From fa080de7afc95aa1c19a6e6fc0e0708ced2eadc4 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 8 Jul 2015 20:46:52 -0400 Subject: Initial commit --- noao/imred/ccdred/doc/mkillumflat.hlp | 101 ++++++++++++++++++++++++++++++++++ 1 file changed, 101 insertions(+) create mode 100644 noao/imred/ccdred/doc/mkillumflat.hlp (limited to 'noao/imred/ccdred/doc/mkillumflat.hlp') diff --git a/noao/imred/ccdred/doc/mkillumflat.hlp b/noao/imred/ccdred/doc/mkillumflat.hlp new file mode 100644 index 00000000..8288fb85 --- /dev/null +++ b/noao/imred/ccdred/doc/mkillumflat.hlp @@ -0,0 +1,101 @@ +.help mkillumflat Oct88 noao.imred.ccdred +.ih +NAME +mkillumflat -- Make illumination corrected flat fields +.ih +USAGE +mkillumflat input output +.ih +PARAMETERS +.ls input +List of input flat field images to be illumination corrected. +.le +.ls output +List of output illumination corrected flat field images. +If none is specified or if the name is the same as the +input image then the output image replaces the input image. +.le +.ls ccdtype = "flat" +CCD image type to select from the input images. +.le +.ls xboxmin = 5, xboxmax = 0.25, yboxmin = 5, yboxmax = 0.25 +Minimum and maximum smoothing box size along the x and y axes. The +minimum box size is used at the edges and grows to the maximum size in +the middle of the image. This allows the smoothed image to better +represent gradients at the edge of the image. If a size is less then 1 +then it is interpreted as a fraction of the image size. If a size is +greater than or equal to 1 then it is the box size in pixels. A size +greater than the size of image selects a box equal to the size of the +image. +.le +.ls clip = yes +Clean the input images of objects? If yes then a clipping algorithm is +used to detect and exclude objects from the smoothing. +.le +.ls lowsigma = 2.5, highsigma = 2.5 +Sigma clipping thresholds above and below the smoothed illumination. +.le +.ls divbyzero = 1. +The illumination flat field is the ratio of the flat field to a +smoothed flat field. This may produce division by zero. A warning is +given if division by zero takes place and the result (the illumination +corrected flat field value) is replaced by the value of this +parameter. +.le +.ls ccdproc (parameter set) +CCD processing parameters. +.le +.ih +DESCRIPTION +First, the input flat field images are processed as needed. Then the +large scale illumination pattern of the images is removed. The +illumination pattern is determined by heavily smoothing the image using +a moving "boxcar" average. The output image is the ratio of the input +image to the illumination pattern. The illumination pattern is +normalized by its mean to preserve the mean level of the input image. + +When this task is applied to flat field images only the small scale +response effects are retained. This is appropriate if the flat field +images have illumination effects which differ from the astronomical +images and blank sky images are not available for creating sky +corrected flat fields. When a high quality blank sky image is +available the related task \fBmkskyflat\fR should be used. Note that +the illumination correction, whether from the flat field or a sky +image, may be applied as a separate step by using the task +\fBmkillumcor\fR or \fBmkskycor\fR and applying the illumination +correction as a separate operation in \fBccdproc\fR. However, creating +an illumination corrected flat field image before processing is more +efficient since one less operation per image processed is needed. For +more discussion about flat fields and illumination corrections see +\fBflatfields\fR. + +The smoothing algorithm is a moving average over a two dimensional +box. The algorithm is unconvential in that the box size is not fixed. +The box size is increased from the specified minimum at the edges to +the maximum in the middle of the image. This permits a better estimate +of the background at the edges, while retaining the very large scale +smoothing in the center of the image. Note that the sophisticated +tools of the \fBimages\fR package may be used for smoothing but this +requires more of the user and, for the more sophisticated smoothing +algorithms such as surface fitting, more processing time. + +To minimize the effects of bad pixels a sigma clipping algorithm is +used to detect and reject these pixels from the illumination. This is +done by computing the rms of the image lines relative to the smoothed +illumination and excluding points exceeding the specified threshold +factors times the rms. This is done before each image line is added to +the moving average, except for the first few lines where an iterative +process is used. +.ih +EXAMPLES +1. Two examples in which a new image is created and in which the +input flat fields are corrected in place are: + +.nf + cl> mkllumflat flat004 FlatV + cl> mkillumflat flat* "" +.fi +.ih +SEE ALSO +ccdproc, flatfields, mkfringecor, mkillumcor, mkskycor, mkskyflat +.endhelp -- cgit