Observatories and observations¶

Observatories glue together Atmosphere, Telescope and Spectrograph properties as the base for observations. They are callable with observing conditions (airmass, seeing, moon brightness) and then create an “observation”.

There are two implementations of Observatory: The SimpleObservatory uses a single spectrograph that can be specified in the constructor, while the QMostObservatory combines all three arms of a 4MOST “LRS” or “HRS” spectrograph, the standard Paranal Atmosphere and the Vista telescope properties.

Observations can get a point like, flat, or sersic targets set, and then estimate the response to a specified exposure time.

See also

The class QMostETC provides the calculation of the exposure time based on a specified ruleset.

class qmostetc.SimpleObservatory(atmosphere, telescope, spectrograph, wl_range=None)¶

Object to put all components together for a simple observatory

This includes the atmosphere, the telescope, and one spectrograph. The object is callable; calling it with airmass, seeing, and moon brightness will create a new SimpleObservation. The atmosphere is used only in the part relevant for the spectrograph.

Parameters:

atmosphereAtmosphere: Atmospheric properties, for example Atmosphere.paranal().
telescopeTelescope: Telescope and fiber throughput, for example Telescope.vista()
spectrographSpectrograph: Spectrograph, may be f.e. built with Spectrograph.qmost()
wl_rangeastropy.units.Quantity: Optional wavelength range [nm] to limit the detection range. If set to None (default), the original range of the spectrograph is used.

Examples

This shows a complete observation of a template-generated point source:

>>> from qmostetc import SEDTemplate
>>> observatory = SimpleObservatory(Atmosphere.paranal(),
...                                 Telescope.vista(),
...                                 Spectrograph.qmost('hrs', 'red'))
>>> obs = observatory(45*u.deg, 1.3*u.arcsec, 'gray')
>>> pck = SEDTemplate('Pickles_G0V')
>>> flux = pck(15*u.ABmag, 'GAIA2r.G')
>>> obs.set_target(flux, 'point')
>>> tbl = obs.expose(300*u.s)
>>> max_snr = (tbl['target'] / tbl['noise']).argmax()
>>> print(f'Best SNR at {tbl[max_snr]["wavelength"]:.3f}'
...       + f' (Signal: {tbl[max_snr]["target"]:.0f};'
...       + f' Noise: {tbl[max_snr]["noise"]:.0f})')
Best SNR at 650.772 nm (Signal: 153 electron; Noise: 14 electron)

limit_wavelength(wl_range)¶

Create a new observatory with a limited wavelength

Parameters:

wl_rangeastropy.units.Quantity: Pair with (lower, upper) limits of the wavelength [nm]

Returns:

SimpleObservatory: A new observatory with the limited wavelengths. If the paramter was None, the original SimpleObservatory object is returned.

property sensitivity¶

Return the sensitivity function

This function is created from

the spectrograph sensitivity,

the spectrograph binwidth,

the telescope throughput,

the telescope area

It converts the photoelectron counts into the flux at the entrance of the telescope.

The values of the array correspond to the wavelenghths that can be retrieved with wavelength().

This plot shows the sensitivity that is used by default for the 4MOST spectrographs:

Returns:

astropy.units.Quantity: Sensitivity factors [erg/(cm² electron)]

Examples

Give the sensitivity array for the blue arm:

>>> observatory = QMostObservatory('lrs')
>>> observatory['blue'].sensitivity
<Quantity [3.55434960e-15, ..., 3.61695626e-15] erg / (electron cm2)>

__call__(airmass, seeing, moon_brightness=None)¶

Start a new observation

Parameters:

airmassfloat or astropy.units.Quantity: Airmass to use (default 1.0). The airmass may also be given as zenith angle [deg].
seeingastropy.units.Quantity: Optical seeing as a single Quantity at zenith [deg].
moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here. If not specified here, the sky spectrum must be given later with SimpleObservation.set_moon_brightness() or with SimpleObservation.set_sky().

Returns:

SimpleObservation: A new observation, to which a target can be set and the exposure time can be applied.

class qmostetc.SimpleObservation(observatory, airmass, seeing, moon_brightness=None)¶

Set up an observation with certain ambient conditions and pointing

Parameters:

observatorySimpleObservatory: Observatory that cause this observation
airmassfloat or astropy.units.Quantity: Airmass to use (default 1.0). The airmass may also be given as zenith angle [deg].
seeingastropy.units.Quantity: Optical seeing as a single Quantity at zenith [deg].
moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here. If not specified here, the sky spectrum must be given later with SimpleObservation.set_moon_brightness() or with SimpleObservation.set_sky().

