Grism Image

The next two numbers are the wavelength range to consider during the actual spectral extraction process. Those two numbers are in microns. The next four numbers define the dispersion relation, parameterized with four parameters a0, a1, a2, and a3. They are used to compute the wavelength $\lambda$in microns via

$$\lambda = a0 + a1\cdot x + a2\cdot x^2 + a3\cdot x^3$$ (29)

where x is the x-position in pixels relative to the center of the direct image.

Subsequently, four parameters for the the image distortion (in y direction) b0, b1, b2, and b3 are listed. They parameterize the distortion as:

$$\delta = b0 + b1\cdot r + b2\cdot r^2 + b3\cdot r^3$$ (30)

where

$$r = \sqrt{(x^2 + y^2)}$$ (31)

is the radius position in pixels and $\delta$ is the y-deviation in pixels of the spectrum from a horizontal line.

The next parameter is the angle $\theta$ which specifies a possible small global rotation of the spectrum relative to an exact alignment along a row. The angle is measured counterclockwise in units of degrees.

Following the angle is the spectra offset field. This is the number of pixels in the y direction that the grism image is offset from the direct image.

The next column specifies the name of the flatfield file to be used for extraction of the spectra. The flat fields to be used for the calibration of the spectra can be specified in two different ways. One is a three-dimensional FITS files, the other one is a list of flatfield files. The file type specified in the grism setup is recognised by the name of the file. Any file name which contains the string "fits" will be interpreted as FITS file. All other filenames will be interpreted as ASCII list. The ASCII input file list is assumed to be located in the $CALNICC_BASE/calib directory. It should contain at least 2 rows with 2 columns each separated by a single space. The first column contains the file name of a NICMOS flatfield. The second column contains the effective wavelength of this flatfield. The NICMOS flatfield are assumed to contain the data in extension 1, as those in the STScI generated nref directory do. The flatfield names are assumed to be relative to the $CALNICC_BASE/calib directory.

The next column lists the name of the fits file with a PSF used to search for objects on an image.

The file containing the background estimate is listed next, followed by the a file containing the response specifications for the grism.

Refer directly to $CALNICC_BASE/calib/grismspec.dat for examples.