flat1d: Make flat field by fitting a 1D func. to the lines or columns

Package: generic

Usage

flat1d input output

Parameters

input: Calibration images to be used to make the flat fields. The images may contain image sections. Only the region covered by the section will be modified in the output image.

output: Flat field images to be created or modified. The number of output images must match the number of input images. If an output image does not exist it is first created and initialized to unit response.

axis = 1: Axis along which the one dimensional fitting is done. Axis 1 corresponds to fitting the image lines and axis 2 corresponds to fitting the columns.

interactive = yes: Set the fitting parameters interactively?

sample = "*": Lines or columns to be used in the fits.

naverage = 1: Number of sample points to combined to create a fitting point. A positive value specifies an average and a negative value specifies a median.

function = spline3: Function to be fit to the image lines or columns. The functions are "legendre" (legendre polynomial), "chebyshev" (chebyshev polynomial), "spline1" (linear spline), and "spline3" (cubic spline). The functions may be abbreviated.

order = 1: The order of the polynomials or the number of spline pieces.

low_reject = 2.5, high_reject = 2.5: Low and high rejection limits in units of the residual sigma.

niterate = 1: Number of rejection iterations.

grow = 1.: When a pixel is rejected, pixels within this distance of the rejected pixel are also rejected.

minflat = 0.: When the fitted value is less than the value of this parameter the flat field value is set to unity.

graphics = "stdgraph": Graphics device for interactive graphics output.

cursor = "": Graphics cursor input

Description

Flat fields are created containing only the small scale variations in the calibration images. The large scale variations in the images are modeled by fitting a function to each image line or column with deviant pixel rejection. The flat field values are obtained by taking the ratio of the image values to the function fit. However, if the fitted value is less than the parameter minflat the flat field value is set to unity.

The function fitting parameters may be set interactively when the interactive flag is set using the interactive curve fitting package icfit. The cursor mode commands for this package are described in a separate help entry under "icfit". For two dimensional images the user is prompted for the sample line or column or a blank-separated range to be averaged and graphed. Note that the lines or columns are relative the input image section; for example line 1 is the first line of the image section and not the first line of the image. Any number of lines or columns may be examined. When satisfied with the fit parameters the user responds with a carriage return to the line or column prompt. The function is then fit to all the lines or columns and the flat field ratios are determined.

If the output image does not exist initially it is created with the same size as the input image without an image section and initialized to unit response. Subsequently the flat field data modifies the pixel values in the output image. Input image sections may be used to restrict the region in which the flat field response is determined leaving the rest of the output image unmodified. This ability is particularly useful when dealing with multi-aperture data.

This task is very similar to fit1d with the addition of the parameter minflat and the deletion of the parameter type which is always "ratio".

Examples

1. Create a flat field from the calibration image "quartz" with the spectrum running along the lines. Exclude the first and last columns, use a spline fit of 25 pieces (a width of 32 pixels over 800 columns), and set grow to 4 pixels.

cl> flat1d quartz flat order=25 sample="2:799" grow=4 \
>>> interactive=no

or

cl> flat1d quartz[2:799,*] flat order=25 grow=4 inter-

The fitting parameters may be set interactively in which case the fitting parameters need not be specified. The command would be

cl> flat1d quartz flat
quartz: Fit column = 1 10
quartz: Fit column =

The user selects sample columns to be fit interactively with the interactive curve fitting package. When satisfied with the fit parameters respond with a carriage return to the prompt. The function is then fit to all the columns and the flat field ratios are determined.

2. As an example for multi-slit spectra the locations of the slits are determined and a file containing the image sections is created. Since there must be the same number of output images another file containing the output images is also created. For example the files might contain

  File quartzs                  File flats
_______________                 __________
quartz[23:40,*]                    flat
quartz[55:61,*]                    flat
quartz[73:84,*]                    flat

A flat field for the slits is then obtained with the command

cl> flat1d @quartzs flats axis=2

Revisions

FLAT1D V2.10.3: The image header keyword "CCDMEAN = 1." is now added or updated.

Bugs

The creation of multi-slit files and the need for an equal number of repeated output files is annoying. It will be worked on in the future.