From 40e5a5811c6ffce9b0974e93cdd927cbcf60c157 Mon Sep 17 00:00:00 2001 From: Joe Hunkeler Date: Tue, 11 Aug 2015 16:51:37 -0400 Subject: Repatch (from linux) of OSX IRAF --- noao/twodspec/apextract/doc/apbackground.hlp | 79 ++++++++++++++++++++++++++++ 1 file changed, 79 insertions(+) create mode 100644 noao/twodspec/apextract/doc/apbackground.hlp (limited to 'noao/twodspec/apextract/doc/apbackground.hlp') diff --git a/noao/twodspec/apextract/doc/apbackground.hlp b/noao/twodspec/apextract/doc/apbackground.hlp new file mode 100644 index 00000000..93a49e42 --- /dev/null +++ b/noao/twodspec/apextract/doc/apbackground.hlp @@ -0,0 +1,79 @@ +.help apbackground Aug90 noao.twodspec.apextract + +.ce +Background Determination + + +Data from slit spectra allow the determination and subtraction +of the background sky using information from regions near the object +of interest. Background subtraction may also apply to cases of +scattered light though other techniques for scattered light removal +may be more appropriate. The APEXTRACT package provides for determining +the background level at each wavelength (line or column along the dispersion +axis) from a set of regions and extrapolating and subtracting the +background at each pixel extracted from the object profile. The +type of background used during extraction is specified by the parameter +\fIbackground\fR. If the value "none" is used then no background is +subtracted and any background parameters defined for an aperture are +ignored. If the value is "average", "median", "minimum" or "fit" then a +background is determined, including a variance estimate when using variance +weighted extraction (see \fIapvariance\fR), and the subtracted background +spectrum may be output if the \fIextras\fR parameter is set. + +The basic aperture definition structure used in the APEXTRACT package +includes associated background regions and fitting parameters. The +background regions are specified by a list of colon delimited ranges +defined relative to the center of the aperture. There are generally +two ranges, one on each side of the object, though one sided or more +complex sets may be used to avoid contaminated or missing parts +of the slit. The default ranges are defined by the parameter +\fIb_sample\fR. Often the ranges are better set graphically using a +cursor by invoking the 'b' option of the aperture editor. + +If the background type is "average", "median", or "minimum" then pixels +occupying these regions are averaged, medianed, or the minimum found to +produce a single background level for all object pixels at each wavelength. +Note that the "average" choice does not exclude any pixels which may +yield a background contaminated by cosmic rays. The "median" or "minimum" +is recommended instead. + +If the background type is "fit" then a function is fit to the pixels in the +background regions using the ICFIT options (see \fBicfit\fR). The +parameter \fIb_naverage\fR may be used to compute averages or medians of +groups or all of the points within each sample region. The fit is defined +by a function type \fIb_function\fR; one of legendre polynomial, chebyshev +polynomial, linear spline, or cubic spline, and function order +\fIb_order\fR (number of polynomial terms or spline pieces). An +interactive rejection of grossly deviant points from the fit may also be +used. The fitted function can define a constant, sloped, or higher order +background for the object pixels. + +Note that the background setting function, the 'b' key in \fBapedit\fR, +may be used to set the background regions for all the background options +but it will always show the result of a fit regardless of the background +type. + +After determining a background by averaging, medianing, minimizing, or +fitting, a box car smoothing step may be applied. The box car size is +given by the parameter \fIskybox\fR. When the number of available +background pixels is small, due to a small slit for instance, the noise +introduced to the extracted object spectrum may be unsatisfactorily large. +By smoothing the background one can reduce the noise when the background +consists of a smooth continuum. The trade-off, however, is that near sharp +features the smoothing will smear the features out and give a poorer +subtraction of these features. One could extract both the object and +background separately and apply a background smoothing separately using +other image processing tools. However, this is not possible for variance +weighted extraction because of the intimate connection between the +background levels, the profile determination, and the variance estimates +based on both. Thus, this smoothing feature is included. + +The background determined by the methods outlined above is actually +subtracted as a separate step during extraction. The background +is also used during profile fitting when cleaning or using variance +weighted extraction. See \fBapvariance\fR and \fBapprofile\fR for +further discussion. +.ih +SEE ALSO +approfile apvariance apdefault icfit apall apsum +.endhelp -- cgit