Wavelength Tilts
Overview
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:
Extract an arc spectrum down the center of each slit/order
Loop on slits/orders
Trace the arc lines (fweight is the default)
Fit the individual arc lines
2D Fit to the offset from pixcen
Save
See the WaveTilts Notebook for some examples (BEWARE, this may be out of date).
QA
The code will output a residual plot of the 2D fit to offsets. It should be possible to achieve an RMS < 0.05 pixels.
Settings
Threshold
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.:
[calibrations]
[[tilts]]
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.:
[calibrations]
[[tilts]]
spat_order = 4