From: Kenneth Elliott
Sent: 10 August 2018 14:15
Subject: Re: 3rd order polynomials
This is how you use the higher order co-efficients with PCSpectra
Original (linear values entered into PCSpectra calibration file):
How to enter higher order co-efficients, x2, x3 and x4, into PCSpectra calibration file:
70.255, 0.6474, 0.001, 0.000000013, 0.00000000000121
Alternatively, scientific format can be used for co-efficients e.g.
7.0255E1, 6.474E-1, 1E-3, 1.3E-8, 1.21E-12
Pitfalls of using higher order co-efficients in PCSpectra:
There are two pitfalls
1: You need to have lots more lines if you use higher order fits and any fits outside the wavelength range you have got calibration lines may be wildly wrong as the equation has no constraints, so its fine with the Argon lamp.
2: you need lots of precision in the coefficients to get the accuracy, otherwise if it is multiplied by X**4 it could be wildly out.
How to improve accuracy/usefulness of higher order coefficients with PCSpectra:
Three different ways you could do this:
1. Astrosurf French site which has line idents and fitting programs, but I gave up trying to use it as, although the program runs in English, if there is an error then the error messages are in French.
2. Using STARLINK (UK) software – on Linux and needs a fair level of skill to get going.
3. The HST software both on Linux – on Linux and needs a fair level of skill to get going.