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###

*immodel*
- subtracting a King model

**In the following example we use a file which is scheduled for
release in the archive at the end of 1994 (rh800021). Although it is
possible to make subtractions on photon arrays, we have chosen to
first smooth the data because it is easier to evaluate the residuals
and intervening steps.
**

**We use imsmooth (section 6.4) to extract a 512****512 array at full pixel
resolution (0.5**
**) with a minimal (compared to the HRI PRF) Gaussian
smoothing function; sigma = 4.24 p, producing 5**
** FWHM. We then evaluate
this image, determining the center of the cluster gas distribution and
measure the total number of counts in order to assign an intensity to the
model. NB: imsmooth conserves counts.
**

**Running immodel, we provide the following parameters:
**

input the array created by imsmooth
func King
arg1 70 this is in pixels, so the core radius is 35"
arg2 1.75 for a beta value of 3/4, the exponent is 1.75
normalize yes
scale -1 this is a global scaling; negative sign means subtract.
sources 257,256,5000 pixel coordinates and amplitude.

**The normalization of intensity is somewhat obscure. The 5000 number
is not counts; see below. The residual map can be examined with imcontour,
tvproj, and the like to see if the model parameters need changing.
**

**Another way to accomplish the same result is afforded by changing the
input specification to ``512 512" and the scale parameter to +1. This
creates an output file with only the model present, thereby allowing
one to measure the effective number of counts in the model with
imcnts. In our example we specified 5000 as the intensity of the
model. Measuring the effective counts in a circle with r=250 pix
(i.e. almost the entire array), we find a value of 9342. Hence the
term ``obscure" used earlier. Since our data had approximately 10,700
counts in the same circle, we used a scaling factor (SF=1.145)
with imcalc in order to minimize the residual:
**

imcalc
input RESIDUAL = SMOOTHED_DATA - SF * MODEL

**The help file for immodel lists other functions available as well as
options for user supplied functions.
**

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*rsdc@cfa.harvard.edu *

1998-06-10