shiftfind -- Finds shift between images by examining cross-correlation images.
shiftfind cclist outfile
- cclist [file name list/template]
- Cross-correlation image list/template.
- outfile [file name]
- Output ASCII file with results, in format appropriate for task avshift.
- (xshift, yshift = INDEF) [real]
- Shift amount of the search box in image. Note that this is the shift needed to bring the image TO the reference image. If INDEF, the search box is centered in the image.
- (boxsize = INDEF) [real]
- Size of box where to search for peak. If INDEF, the full image is scanned (thus xshift,yshift are not used).
- (fwhm = 7.) [real]
- FWHM of the Gaussian used to fit cross-correlation peaks.
- (ellip = 0.05) [real]
- Ellipticity (1 - b/a) of the Gaussian used to fit cross-correlation peaks.
- (pa = 45.) [real]
- Position angle (in degress) of the Gaussian used to fit cross-correlation peaks. Position angle increases counterclockwise from the +x direction.
- (fitbox = 7) [int]
- Size of box where the 2-D Gaussian fit takes place.
- (kellip = "yes") [bool]
- Keep ellipticity fixed during fit ?
- (kpa = "yes") [bool]
- Keep position angle fixed during fit ?
- (tempdir = "tmp$") [path name]
- Path to directory where temporary files are kept.
Shiftfind locates the peak in a cross-correlation image, and from the offset of the peak from the center of the image, calculates the shift betweent the two cross-correlated images. If the maximum of the cross-correlation is the highest point in the image, then shiftfind will automatically acquire the correct peak. However, if, for instance, cosmic rays or other noise sources have produced false peaks, the user can restrict the search using xshift, yshift and boxsize.
Shiftfind first locates the brightest pixel in the search box, and then attempts to fit a Gaussian to the peak, performing the fit over the smaller fitbox. The size of the fitbox should probably not be larger than the fwhm, as the Gaussian is, in this case, a proxy for a parabolic fit to the peak of the cross-correlation.
Here a user wishes to locate the shift from a cross-correlation image pos2x6.fits.
di> shiftfind pos2x6.fits pos2x6.out Backg = 0. (0.) Chi-sq = 1.657710E10 Resid = 124663.6 Ampl = 13773156. (31377.6) Xcent = 334.7148 (0.03916022) Ycent = 335.5871 (0.03108041) FWHM = 18.27402 (0.2749151) Ellip = 0.05 (0.) Theta = 45. (0.) Line 1 created in output table. 0.95 CPU seconds, 0.03 elapsed minutes. Crosscor: pos2x6.fits Xshift = -8.7148 Error = 0.0392 Yshift = -9.5871 Error = 0.0311
In this case, the output file, pos2x6.out, contains a single line:
pos2x6.fits -8.7148 0.0392 -9.5871 0.0311
This task was written by I.Busko in collaboration with A. Fruchter