Spectrograph simulation

class qmostetc.Spectrograph(wavelength, widths, efficiency, fwhm=None, dark_flux=<Quantity 0. electron / s>, readout_noise=<Quantity 0. electron>, gain=<Quantity 1. electron / adu>, saturation=<Quantity inf adu>)

Spectrograph properties

The spectrograph is used to convert the fiber photon flux into an electron flux on the CCD bins. It also holds the dark current, readout noise and the ADC gain to calculate the response on an exposure. The main function here is rebin(), which does all the necessary steps.

The most convenient way to create a 4MOST standard spectrograph is the function Spectrograph.qmost.

Parameters:

wavelengthastropy.units.Quantity

Left borders of wavelenght bins [nm]

widthsastropy.units.Quantity

Bin widths for each wavelength bin [nm]

efficiencyastropy.units.Quantity

Spectrograph specific efficiency for each wavelength bin [electron/photon]

fwhmastropy.units.Quantity

Resolution (FWHM) for each wavelength bin [nm] (optional)

dark_currentastropy.units.Quantity

Dark current [electron/s] (optional)

readout_noiseastropy.units.Quantity

Readout noise [electron] (optional)

gainastropy.units.Quantity

ADC gain [electron/adu] (optional)

saturationastropy.units.Quantity

Saturation value [adu] (optional)

Examples

This example uses the green arm of the LRS to rebin a single line at 600 nm:

>>> import numpy as np
>>> import astropy.units as u
>>> from qmostetc import Spectrograph, Spectrum
>>> spec = Spectrograph.qmost('lrs', 'green')
>>> wl = np.arange(590., 610., 0.01) * u.nm
>>> line = np.zeros(wl.shape) * u.ph / (u.nm * u.s)
>>> line[1000] = 1e3 * u.ph / (u.nm * u.s)
>>> for i in range(1000-2,1000+3):
...     print(f'{wl[i]:.2f}: {line[i]:.2g}')
599.98 nm: 0 ph / (nm s)
599.99 nm: 0 ph / (nm s)
600.00 nm: 1e+03 ph / (nm s)
600.01 nm: 0 ph / (nm s)
600.02 nm: 0 ph / (nm s)
>>> flux = spec.rebin(Spectrum(wl, line))
>>> mx = np.argmax(flux.flux)
>>> for i in range(mx-3,mx+4):
...     print(f'{flux.wavelength[i]:.2f}: {flux.flux[i]:.2g}')
599.88 nm: 0.03 electron / s
599.92 nm: 0.23 electron / s
599.95 nm: 0.78 electron / s
599.98 nm: 1.2 electron / s
600.02 nm: 0.78 electron / s
600.05 nm: 0.23 electron / s
600.08 nm: 0.03 electron / s
>>> print(f'Total flux: {flux.flux.sum():.1f}')
Total flux: 3.2 electron / s
>>> print('Wavelength average: '
...      + f'{np.average(flux.wavelength, weights=flux.flux):.2f}')
Wavelength average: 599.98 nm

as_filter()

Return the efficiency of the spectrograph as a filter

This is used as filter to calculate the arm magnitudes

Returns:

Spectrum

The efficiency of the spectrograph

Examples

Show the magnitude of a spectrum in the LRS/blue arm:

>>> import astropy.units as u
>>> from qmostetc import Spectrograph, SEDTemplate
>>> spec = SEDTemplate('Pickles_G0V')(15*u.ABmag, 'GAIA_GAIA2r.G')
>>> lrs_green = Spectrograph.qmost('lrs', 'green')
>>> spec.get_mag(u.ABmag, lrs_green.as_filter())
<Magnitude 14.89... mag(AB)>

limit_wavelength(wl_range)

Create a new Spectrograph with a limited wavelength

Parameters:

wl_rangeastropy.units.Quantity

Pair with (lower, upper) limits of the wavelength [nm]

Returns:

Spectrograph

A new spectrograph with the limited wavelengths. If the paramter was None, the original Spectrograph object is returned.

rebin(spectrum)

Rebin a given spectrum

This takes a flux spectrum (per nm) at the fiber input (i.e. with atmospheric transmission, telescope area and efficiency, fiber area and fiber coupling applied) and calculates the measured flux spectrum (per bin) at the CCD.

