SLIDEFIL

Name

slidefil

Purpose

Sliding spatial filter on time series data.

Description

This program scan a data stream looking for short duration, non-random
excursions. Primarily designed for scanning stellar occultation data,
you provide the data (y), the independent variable (x) and the filter
widths. The equivalent width within the sliding window is computed
and returned alone with the x location for the window. The spacing in
x does not need to be precisely constant but should not vary by much
across the window.
The data are scanned from the specified starting and ending POINT numbers.
This allows for x being double-valued with respect to the point index.
The sliding aperture looks like:
|----------------------|+++++X+++++|---------------------|
^ ^ ^ center of window
| edge of sliding window (hhw from center)
+- edge of background window (bhw from center)
A polynomial is fitted to the background points (-), default order is 1.
This polynomial extrapolated across the "gap" (+). This extrapolation
is either divided into the window points (default), or subtracted (and
and 1.0 is added back. The sum of the window points multipled by the
local dx is then the equivalent width of the signal in the window.
This filter is designed for scanning normalized data (full intensity=1
and no intensity=0). The returned values of equivalent width are in
the same units as x.

Calling Sequence

slidefil,x,y,bhw,hhw,start,stop,xout,ew

Inputs

x - Independent vector (such as distance in km).
y - Data vector.
bhw - Half width (in units of x) of inner sliding window.
hhw - Half width (in units of x) of outer (background) window.
start - Point number (not x) at start of scan
stop - Point number (not x) near end of scan (won't go past, may stop
up to hhw-1 points early).
If either start or stop are too close to the edge for a full window,
the start and stop are adjusted inward as needed.

Optional Input Parameters

Keyword Input Parameters

DX - This is the derivative of X. If the length is not the same
as X, or, if it is undefined, the derivative is computed
and returned to the supplied argument. If the length
matches on the input, then this variable is assumed to
be the derivative and not recomputed. This can save
significant CPU time on large vectors.
FULLPLOT - Flag, enable full plotting of all data.
NOPLOT - Flag, supress all plots.
ORDER - Order of polynomial to fit to background (def=1)
SILENT - Flag, if true suppresses all printout to the IDL "console"
SUBTRACT - Flag, if true - background is subtracted, otherwise background
is ratioed.
XUNITS - String that describes the units of X (default=km)

Outputs

Keyword Output Parameters

IDXOUT - point index number for output points.

Common Blocks

Side Effects

Restrictions

Procedure

Modification History

1995/03/28 - Written by Marc W. Buie, Lowell Observatory
2009/10/02, MWB, removed obsolete Poly_fit arguments