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Add offset to spectral transformations to enable shifted slits #875

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Add offset to spectral transformations to enable shifted slits #875

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fc6b039
Add offset to spectral transformations to enable shifted slits
oczoske Feb 3, 2026
95aaa7a
Initialise offsets in SpectralTrace
oczoske Feb 4, 2026
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15 changes: 14 additions & 1 deletion scopesim/effects/spectral_trace_list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -121,13 +121,14 @@ def __init__(self, **kwargs):
"x_colname": "x",
"y_colname": "y",
"s_colname": "s",
"offset_x": 0, # [mm] in detector plane
"offset_y": 0, # [mm] in detector plane
"wave_colname": "wavelength",
"center_on_wave_mid": False,
"dwave": 0.002, # [um] for finding the best fit dispersion
"invalid_value": None, # for dodgy trace file values
}
self.meta.update(params)

# Parameters that are specific to the subclass
self.meta.update(self._class_params)
self.meta.update(kwargs)
Expand Down Expand Up @@ -239,6 +240,18 @@ def apply_to(self, obj, **kwargs):
logger.info("Making cube")
obj.cube = obj.make_hdu()

# Check whether an offset slit is used. If so, recompute spectral traces.
offset_x = obj.cube.header["CRVAL1D"]
offset_y = obj.cube.header["CRVAL2D"]
if (offset_x != self.meta["offset_x"] or
offset_y != self.meta["offset_y"]):
logger.debug("Recomputing spectral traces for offset (%.1g, %.1g)",
offset_x, offset_y)
self.meta["offset_x"] = offset_x
self.meta["offset_y"] = offset_y
self.make_spectral_traces()
self.update_meta()

spt = self.spectral_traces[obj.trace_id]
obj.hdu = spt.map_spectra_to_focal_plane(obj)

Expand Down
9 changes: 7 additions & 2 deletions scopesim/effects/spectral_trace_list_utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,8 @@ class SpectralTrace:
"x_colname": "x",
"y_colname": "y",
"s_colname": "s",
"offset_x": 0,
"offset_y": 0,
"wave_colname": "wavelength",
"dwave": 0.002,
"aperture_id": 0,
Expand Down Expand Up @@ -119,9 +121,12 @@ def compute_interpolation_functions(self):
Focal plane coordinates are `x` and `y`, in mm. Slit coordinates are
`xi` (spatial coordinate along the slit, in arcsec) and `lam`
(wavelength, in um).

The interpolation functions include a shift in the focal-plane
coordinates, determined from the CRVAL of the source FOV.
"""
x_arr = self.table[self.meta["x_colname"]]
y_arr = self.table[self.meta["y_colname"]]
x_arr = self.table[self.meta["x_colname"]] + self.meta["offset_x"]
y_arr = self.table[self.meta["y_colname"]] + self.meta["offset_y"]
xi_arr = self.table[self.meta["s_colname"]]
lam_arr = self.table[self.meta["wave_colname"]]

Expand Down
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