i've been given a parabolic mirror, which i have scanned with a CMM.

i now have XY point data in Excel for a cross section of the mirror, does anyone know how i can find the quadratic constants to best fit my data?

(Y = aX^2 + bX + c): i want to know a b and c…

i was hoping i could use goal seek / solver, or even the quadratic trendline functions, but they've all given up.

please use short words in your replies, i'm a bit thick, thankyou.

solve it on paper?

that would take the rest of time…

(for the accuracy i'm aiming for, i've got close to a million combinations of a b and c, and over 200 data points – are you suggesting i sit down and try 200million calculations?)

plot it on an XY graph then right-click any plotted point and select "Add Trendline" then select Polynomial and select the tick boxes for "Display Equation" and indeed "Display R2"

That will give you the best fitting values of a, b, and c

That should do it – why do you say it didn't work?

er, i tried that, and the trend line i'm given looks nothing like my data.

send me the data to reggiegasket [at] yahoo.co.uk and I'll have a look at it.

Thinks Reggie is either very motivated by helping people or has way too much time on his hands!

it's a slow afternoon…. 8)

I analyse data for a living though, so it shouldn't burn much time….

check your mail Adam