.help fitskypars May00 noao.digiphot.daophot
.ih
NAME
fitskypars - edit the sky fitting algorithm parameters
.ih
USAGE
fitskypars
.ih
PARAMETERS
.ls salgorithm = "mode"
The sky fitting algorithm.  The sky fitting options are:
.ls constant
Use a user supplied constant sky value.
This algorithm is useful for measuring large resolved objects on flat
backgrounds such as galaxies or comets.
.le
.ls file
Read sky values from a text file. This option is useful for importing
user determined sky values into DAOPHOT.
.le
.ls mean
Compute the mean of the sky pixel distribution. This algorithm is useful
for computing sky values in regions with few background counts.
.le
.ls median
Compute the median of the sky pixel distribution. This algorithm is a useful
for computing sky values in regions with rapidly varying sky backgrounds
and is a good alternative to "centroid".
.le
.ls mode
Compute the mode of the sky pixel distribution using the mean and median.
This is the recommended algorithm for DAOPHOT users measuring stellar objects in
crowded stellar fields. Mode may not perform well in regions with
rapidly varying sky backgrounds.
.le
.ls centroid
Compute the intensity weighted mean of the sky pixel histogram. This algorithm
is reasonably robust in regions with rapidly varying or crowded sky backgrounds
and is a good alternative to "median".
.le
.ls gauss
Fit a Gaussian function to the sky pixel histogram using non-linear least-
squares techniques to determine the peak. 
.le
.ls ofilter
Optimally filter the sky pixel histogram using a triangular weighting
function to determine the peak.
.le
.ls crosscor
Compute the peak of the cross-correlation function of the pixel distribution
and a Gaussian noise function to determine the peak.
.le
.ls histplot
Mark the peak of the sky pixel histogram with the graphics cursor.
This algorithm is useful for making careful interactive sky measurements
for a small number of objects in complicated regions or for checking the
behavior of other sky algorithms. 
.le
.ls radplot
Mark the sky level on a radial profile plot with the graphics cursor.
This algorithm is useful for making careful interactive sky measurements
for a small number of objects in complicated regions or for checking the
behavior of other sky algorithms. 
.le
.le
.ls annulus = 10.0  (scale units)
The inner radius of the annular sky fitting region in units of the DATAPARS
scale parameter.
.le
.ls dannulus = 10.0  (scale units)
The width of the annular sky fitting region in units of the DATAPARS scale
parameter.
.le
.ls skyvalue = 0.0
The constant for constant sky subtraction.
.le
.ls smaxiter = 10
The maximum number of iterations performed by the sky fitting algorithm.
Smaxiter is required by the "gauss" and "ofilter" sky fitting algorithms.
.le
.ls sloclip = 0.0, shiclip = 0.0 (percent)
The high and low side clipping parameters in percent of the total number
of pixels. If either of these parameters > 0.0 then the specified
percentage of the pixels will be removed from the sky pixel distribution
before any sky fitting is done.
.le
.ls snreject = 50
The maximum number of sky pixel rejection cycles.
.le
.ls sloreject = 3.0, shireject = 3.0
The k-sigma clipping factors for the pixel rejection  phase of the
sky fitting algorithm. Sloreject and shireject are in units of the
computed sky sigma.
.le
.ls khist = 3.0
The k-sigma clipping factor for computing the sky pixels histogram. Khist is in
units of sigma of the local sky pixel distribution.  The histogram will be
2.0 * khist * sigma wide.  Khist is used by the "centroid", "gauss",
"crosscor", "ofilter", and "histplot" sky fitting algorithms.
.le
.ls binsize = 0.10
The width of a single bin of the sky pixel histogram.  Binsize is in units of
the sigma of the local sky pixel distribution. Binsize is used by the
"centroid", "gauss", "crosscor", "ofilter", and "histplot" sky fitting
algorithms.
.le
.ls smooth = no
Boxcar smooth the sky pixel histogram before computing a sky value.
Smooth is used by the "centroid", "gauss", "crosscor", "ofilter", and
"histplot" sky fitting algorithms.
.le
.ls rgrow = 0.0  (scale units)
The region growing radius for pixel rejection in the sky region in units
of the DATAPARS scale parameter. When a bad sky_pixel is detected, all pixels
within rgrow / scale pixels of the bad pixel will be rejected. If rgrow is
0.0 region growing is disabled.
.le
.ls mksky = no
Mark the sky annuli on the displayed image ?
.le
.ih
DESCRIPTION
The sky fitting algorithm parameters control the action of the sky fitting
algorithms. The default parameter settings should give reasonable results in
the majority of cases.  Several of the sky fitting parameters scale with
image scale, \fIscale\fR which is data dependent.
\fIScale\fR is defined in the DATAPARS parameter set.

Sky pixels in an annular region of inner radius \fIannulus / scale\fR pixels
and a width of \fIdannulus / scale\fR pixels are extracted from the IRAF image.
If the \fIscale\fR parameter is defined in terms of the number of half-width
at half-maximum of the point spread function per pixel, then single values of
annulus and dannulus will work well for images with different seeing and
detector characteristics.

Pixels outside of the good data range specified by \fIdatamin\fR and
\fIdatamax\fR are rejected from the sky pixel distribution. After bad
data rejection \fIPloclip\fR and \fIphiclip\fR percent pixels are rejected
from the low and high sides of the sorted pixel distribution before any
sky fitting is done.

Sky values are computed using the sky fitting algorithm specified by
\fIsalgorithm\fR. The default value is "centroid". If \fIsalgorithm\fR
= "mean", "median" or "mode", 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. The computed histogram is \fIkhist\fR * sigma wide
with a bin width of \fIbinsize\fR * sigma  where sigma is the computed
standard deviation of the sky pixels for each object. If \fIsmooth\fR = yes,
boxcar smoothing is performed on the computed histogram before sky fitting.
The mode of the histogram is  computed using, a non-linear least squares
fit to a Gaussian (salgorithm = "gauss"), optimal filtering of the histogram
(salgorithm = "ofilter"), computing the centroid of the histogram
(salgorithm = "centroid"), or by cross-correlation techniques
(salgorithm = "crosscor").

Two interactive methods of fitting sky are also available. If \fIsalgorithm\fR
is "radplot" or "histplot", the user must interactively set
the value of the sky using a radial profile or a histogram plot.

Pixels which deviate from the sky value by more than \fIkreject times the
computed sky sigma are rejected from the fit. If \fIrgrow\fR > 0, pixels
within a radius of rgrow / scale of the rejected pixel are also rejected from
the fit. The rejection procedure iterates until no further pixels are rejected,
all pixels are rejected, or the maximum number of rejection cycles
\fIsnreject\fR iterations is reached.

.ih
EXAMPLES

1. List the sky fitting parameters.

.nf
	da> lpar fitskypars
.fi

2. Edit the sky fitting parameters.

.nf
	da> fitskypars
.fi

3. Edit the FITSKYPARS parameters from with the PHOT task.

.nf
    da> epar phot

	... edit a few phot parameters

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

	... edit the fitskypars parameters and type :wq

	... finish editing the phot parameters and type :wq
.fi

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

.nf
    da> epar fitskypars

	... type ":w skynite1.par"  from within epar
.fi
.ih
TIME REQUIREMENTS
.ih
BUGS
.ih
SEE ALSO
epar,lpar,datapars,phot
.endhelp