Attributes:

observatorySimpleObservatory: Observatory for this observation
skyastropy.units.Quantity: Sky emission measured at the fiber [photon/(s m² nm)]. The corresponding wavelengths are in Atmosphere.wavelength.
airmassfloat: Airmass
seeingastropy.units.Quantity: Target seeing [deg]
exposuresdict of (str, Exposure): Dictionary with the spectrograph (arm) name as key and the corresponding Exposure as value.

set_moon_brightness(moon_brightness)¶

Set a new sky spectrum based on a moon brightness

Parameters:

moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here.

set_sky(sky)¶

Set the sky brackground

This is an alternative for SimpleObservation.set_moon_brightness() to set the sky background.

Parameters:

fluxSpectrum: Photon flux of the sky [ph/(s cm² nm arcsec²].

set_target(flux, target_shape, sersic_radius=None, sersic_index=None)¶

Set the target object

Parameters:

fluxSpectrum: Photon flux of the target [ph/(s cm² nm) for point sources, and ph/(s cm² nm arcsec²) for extended sources].
target_shapestr: Target shape. One of ‘flat’, ‘point’, ‘sersic’.
sersic_radiusastropy.units.Quantity: Sersic effective radius (only for Sersic target shape) [arcsec].
sersic_indexfloat: Sersic index (only for Sersic target shape).

expose(exptime, nexp=1, var=None)¶

Create the result for a defined exposure time

Parameters:

exptimeastropy.units.Quantity

Exposure time [s]

nexpint

Number of exposures. Defaults to 1.

varstr or None

Requested column (default None). If this is used, only a single quantity array is returned containing just the values of this column. Allowed values:

target: Target signal count [electron]

sky: Sky background count [electron]

noise: Noise count [electron]

bg_noise_flux: Background noise flux [electron/s]

snr: Signal to noise ratio

Returns:

astropy.table.QTable (if no columns were specified)

Resulting table with the following columns:

wavelength: Left borders of wavelenght bins [nm]

binwidth: Wavelength bin width [nm]

efficiency: Spectrograph efficiency [electron/photon]

gain: Spectrograph gain [electron/adu]

target: Target signal count [electron]

sky: Sky background count [electron]

dark: CCD dark current [electron]

ron: CCD readout noise [electron]

noise: Noise count [electron]

astropy.units.Quantity (if a specific column was specified)

Resulting array for the requested column

property wavelength¶: astropy.units.Quantity : Left borders of wavelenght bins [nm]

class qmostetc.QMostObservatory(spectrograph, spectral_binning=1)¶

Object to put all components together for 4MOST

This includes the atmosphere, the telescope, and the spectrograph. The object is callable; calling it with airmass, seeing, and moon brightness will create a new QMostObservation.

Parameters:

spectrographstr: Spectrograph name (lrs or hrs)
spectral_binningint: Spectral binning for CCD readout (1, 2 or 4). The spacial binning during readout is fixed to 1, the number of spacial pixels during spectrum extraction is fixed to 5.

Examples

This shows a complete observation of a template-generated point source:

>>> from qmostetc import SEDTemplate
>>> observatory = QMostObservatory('hrs')
>>> obs = observatory(45*u.deg, 1.3*u.arcsec, 'gray')
>>> pck = SEDTemplate('Pickles_G0V')
>>> flux = pck(15*u.ABmag, 'GAIA2r.G')
>>> obs.set_target(flux, 'point')
>>> tbl = obs.expose(300*u.s)
>>> max_snr = (tbl['target'] / tbl['noise']).argmax()
>>> print(f'Best SNR at {tbl[max_snr]["arm"]} arm,'
...       + f' {tbl[max_snr]["wavelength"]:.3f}'
...       + f' (Signal: {tbl[max_snr]["target"]:.0f};'
...       + f' Noise: {tbl[max_snr]["noise"]:.0f})')
Best SNR at red arm, 650.772 nm (Signal: 153 electron; Noise: 14 electron)

Similarly, this is a complete observation for a template-generated flat source:

>>> from qmostetc import SEDTemplate
>>> observatory = QMostObservatory('lrs')
>>> obs = observatory(45*u.deg, 1.3*u.arcsec, 'gray')
>>> kin = SEDTemplate('Kinney_ell')
>>> flux = kin(15*u.ABmag, 'GAIA2r.G')
>>> flux = flux / u.arcsec**2  # The flat flux is per arcsec
>>> obs.set_target(flux, 'flat')
>>> tbl = obs.expose(300*u.s)
>>> max_snr = (tbl['target'] / tbl['noise']).argmax()
>>> print(f'Best SNR at {tbl[max_snr]["arm"]} arm,'
...       + f' {tbl[max_snr]["wavelength"]:.3f}'
...       + f' (Signal: {tbl[max_snr]["target"]:.0f};'
...       + f' Noise: {tbl[max_snr]["noise"]:.0f})')
Best SNR at red arm, 816.189 nm (Signal: 6350 electron; Noise: 80 electron)

