ndprep -- Make a neutral density filter calibration image
ndprep filter_curve output
A neutral density (ND) filter curve is converted to a calibration image with the same size and wavelength range as the images to be calibrated. A list of standard neutral density curves is typed if the filter curve name is given as ?. The ND curves are text files containing wavelength and filter transmission pairs. Comments begin with #. A plot of the ND curve can be obtained using graph .
The ND curve is first interpolated to a one dimensional image of nw wavelength points with starting wavelength wO and wavelength increment dw using the task sinterp . The wavelength parameters must be in the same units as the filter curves (currently Angstroms) even if the final calibration image is to be in logarithmic wavelength intervals. If logarithmic wavelength format is specified the image is rebinned over the same wavelength range with the same number of points using the task dispcor . The rebinning may include flux conservation to account for the changing size of pixels or simply interpolate. Note that flux conservation will change the apparent shape of the ND curve.
If the number of points across the dispersion, nspace is zero then the final calibration image is one dimensional. If it is greater than zero the one dimensional ND image is expanded to the specified number of spatial points with the dispersion axis specified by the parameter dispaxis (1 = dispersion along the lines, 2 = dispersion along the columns).
To get a list of standard ND filter curves:
cl> ndprep ?
To graph the ND filter curve:
cl> graph onedstds$ctio/nd1m.100mag.dat
Naturally, if a calibration image is made then the image plotting tasks such as graph , implot , and splot may also be used.
To make a one dimensional ND calibration spectrum:
cl> ndprep w0=4000 dw=1.2 nw=512 Input ND filter curve: onedstds$ctio/nd1m.100mag.dat Output calibration image: NDimage
To make a two dimensional ND calibration spectrum in logarithmic wavelength:
cl> ndprep w0=4000 dw=1.2 nw=512 nspace=200 log+ Input ND filter curve: onedstds$ctio/nd4m.u000mag.dat Output calibration image: NDimage