Conventions

Units

New scientific code should prefer Astropy quantities for physical inputs and outputs. This is particularly important in the simulator, where focal lengths, fiber diameters, groove densities, wavelengths, detector pixel sizes, gain, and noise terms otherwise have easy-to-miss implicit units.

Common conventions are:

Scientific quantities

Quantity	Preferred representation
wavelength	Astropy length quantity; Angstrom is the common internal spectral unit
input f_lambda	erg / (s cm2 Angstrom)
detector charge	electrons
detector output	ADU after gain/bias application
gain	electrons / ADU
read noise	electrons per pixel
dark current	electrons / second per pixel
detector locations/widths	pixels

Array orientation

Detector images follow NumPy convention image[y, x] with shape (ny, nx). The dispersion direction is represented by the detector x coordinate in the simulator, while the individual fiber traces are separated along y.

Spectral arrays

A single spectrum uses one wavelength array and one flux-density array of the same length. A multi-fiber simulator input uses one common wavelength array and a flux-density matrix shaped (fiber_count, n_wavelength).

For the ETC, the wavelengths in a two-column input spectrum are observer-frame quantities. Requested wavelength-bin centers and bin sizes are also specified in the observer frame. The ETC does not apply a redshift correction, so rest-frame templates must be transformed before they are supplied to the calculator.

Wavelength direction

Whether wavelength increases toward larger or smaller detector x should not be inferred from the sign of a hand-entered dispersion. The physical simulator has an explicit wavelength_increases_with_x choice and computes the wavelength mapping from the grating geometry.

Masks

Masks indicate data that should not contribute as valid measurements. NaN should not be used as a substitute for a mask when the underlying CCD container already supports a mask and uncertainty.