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diff --git a/noao/imred/ccdred/doc/mkskyflat.hlp b/noao/imred/ccdred/doc/mkskyflat.hlp new file mode 100644 index 00000000..d28e2301 --- /dev/null +++ b/noao/imred/ccdred/doc/mkskyflat.hlp @@ -0,0 +1,110 @@ +.help mkskyflat Feb88 noao.imred.ccdred +.ih +NAME +mkskyflat -- Make sky corrected flat field images +.ih +USAGE +mkskyflat input output +.ih +PARAMETERS +.ls input +List of blank sky images to be used to create sky corrected flat field +calibration images. +.le +.ls output +List of output sky corrected flat field calibration images (called +sky flats). 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 = "" +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 iillumination. +.le +.ls ccdproc (pset) +CCD processing parameter set. +.le +.ih +DESCRIPTION +A sky corrected flat field calibration image, called a sky flat, is a +flat field that when applied to observations of the sky have no large +scale gradients. Flat field images are generally obtained by exposures +to lamps either illuminating the telescope field or a surface in the dome +at which the telescope is pointed. Because the detector is not illuminated +in the same way as an observation of the sky there may be large +scale iillumination patterns introduced into the observations with such +a flat field. To correct this type of flat field a blank sky observation +(which has been divided by the original flat field) is heavily smoothed +to remove the noise leaving only the residual large scale iillumination +pattern. This iillumination pattern is divided into the original flat +field to remove this residual. + +The advantage of creating a sky flat field is that when processing +the observations no additional operations are required. However, +if the observations have already been processed with the original +flat field then the residual iillumination pattern of blank sky +calibration images may be created as an iillumination correction +to be applied by \fBccdproc\fR. Such a correction is created by the +task \fBmkskycor\fR. If a good blank sky image is not +available then it may be desirable to remove the iillumination pattern +of the flat field image using \fBmkillumflat\fR or \fBmkillumcor\fR +provided the sky observations are truly uniformly illuminated. +For more on flat fields and iillumination corrections see \fBflatfields\fR. + +The input, blank sky images are first processed, based on the +\fBccdproc\fR parameters, if needed. These parameters also determine +the flat field image to be used in making the sky flat. The residual +iillumination pattern is determined by heavily smoothing the image using +a moving "boxcar" average. The effects of objects in the input image +may be minimized by using a sigma clipping algorithm to detect and +exclude the objects from the average. The output image is ratio of the +flat field image, for the same subset as the input image, to the +residual iillumination pattern determined from the processed blank sky +input image. The iillumination pattern is normalized by its mean to +preserve the mean level of the flat field image. + +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. + +Blank sky images may not be completely blank so a sigma clipping +algorithm may be used to detect and exclude objects from the +iillumination pattern. This is done by computing the rms of the image +lines relative to the smoothed background 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 sky images are converted to sky flats are: + +.nf + cl> mkskyflat sky004 Skyflat + cl> mkskyflat sky* "" +.fi +.ih +SEE ALSO +ccdproc, flatfields, mkfringecor, mkillumcor, mkillumflat, mkskycor +.endhelp |