Wavelength Tilts


To construct a wavelength image that assigns a wavelength value to every pixel in the science frame, one must measure the tilts of the arc lines (or sky lines) across the slits/orders.

This process is organized by the BuildWaveTilts class, which is primarily a wrapper to methods in tracewave.

Here is the code flow:

  1. Extract an arc spectrum down the center of each slit/order

  2. Loop on slits/orders

    1. Trace the arc lines (fweight is the default)

    2. Fit the individual arc lines

    3. 2D Fit to the offset from pixcen

    4. Save

See the WaveTilts Notebook for some examples (BEWARE, this may be out of date).


The code will output a residual plot of the 2D fit to offsets. It should be possible to achieve an RMS < 0.05 pixels.



Significance threshold of an arc or sky line for analysis. The default is 20. You may wish to lower this parameter to include more lines, especially if you are short on lines near the spectral edges of the slit/order, e.g.:

        tracethresh = 10.

We tune this parameter for the various instruments.

Spatial Order

Order of the function (default is Legendre) that is fit to each arc line across the slit/order (spatial). Very long slits will likely require order=4 or higher, e.g.:

        spat_order = 4