Reference / Survey properties

Survey properties¶

This page lists the wavelength range, the resolution and the line spread function (LSF) of each survey whose spectra DOROTHY reads. Each section links to its sources.

Overview¶

Survey	Wavelengths	Resolution R	LSF representation
DESI	3600–9800 Å	2000–5000	Resolution matrix per spectrum
BOSS	3600–10,400 Å	1560–2650	Gaussian \(\sigma\) per pixel (`WAVEDISP`)
LAMOST LRS	3700–9000 Å	1800 at 5500 Å	None published
LAMOST MRS	4950–5350 Å and 6300–6800 Å	7500 at 5163 Å and 6593 Å	None published

The catalogue stores each survey on the wavelength grid of its input files (How the catalogue is built). DOROTHY does not use the LSF.

Resolution and line width¶

\[\begin{split} \begin{aligned} R &= \frac{\lambda}{\Delta\lambda} , \\ \sigma_\lambda &= \frac{\Delta\lambda}{2\sqrt{2\ln 2}} \approx \frac{\Delta\lambda}{2.355} , \\ \sigma_v &= \frac{c}{2.355\,R} \approx \frac{127{,}300\ \mathrm{km\,s^{-1}}}{R} . \end{aligned} \end{split}\]

\(\Delta\lambda\) is the full width at half maximum (FWHM) of the LSF at wavelength \(\lambda\). \(\sigma_\lambda\) is the standard deviation of a Gaussian with that FWHM, and \(\sigma_v\) the same width as a velocity, with \(c = 299{,}792\ \mathrm{km\,s^{-1}}\). For example, \(R = 1800\) gives \(\sigma_v \approx 71\ \mathrm{km\,s^{-1}}\), and \(R = 7500\) gives \(\sigma_v \approx 17\ \mathrm{km\,s^{-1}}\).

DESI¶

DESI has three spectrograph channels, whose wavelength ranges overlap (DESI Collaboration et al. 2022).

Channel	Wavelengths
Blue (B)	3600–5930 Å
Red (R)	5600–7720 Å
Infrared (Z)	7470–9800 Å

The resolution rises with wavelength (DESI spectrograph).

Wavelengths	R
3600–5550 Å	2000–3200
5550–6560 Å	3200–4100
6560–9800 Å	4100–5000

Each spectrum has its own resolution matrix, stored as its diagonals in the B_RESOLUTION, R_RESOLUTION and Z_RESOLUTION extensions of the coadd files, one per channel.

BOSS¶

BOSS has two channels, split by a dichroic at about 6000 Å (BOSS spectrograph).

Channel	Wavelengths (at least)	R
Blue	3600–6350 Å	1560 at 3700 Å to 2270 at 6000 Å
Red	5650–10,000 Å	1850 at 6000 Å to 2650 at 9000 Å

The WAVEDISP extension (HDU 4) of the spCFrame files holds the LSF at every pixel. It is the \(\sigma\) of a fitted Gaussian, in pixels of the log-wavelength grid. A pixel spans 10⁻⁴ in log₁₀ λ. The wavelengths are in vacuum.

LAMOST LRS¶

LAMOST low-resolution spectra cover 3700–9000 Å with R = 1800 at 5500 Å (DR9 data description). The pixels are log-linear, with a spacing of 10⁻⁴ in log₁₀ λ, and the wavelengths are in vacuum. The data release publishes no LSF.

LAMOST MRS¶

LAMOST medium-resolution spectra have two arms: blue, 4950–5350 Å, and red, 6300–6800 Å. Each arm has R = 7500 at its reference wavelength: 5163 Å for blue and 6593 Å for red (DR7 data description). The wavelengths are in vacuum. The data release publishes no LSF. The catalogue stores both arms (File formats).

Sources¶

The values on this page come from these sources. The survey papers are DESI Collaboration et al. (2022) (DESI), Smee et al. (2013) (BOSS) and Cui et al. (2012), Liu et al. (2020) (LAMOST).

Survey	Values	LSF data
DESI	DESI Collaboration et al. (2022), Figure 14 (channel ranges); DESI spectrograph; Guy et al. 2023, arXiv:2209.14482	coadd file data model
BOSS	BOSS spectrograph	spCFrame data model
LAMOST LRS	LAMOST DR9 low-resolution data description	None
LAMOST MRS	LAMOST DR7 medium-resolution data description	None