPEC calculates the correction as described by Grady and Garhart (IUE NASA Newsletter 37) and Ake (IUE NASA Newsletter 19). See the IUE Image Processing Manual (page 7-8 in Version 2.0, page 6-9 in Version 1.1) for a discussion of the echelle blaze correction.