__call__(airmass, seeing, moon_brightness=None)¶

Start a new observation

Parameters:

airmassfloat or astropy.units.Quantity: Airmass to use (default 1.0). The airmass may also be given as zenith angle [deg].
seeingastropy.units.Quantity: Optical seeing as a single Quantity at zenith [deg].
moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here. If not specified here, the sky spectrum must be given later with QMostObservation.set_moon_brightness() or with QMostObservation.set_sky().

Returns:

QMostObservation: A new observation, to which a target can be set and the exposure time can be applied.

class qmostetc.QMostObservation(observatory, airmass, seeing, moon_brightness)¶

Set up an observation with certain ambient conditions and pointing

Parameters:

observatoryQMostObservatory: Observatory that caused this observation
airmassfloat or astropy.units.Quantity: Airmass to use (default 1.0). The airmass may also be given as zenith angle [deg].
seeingastropy.units.Quantity: Optical seeing as a single Quantity at zenith [deg].
moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here. If not specified here, the sky spectrum must be given later with QMostObservation.set_moon_brightness() or with QMostObservation.set_sky().

Attributes:

observatoryQMostObservatory: Observatory for this observation
skyastropy.units.Quantity: Sky emission measured at the fiber [photon/(s m² nm)]. The corresponding wavelengths are in Atmosphere.wavelength.
airmassfloat: Airmass
seeingastropy.units.Quantity: Target seeing [deg]
exposuresdict of (str, Exposure): Dictionary with the spectrograph (arm) name as key and the corresponding Exposure) as value.

set_moon_brightness(moon_brightness)¶

Set a new sky spectrum based on the moon brightness

Parameters:

moon_brightnessstr or astropy.units.Quantity: Moon brightness. One of 'superbright', 'bright', 'gray' or 'dark'. Alternatively, the moon-sun separation [deg] can be given directly here.

set_sky(sky)¶

Set the sky brackground

This is an alternative for QMostObservation.set_moon_brightness() to set the sky background.

Parameters:

fluxSpectrum: Photon flux of the sky [ph/(s cm² nm arcsec²].

set_target(flux, target_shape, sersic_radius=None, sersic_index=None)¶

Set the target object

Parameters:

fluxSpectrum: Photon flux of the target [ph/(s cm² nm) for point sources, and ph/(s cm² nm arcsec²) for extended sources].
target_shapestr: Target shape. One of ‘flat’, ‘point’, ‘sersic’.
sersic_radiusastropy.units.Quantity: Sersic effective radius (only for Sersic target shape) [arcsec].
sersic_indexfloat: Sersic index (only for Sersic target shape).

property mean_loss¶

Return the mean atmospheric and telescope transmission loss

This is set by the set_target() and can be used to calculate a kind of “fiber magnitude”.

Note that this is very approximate, as it just averages the average transmission for each individual observation. It also just includes the atmospheric losses and the geometrical fiber fraction, no optical or other losses.

Examples

Calculate the fiber magnitude for a standard Pickles observation:

>>> from qmostetc import SEDTemplate
>>> observatory = QMostObservatory('lrs')
>>> obs = observatory(1.05, 0.8*u.arcsec, 'gray')
>>> template = SEDTemplate('Kinney_s0')
>>> mag = 20*u.ABmag
>>> target_spectrum = template(mag, 'GAIA2r.G')
>>> obs.set_target(target_spectrum, 'sersic', 1*u.arcsec, 1)
>>> print(mag + u.Magnitude(obs.mean_loss*u.one))
21.89... mag(AB)

expose(exptime, nexp=1)¶

Create the result table for a defined exposure time

Parameters:

exptimeastropy.units.Quantity: Exposure time [s]
nexpint: Number of exposures. Defaults to 1.

Returns:

astropy.table.QTable

Resulting table with the following columns:

arm : Spectrograph arm (blue, green, or red)

wavelength: Left borders of wavelenght bins [nm]

binwidth: Wavelength bin width [nm]

efficiency: Spectrograph efficiency [electron/photon]

gain: Spectrograph gain [electron/adu]

target: Target signal count [electron]

sky: Sky background count [electron]

dark: CCD dark current [electron]

ron: CCD readout noise [electron]

noise: Noise count [electron]

Observatories and observations¶

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