The following effects are applied here:

• Rebinning to the final spectral bins, and applying the bin width

• Spectral resolution (optional)

• Spectrograph througput and the CCD quantum efficiency

The exposure time is not applied here.

The following plot shows how the rebinning is applied on the 4MOST spectrograph arms:

Parameters:

spectrumSpectrum

Input spectrum [ph / (nm s)]

Returns:

astropy.units.Quantity

Measured flux per bin on CCD [electron/s]

static qmost(spectrograph, arm, spectral_binning=1, fname=None)

Standard 4MOST spectrograph

The spectrograph data are taken from a FITS provided by TOAD. If no file name is give, the data file provided with this package is used.

Parameters:

spectrographstr

Spectrograph name. Either “lrs” or “hrs”.

armstr

Spectrograph arm name. One of “blue”, “green”, “red”.

spectral_binningint

Spectral binning for CCD readout. The spacial binning during readout is fixed to 1, the number of spacial pixels during spectrum extraction is fixed to 5.

fnamepathlib.Path or str

TOAD file name. Defaults to the file provided with TOAD.

Returns:

Spectrograph

Standard spectrograph with the data of the given data file

Default 4MOST spectrograph properties

The spectrograph properties together with the telescope properties are generated with TOAD in a separate project. All data are stored in the file qmostetc/data/TOAD_4FS_ETC_interface_data.fits which is a FITS file with several extensions. The extensions relevant for the spectrographs are:

Material

The transmission of the system, including telescope mirrors and spectrograph. Relevant columns:

• spectrograph: Spectrograph (lrs or hrs)

• spectrograph_arm: Spectrograph arm (one of blue, green, red)

• design_wave_interval: Wavelength interval the spectrograph arm is designed for [nm].

• wavelength_vector: Wavelength information in [nm].

• transmission_vector: Transmission curve [counts/photon]

• extraction_vector: Efficiency to extract the spectral bins from the image.

The spectrograph takes the product of transmission and extraction to convert the photon flux into counted electrons.

Design wavelength intervals:

• hrs / blue: 392.6 nm … 435.5 nm

• hrs / green: 516.0 nm … 573.0 nm

• hrs / red: 610.0 nm … 679.0 nm

• lrs / blue: 370.0 nm … 554.0 nm

• lrs / green: 524.0 nm … 721.0 nm

• lrs / red: 691.0 nm … 950.0 nm

Resolution

Spectrograph resolution curve. Relevant columns:

• spectrograph: Spectrograph (lrs or hrs)

• spectrograph_arm: Spectrograph arm (one of blue, green, red)

• wavelength_vector: Wavelength vector [nm]

• fwhm_vector: Full Width Half Maximum vector [nm]

Dispersion

This binary table contains the pixel position on the detector. The columns which are relevant are:

• spectrograph: Spectrograph (lrs or hrs).

• spectrograph_arm: Spectrograph arm (one of blue, green, red).

• wavelength_vector: Wavelength information in [nm].

• pixel_width_vector: Spectral width of the pixels [nm].

Detector

CCD properties. The columns relevant for the ETC are:

• spectrograph: Spectrograph (lrs or hrs)

• spectrograph_arm: Spectrograph arm (one of blue, green, red)

• dark_current: The dark current in electrons per pixel per second at the operation temperature of the instrument [count/s].

• read_noise: The read noise per pixel, including the amplifier electronics [count].

• gain: ADU gain [count/adu]

• saturation: Saturation value [adu]

spectrograph	spectrograph_arm	dark_current	read_noise	gain	saturation
hrs	blue	0.000173	2.77646	1.0515	196193
hrs	green	0.000308	2.7193525	1.0623	194814
hrs	red	0.000225	2.31	1.0726	193114
lrs	blue	0.000240	2.3729325	1.0681	199112
lrs	green	0.000113	2.3781925	1.0734	192897
lrs	red	0.000312	2.2213125	1.0633	187379