Noise

Add foreground emission to datacubes.

NoiseModel

Source code in fastbox/noise.py

class NoiseModel(object):

    def __init__(self, box):
        """
        An object to manage the addition of noise on top of a realisation 
        of a density field in a box.

        Parameters:
            box (CosmoBox):
                Object containing a simulation box.
        """
        self.box = box


    def realise_radiometer_noise(self, Tinst, tp, fov, Ndish, redshift=None):
        """
        Realisation of white noise based on the radiometer equation.
        The instrument temperature is added to a frequency-dependent sky 
        temperature, T_sky ~ 60 K (nu / 300 MHz)^-2.5. 

        Parameters:
            Tinst (float):
                Instrument temperature, in Kelvin. This is added to the sky 
                temperature to form the total system temperature.

            tp (float):
                Total integration time per pointing, in hours.

            fov (float):
                Field of view (i.e. solid angle per pointing) in deg^2.

            Ndish (int):
                Number of dishes/receivers combined to reach effective noise level.

            redshift (float, optional):
                Redshift to evaluate the centre of the box at. Default: Same value 
                as `self.box.redshift`.

        Returns:
            noise (array_like):
                Realisation of noise in datacube. Units in mK.
        """
        # Get frequency array and channel width (MHz)
        freqs = self.box.freq_array(redshift=redshift)
        dnu = np.abs(freqs[1] - freqs[0])

        # Convert time per pointing to seconds
        tp *= 3600. # hrs to sec

        # Calculate time per angular resolution element (i.e. per pixel)
        ang_x, ang_y = self.box.pixel_array(redshift=redshift)
        dtheta = ang_x[1] - ang_x[0] # pixel angular size, degrees
        t_res = tp * dtheta**2. / fov # fov also in deg^2

        # Get Tsys as a function of frequency
        Tsky = 60e3 * (freqs / 300.)**(-2.5) # mK
        Tsys = Tinst*1e3 + Tsky # mK

        # Noise rms from the radiometer equation (fn. of frequency only)
        sigma_rms = Tsys / np.sqrt(Ndish * t_res * (dnu * 1e6))

        # Generate unit white noise and multiply by noise rms
        noise = np.random.normal(0., 1., self.box.Kx.shape)
        noise *= sigma_rms[np.newaxis,np.newaxis,:]
        return noise

__init__(self, box) special

An object to manage the addition of noise on top of a realisation of a density field in a box.

Parameters:

Name	Type	Description	Default
box	CosmoBox	Object containing a simulation box.	required

Source code in fastbox/noise.py

def __init__(self, box):
    """
    An object to manage the addition of noise on top of a realisation 
    of a density field in a box.

    Parameters:
        box (CosmoBox):
            Object containing a simulation box.
    """
    self.box = box

realise_radiometer_noise(self, Tinst, tp, fov, Ndish, redshift=None)

Realisation of white noise based on the radiometer equation. The instrument temperature is added to a frequency-dependent sky temperature, T_sky ~ 60 K (nu / 300 MHz)^-2.5.

Parameters:

Name	Type	Description	Default
Tinst	float	Instrument temperature, in Kelvin. This is added to the sky temperature to form the total system temperature.	required
tp	float	Total integration time per pointing, in hours.	required
fov	float	Field of view (i.e. solid angle per pointing) in deg^2.	required
Ndish	int	Number of dishes/receivers combined to reach effective noise level.	required
redshift	float	Redshift to evaluate the centre of the box at. Default: Same value as self.box.redshift.	None

Returns:

Type	Description
noise (array_like)	Realisation of noise in datacube. Units in mK.

Source code in fastbox/noise.py

def realise_radiometer_noise(self, Tinst, tp, fov, Ndish, redshift=None):
    """
    Realisation of white noise based on the radiometer equation.
    The instrument temperature is added to a frequency-dependent sky 
    temperature, T_sky ~ 60 K (nu / 300 MHz)^-2.5. 

    Parameters:
        Tinst (float):
            Instrument temperature, in Kelvin. This is added to the sky 
            temperature to form the total system temperature.

        tp (float):
            Total integration time per pointing, in hours.

        fov (float):
            Field of view (i.e. solid angle per pointing) in deg^2.

        Ndish (int):
            Number of dishes/receivers combined to reach effective noise level.

        redshift (float, optional):
            Redshift to evaluate the centre of the box at. Default: Same value 
            as `self.box.redshift`.

    Returns:
        noise (array_like):
            Realisation of noise in datacube. Units in mK.
    """
    # Get frequency array and channel width (MHz)
    freqs = self.box.freq_array(redshift=redshift)
    dnu = np.abs(freqs[1] - freqs[0])

    # Convert time per pointing to seconds
    tp *= 3600. # hrs to sec

    # Calculate time per angular resolution element (i.e. per pixel)
    ang_x, ang_y = self.box.pixel_array(redshift=redshift)
    dtheta = ang_x[1] - ang_x[0] # pixel angular size, degrees
    t_res = tp * dtheta**2. / fov # fov also in deg^2

    # Get Tsys as a function of frequency
    Tsky = 60e3 * (freqs / 300.)**(-2.5) # mK
    Tsys = Tinst*1e3 + Tsky # mK

    # Noise rms from the radiometer equation (fn. of frequency only)
    sigma_rms = Tsys / np.sqrt(Ndish * t_res * (dnu * 1e6))

    # Generate unit white noise and multiply by noise rms
    noise = np.random.normal(0., 1., self.box.Kx.shape)
    noise *= sigma_rms[np.newaxis,np.newaxis,:]
    return noise