The Maryland-Magellan Tunable Filter
MMTF Wavelength Calculator (WCALC)

MMTF on-Hα (red), off-Hα (yellow) and HST near-UV (blue)
image of filaments in galaxy cluster Abell 1795. The off-band
exposure was taken 60Å redward of Hα in order to avoid the
[N II] λ6583 line. (McDonald & Veilleux, 2009)

For general use:

 Choose filter 5100 Å 5300 Å 6400 Å 6600 Å 6800 Å 7050 Å 8150 Å 8650 Å 9150 Å Choose coarse Z -2 +1 +3
 A B Z R F λ(R)

To track wavelength drift:

 Aref ΔA

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)
```

WCALC 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.