The fors_trace_flat recipe

fors_trace_flat

Synopsis

Determine spectral curvature model

Description

This recipe is used to trace the edges of MOS/MXU flat field slit spectra and determine the spectral curvature solution. The input master flat field image, product of the recipe fors_flat, is expected to be oriented with horizontal dispersion direction and red wavelengths on the right side.

The input slits location table should be the product of the recipe fors_detect_spectra.

The input master flat image is shifted one pixel down and is subtracted from the original image. The result is a vertical gradient map. Next, the negative values are forced positive, to obtain an absolute gradient map. The map is passed with a horizontal median filter, and after that the gradient peaks are traced starting from the slits positions listed in the input slits location table. The number of pixels to the left and to the right of the reference pixel is trivially derived from the specified spectral range and spectral dispersion.

The output spectral curvature table contains the coefficients of the polynomial fitting of the found traces, while the output trace table contains the traced spectral edges positions in CCD (Y) coordinates for each spectrum, and their comparison with their modeling. A spatial map is also created, where to each CCD pixel is assigned the value of the spatial coordinate along the slit (in pixel). For more details please refer to the FORS Pipeline User’s Manual.

Note that specifying an input GRISM_TABLE will set some of the recipe configuration parameters to default values valid for a particular grism.

Again, see the pipeline manual for more details.

In the table below the MXU acronym can be alternatively read as MOS.

Input files

DO category:               Type:       Explanation:         Required:
MASTER_SCREEN_FLAT_MXU     Calib       Master flat frame       Y
SLIT_LOCATION_DETECT_MXU   Calib       Slits location          Y
GRISM_TABLE                Calib       Grism table             .

Output files

DO category:               Data type:  Explanation:
CURV_TRACES_MXU            FITS table  Flat field tracings
CURV_COEFF_MXU             FITS table  Spectral curvature table
SPATIAL_MAP_MXU            FITS image  Map of spatial coordinate

Constructor

cpl.Recipe("fors_trace_flat")

Create an object for the recipe fors_trace_flat.

import cpl
fors_trace_flat = cpl.Recipe("fors_trace_flat")

Parameters

fors_trace_flat.param.dispersion

Expected spectral dispersion (Angstrom/pixel) (float; default: 0.0) [default=0.0].

fors_trace_flat.param.startwavelength

Start wavelength in spectral extraction (float; default: 0.0) [default=0.0].

fors_trace_flat.param.endwavelength

End wavelength in spectral extraction (float; default: 0.0) [default=0.0].

fors_trace_flat.param.cdegree

Degree of spectral curvature polynomial (int; default: 0) [default=0].

fors_trace_flat.param.cmode

Interpolation mode of curvature solution applicable to MOS-like data (0 = no interpolation, 1 = fill gaps, 2 = global model) (int; default: 1) [default=1].

The following code snippet shows the default settings for the available parameters.

import cpl
fors_trace_flat = cpl.Recipe("fors_trace_flat")

fors_trace_flat.param.dispersion = 0.0
fors_trace_flat.param.startwavelength = 0.0
fors_trace_flat.param.endwavelength = 0.0
fors_trace_flat.param.cdegree = 0
fors_trace_flat.param.cmode = 1

You may also set or overwrite some or all parameters by the recipe parameter param, as shown in the following example:

import cpl
fors_trace_flat = cpl.Recipe("fors_trace_flat")
[...]
res = fors_trace_flat( ..., param = {"dispersion":0.0, "startwavelength":0.0})

See also

cpl.Recipe for more information about the recipe object.

Bug reports

Please report any problems to Carlo Izzo. Alternatively, you may send a report to the ESO User Support Department.