MMTF Fabry-Perot Wavelength Calculator

W0 = A + B*Z
W(R) = W0 / sqrt(1 + R^2/F^2)

W0 is the wavelength on the optical axis

W(R) is the wavelength at a radius R (in pixels) from the axis

Z is the Z-parameter that controls the etalon spacing

A and B are the wavelength solution as a function of Z

F is the focal length of the spectrograph camera (in pixels); F~23600 for the IMACS f/2 camera.

Note that the wavelength decreases at greater radius. An example use of the calculator: You have a wavelength calibration that gives B=0.35 and F=23600. You take a calibration lamp exposure of the Neon 6678 Angstrom line at Z=550, and measure the radius to be 1556 pixels. Enter these numbers and hit "compute A" to get A=6500, which is now the zeropoint of the wavelength-z relation. Your target is a galaxy emission line predicted to be at W=6690 Angstroms, and the galaxy is positioned R=500 pixels from the optical axis. Type these numbers in and hit "Compute Z" to get the Z you'll need to send to the instrument (Z=547). Then change R to 1000 and compute W to see what wavelength is transmitted further from the optical axis (W=6685.5).

This calculator only knows about R measured directly from the optical axis. A calculator that knows about the geometry of a CCD mosaic is more elaborate to implement in Javascript, and is under development. You must have Javascript enabled to use the calculator. You can save this HTML page and use the calculator offline. Note that there is no sanity checking on the input values.

Benjamin Weiner, bjw at as.arizona.edu, revised October 2006