This JavaScript calculator solves the λ-Z equation of a Fabry-Perot tunable filter for a single unknown. The λ-Z equation is:

λ(0) = A + B z
λ(R) = λ(0) / √(1 + R2/F2)

where λ(0) = wavelength on the optical axis
      λ(R) = wavelength at radius R (in pixels) from the axis
             z = (electronic) etalon spacing, Zfine
             F = focal length of the spectrograph camera (in pixels)
                 (Depends on filter and coarse Z)

FPCALC can be used for:

• computing the plate spacing necessary to achieve a certain wavelength, given the wavelength solution;
• adjusting the wavelength solution for zero-point drift;
• computing the wavelength at different optical radii;
• and other problems.

An example use of the calculator: Your target is a galaxy emission line predicted to be at λ=6690 Å, and the galaxy is positioned R=500 pixels from the optical axis. Type these numbers in and hit "Compute Z" to get the electronic spacing you'll need to send to the instrument. Then change R to 1000 and compute λ to see what wavelength is transmitted further from the optical axis.