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SUMANN

sumann

## Purpose (one Line)

Integrate over an annulus.

## Description

This procedure computes the first two image moments of the pixels
contained within the input annulus. The position and inner and outer
radii define the annulus. Each pixel in the input image is then
assigned a weighting value which is its areal overlap between the pixel
and the annulus. This weight varies from 0 to 1 and is the precise
analytic area of overlap (computed by pixwt.pro). Of course, this
program doesn't do the computation for all pixels, only those near
the edge.
For historical reasons, the moments are split into two components:
The moments of all negative pixels in the annulus and the moments of
all positive pixels in the annulus. Therefore, to get the true total
number of counts you must add possum+negsum. Likewise, to get the
true center-of-mass (center-of-light) moment in either x or y, you
should add the positive and negative moments, ie.,
xmom = xcen + posxmom/possum + negxmom/negsum
ymom = ycen + posymom/possum + negymom/negsum
These numbers are returned separately because there are other reasons
for wanting the distributions separated. For example, one use of this
routine is to compute the position of a star in an image. For this
you would make two calls. The first is a call with back=0. centered
(roughly) on the object. you would then set inradious to be larger
than the object and outradius to something larger still. The sky
(background) signal would then be possum+negsum. Note this is NOT
how I use this routine for photometry, it's meant as an example ONLY.
to be just larger than the star. This time, set back to the
background found in the first call. The object brightness is then
possum+negsum counts above sky. Then, I use just posxmom and posymom
as the position of the object. Strictly speaking I should add the
negative moments, but, I know that the negative stuff is most likely
to be sky or bad pixels and irrelevant for the object location.
Note: the use of this routine for computing sky signals has been
superceded by a very different algorithm. basphote.pro uses a
combination of getannul.pro and robomean.pro to compute good sky
background values. This routine is used mostly for object counts and

## Category

CCD data processing

## Calling Sequence

possum, negsum, posxmom, negxmom, posymom, negymom

## Inputs

image : CCD image array.
xcen,ycen : Center of annulus.
back : Background to subtract from each pixel.

## Outputs

totweight : Area of annulus.
possum, negsum : Sums of positive and negative pixels.
posxmom, negxmom : Positive and negative x moments relative to xcen.
posymom, negymom : Positive and negative y moments relative to ycen.

## Modification History

Ported by Doug Loucks, Lowell Observatory, 1992 Oct, from the
C-language version written by Marc Buie.
April, 1993. DWL. Replaced the inner FOR loop with vectors.
98/09/21, MWB, optimizations
98/09/29, MWB, fixed nasty bug introduced during optimization.
2011/11/30, MWB, added output keywords XVAL, YVAL, WVAL

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