NAME

skypars - Edit the sky fitting algorithm parameters

USAGE

skypars

PARAMETERS

smode = "concentric

The default sky measurement mode. The options are:

concentric

Measure the sky in a region concentric with the object.

offset

Measure the sky in a region offset from the object.

sgeometry = "circle"

The default geometry of the sky region. The options are "circle", "ellipse", "rectangle", and "polygon".

srannulus = 15.(iscale units)

The inner semi-major axis of the annular sky fitting region in units of impars.iscale parameter. Srannulus can be 0.0. If the sky region is a concentric polygon the inner boundary of the user defined polygon is pushed outward by srannulus iscale units; if it is an offset polygon srannulus is ignored.

swannulus = 10.(iscale units)

The width of the annular sky fitting region in units of the impars.iscale parameter.

saxratio = 1.0

The default ratio of the minor to major axis of the sky region. This parameter does not apply to circles or polygons.

sposangle = 0.(degrees)

The default position angle of the sky region in degrees counter-clockwise from the positive x axis. This parameter does not apply to circles or polygons.

salgorithm = "hcentroid"

The sky fitting algorithm. The options are:

none

Use 0.0 for the sky background.

constant

Use the current value of the sconstant parameter for the sky background.

mean

Estimate the mean of the sky pixel distribution.

median

Estimate the median of the sky pixel distribution.

mode32

Estimate the mode of the sky pixel distribution using the mode = 3 * median - 2 * mean algorithm.

hcentroid

Estimate the intensity-weighted mean or centroid of the sky pixel histogram. This algorithm is equivalent to estimating the mode of the histogram.

hgauss

Fit a Gaussian function to the sky pixel histogram using non-linear least-squares techniques.

hofilter

Estimate the sky using the optimal filtering algorithm, a triangular weighting function, and the histogram of the sky pixels.

hcrosscor

Estimate the sky value using the cross-correlation function of the sky pixel histogram and a Gaussian noise function with a sigma equal to the computed sigma of the sky pixel distribution.

sconstant

The constant used by the "constant" sky fitting algorithm.

shwidth = 3.(sigma)

The ksigma clipping factor for computing the histogram of the sky pixels. Shwidth is in units of the computed sky sigma. The computed histogram will be 2.0 * shwidth * sigma ADU wide.

shbinsize = 0.(sigma)

The resolution of the sky pixel histogram in units of the computed sky sigma. The number of histogram bins will be 2 * shwidth / shbinsize + 1.

shmooth = yes

Boxcar smooth the sky histogram before computing a sky value?

smaxiter = 10

The maximum number of iterations performed by the "hgauss" and "hofilter" sky fitting algorithms.

sloclip = 0.(percent)

The low-side clipping factor in percentage points of the total number of sky pixels.

shiclip = 0.(percent)

The high-side clipping factor in percentage points of the total number of sky pixels.

snreject = 50

The maximum number of sky pixel rejection cycles.

sloreject = 3.(sigma)

The ksigma low-side clipping factor for the pixel rejection phase of the sky fitting algorithm in units of the computed sky sigma.

shireject = 3.(sigma)

The ksigma high-side clipping factor for the pixel rejection phase of the sky fitting algorithm in units of the computed sky sigma.

srgrow = 0.(iscale units)

The region growing radius for sky pixel rejection in units of iscale. When a bad sky pixel is detected, all pixels within srgrow / impars.iscale pixels will be rejected. If srgrow is 0.0 region growing is not performed.

skymark = yes

Mark the sky region on the displayed image ?

scolormark = "yellow"

The color of the marked sky region. The options are "red", "blue", "green", and "yellow".

DESCRIPTION

The skypars parameters control the action of the sky fitting algorithms. The default values have been chosen for efficiency and to give reasonable results in most cases. Several of the sky fitting parameters scale with the image scale defined by the impars.iscale parameter and the computed standard deviation of the sky pixels sigma.

Xguiphot extracts sky pixels from a region of the image described by the sgeometry , srannlus , swannulus , saxratio , and sposangle parameters. If sgeometry is "polygon" the user must have defined the polygon interactively using the image display and image cursor before making the sky measurement.

The sky region may be offset from the object by setting smode to "offset", otherwise the sky region is assumed to be concentric with the object although it need not have the same geometry.

If sgeometry is "circle", "ellipse", "or "rectangle", and the impars.iscale parameter is defined in terms of the half-width at half-maximum of the point spread function, then single values of the srannulus and swannulus once chosen will work well for computing sky values in images with different seeing and detector characteristics.

Pixels outside of the good data range specified by the impars parameters imindata and imaxdata are rejected before any sky fitting is done. After bad data rejection sloclip and shiclip percent of the remaining sky pixels are rejected from the low and high sides of the sorted sky pixel pixel distribution before any sky fitting is done.

Sky values are computed using the sky fitting algorithm specified by salgorithm . The default value is "hcentroid". If salgorithm = "mean", "median" or "mode32", the sky value is computed directly from the array of sky pixels. The remaining sky fitting algorithms use the histogram of the object sky pixels, where histogram is 2.0 * shwidth * sigma wide and has a resolution of shbinsize * sigma, where sigma is the computed standard deviation of the sky pixels. If hsmooth = yes, boxcar smoothing is performed on the computed histogram before sky fitting. The peak of the histogram is then computed by, a non-linear least squares fit to a Gaussian (salgorithm = "hgauss"), optimal filtering of the histogram (salgorithm = "hofilter"), computing the intensity weighted mean of the histogram (salgorithm = "hcentroid") or by cross-correlation techniques (salgorithm = "hcrosscor").

Pixels which deviate from the sky value by more than sloreject * sigma below or shireject * sigma above the current best fit sky value rejected from the fit. If srgrow > 0, pixels within a radius of srgrow / scale of the rejected pixel are also rejected from the fit. The sky rejection procedure iterates until no more pixels are rejected, all pixels are rejected, or the maximum number of rejection cycles snreject iterations is reached.

If skymark is "yes", the sky region is marked on the images display using the color defined by scolormark .

EXAMPLES

1. List the sky fitting parameters.

xa> lpar skypars

2. Edit the sky fitting parameters.

xa> skypars

3. Edit the skypars parameters and run the xguiphot task.

xa> epar xguiphot

... edit a few xguiphot parameters

... move to the skypars parameter and type :e

... edit the skypars parameters and type :q

... finish editing the remaining psets and task parameters

... type :go to run the task

4. Save the current skypars parameter set in a text file skynite1.par. This can also be done from inside a higher level task as in the above example.

xa> skypars

... edit some parameters

... type ":w skynite1.par"  from within epar

TIME REQUIREMENTS

BUGS

SEE ALSO

epar,lpar,xguiphot,impars,findpars,cenpars,photpars