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4.8 Background and Continuum Intensity Estimation

This section of code determines the maximum continuum and average background DN levels for data utilizing the same rotated raw image as described in Chapter 4.6. This information is stored in the FITS header and history portion of the label as keywords. Although this information is not used in subsequent portions of image processing, it provides the user with a consistent quick-look estimate of the exposure level for a given image. The continuum levels for low- and high-dispersion wavelength calibration and flat-field exposures are set to zero, regardless of the output from the code.

The continuum level is determined in the following manner. Several predefined 2-D regions along the dispersion direction, encompassing both background and spectrum, are sampled. The sample areas are placed so as to avoid most emission lines and were carefully chosen after examining a variety of exposures of stars with different spectral types. The brightest pixels within each zone are averaged together to derive a peak continuum level for that region. The DN averages for each region are compared, and the maximum is chosen as representative of the continuum level. An image is considered to be overexposed (i.e., continuum level set to 255 DN) if 5 or more pixels in any single region are saturated. The approximate low-dispersion wavelengths for each sample area are listed in Table 4.8.

 
 
Table 4.8:  Low-Dispersion Wavelength Regions for DN Measurements (Å)
Continuum
LWP LWR SWP
2120-2285 2405-2480 1255-1300
2595-2685 2565-2710 1310-1365
2850-2910 2850-2925 1830-1880
3070-3120 2985-3035 1925-1975
 
Background
LWP LWR SWP
2120-3120 2405-3035 1255-1975

A measurement of the high-dispersion continuum level is taken in a similar fashion. In this case, the samples correspond to areas centered about the peak of the echelle blaze and span several orders.

Low-dispersion background levels are calculated by averaging the DN values for a line of pixels (see Table 4.8 for the wavelength boundaries) sampled parallel to the dispersion and midway between the large- and small-apertures. For high-dispersion, a swath of interorder pixels adjacent to the respective continuum section is averaged. The background level corresponding to the maximum continuum region is recorded.


next up previous contents
Next: 4.9 High-Dispersion Order Registration Up: 4 Raw Image Screening Previous: 4.7 Serendipitous Spectrum Recognition
Karen Levay
12/4/1997