;Enter 3 points on circle. Find center & radius ;Convert radius in HST PC pixels to arcsec ; pro fit_circle, xy xy= 1.0*xy x1=xy[0] & y1=xy[1] x2=xy[2] & y2=xy[3] x3=xy[4] & y3=xy[5] x12= x1-x2 & x21= - x12 x13= x1-x3 & x31= - x13 y12= y1-y2 & y21= - y12 y13= y1-y3 & y31= - y13 sx13= (x1*x1) - (x3*x3) sy13= (y1*y1) - (y3*y3) sx21= (x2*x2) - (x1*x1) sy21= (y2*y2) - (y1*y1) ; f= 0.50*((sx13*x12)+(sy13*x12)+(sx21*x13)+(sy21*x13)) f= -f/((y31*x12)-(y21*x13)) ; g= 0.5*((sx13*y12)+(sy13*y12)+(sx21*y13)+(sy21*y13)) g= -g/((x31*y12)-(x21*y13)) ; c= (-(x1*x1)) + (-(y1*y1)) + (2*g*x1) + (2*f*y1) r2= (f*f)+(g*g) -c & R=sqrt(r2) print,'Radius =',R,' coordinates of center =',g,f tvcirc,R,g,f tvcirc,8,g,f return end