2021-06-18 14:16:16 -04:00
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subroutine polfit(y,npts,a)
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! Input: y(npts) !Expect npts=4
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! Output: a(1) = baseline
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! a(2) = amplitude
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! a(3) = theta (deg)
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real y(npts)
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real a(3)
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real deltaa(3)
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integer ipk(1)
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save
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! Set starting values:
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a(1)=minval(y)
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a(2)=maxval(y)-a(1)
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ipk=maxloc(y)
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a(3)=(ipk(1)-1)*45.0
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deltaa(1:2)=0.1*(a(2)-a(1))
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deltaa(3)=10.0
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nterms=3
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! Start the iteration
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chisqr=0.
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chisqr0=1.e6
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iters=10
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do iter=1,iters
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do j=1,nterms
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chisq1=fchisq_pol(y,npts,a)
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fn=0.
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delta=deltaa(j)
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10 a(j)=a(j)+delta
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chisq2=fchisq_pol(y,npts,a)
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if(chisq2.eq.chisq1) go to 10
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if(chisq2.gt.chisq1) then
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delta=-delta !Reverse direction
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a(j)=a(j)+delta
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tmp=chisq1
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chisq1=chisq2
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chisq2=tmp
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endif
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20 fn=fn+1.0
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a(j)=a(j)+delta
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chisq3=fchisq_pol(y,npts,a)
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if(chisq3.lt.chisq2) then
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chisq1=chisq2
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chisq2=chisq3
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go to 20
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endif
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! Find minimum of parabola defined by last three points
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delta=delta*(1./(1.+(chisq1-chisq2)/(chisq3-chisq2))+0.5)
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a(j)=a(j)-delta
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deltaa(j)=deltaa(j)*fn/3.
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! write(*,4000) iter,j,a,deltaa,chisq2
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!4000 format(2i2,2(2x,3f8.2),f12.5)
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2021-07-01 11:14:46 -04:00
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enddo ! j=1,nterms
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2021-06-18 14:16:16 -04:00
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chisqr=fchisq_pol(y,npts,a)
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! write(*,4000) 0,0,a,chisqr
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2021-07-01 11:14:46 -04:00
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if(chisqr.lt.1.0) exit
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2021-06-18 14:16:16 -04:00
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if(deltaa(1).lt.0.01*(a(2)-a(1)) .and. deltaa(2).lt.0.01*(a(2)-a(1)) &
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.and. deltaa(3).lt.1.0) exit
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if(chisqr/chisqr0.gt.0.99) exit
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chisqr0=chisqr
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2021-07-01 11:14:46 -04:00
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enddo ! iter
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a(3)=mod(a(3)+360.0,180.0)
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2021-06-18 14:16:16 -04:00
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return
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end subroutine polfit
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real function fchisq_pol(y,npts,a)
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real y(npts),a(3)
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data rad/57.2957795/
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chisq = 0.
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do i=1,npts
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theta=(i-1)*45.0
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yfit=a(1) + a(2)*cos((theta-a(3))/rad)**2
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chisq=chisq + (y(i) - yfit)**2
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enddo
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fchisq_pol=chisq
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return
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end function fchisq_pol
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