Survey¶

The Survey component offers convenient data on the VAST Pilot Survey or the ASKAP telescope.

These include:

• A list of the supported pilot survey epochs in VAST Tools.
• A list of what fields are contained in each pilot survey epoch.
• The centres of each pilot survey tile field.
• The ASKAP observing location.

The Survey component also contains two classes Fields and Image that may be useful to users but are mainly used for internal purposes in other components.

Using the Survey Component¶

Code reference.

Survey Component Functions¶

get_askap_observing_location¶

Code reference.

This function returns the location of the ASKAP telescope in the form of an astropy EarthLocation instance. There are no arguments available to the function. The values returned are the longitude, latitude and height.

from vasttools.survey import get_askap_observing_location

askap_loc = get_askap_observing_location()
print(askap_loc)
#output
(-2556451.02811979, 5096903.35306894, -2848173.88980533) m

get_fields_per_epoch_info¶

Code reference.

This function returns a pandas dataframe containing the field that are present in each epoch along with the SBID value and the date of the observation. There are no arguments available to the function.

from vasttools.survey import get_fields_per_epoch_info

field_info = get_fields_per_epoch_info()
field_info

epoch8_fields = field_info.loc['8']

get_supported_epochs¶

Code reference.

Returns a list of the currently supported VAST Pilot Survey epochs.

from vasttools.survey import get_supported_epochs

epochs = get_supported_epochs()
print(epochs)

['00', '01', '02', '03x', '04x', '05x', '06x', '07x', '08', '09', '10x', '11x', '12', '13']

load_field_centres¶

Code reference.

Returns a pandas dataframe containing three columns:

• field is the name of the field in question.
• centre-ra is the centre right ascension coordinate of the respective field in degrees.
• centre-dec is the centre declination coordinate of the respective field in degrees.

from vasttools.survey import load_field_centres

field_centres = load_field_centres()

load_fields_skycoords¶

Code reference.

Ths function allows the user to load SkyCoord objects for all beam centres for a given epoch.

from vasttools.survey import load_fields_skycoords

epoch_1_scs = load_fields_skycoords('1')

load_fields_file¶

Code reference.

This function allows the user to load the packaged field information file of the requested epoch as a pandas dataframe. The included columns are:

• SBID
• FIELD_NAME
• BEAM
• RA_HMS
• DEC_DMS
• DATEOBS
• DATEEND
• NINT
• BMAJ
• BMIN
• BPA

from vasttools.survey import load_fields_file

epoch_1_fields = load_fields_file('1')

Fields Class¶

Code reference.

The Fields class is designed to act as a representation of the fields contained in an epoch. It is primarily used by the Query component to perform the act of finding matches to provided sky coordinates. The class holds details of the fields at the individual beam level rather than the 'tile field' as a whole.

Initialising a Fields Instance¶

Code reference.

A Fields instance is initialised by declaring the epoch that is wanted to be represented. This is demonstrated below.

from vasttools.survey import Fields

epoch_8 = Fields('8')

Fields Attributes¶

Code reference

The following attributes are available on a Fields instance.

Fields.fields
This is a pandas dataframe which contains the information of each individual beam that makes up each tile which in turn makes up the complete epoch. The columns of the dataframe are:

• SBID The SBID of the observation.
• FIELD_NAME The field name of which the beam belongs to (there are 36 beams in each tile).
• BEAM The designated beam number.
• RA_HMS The right ascension coordinate of the centre of the beam pointing, in sexagesimal format.
• DEC_DMS The declination coordinate of the centre of the beam pointing, in sexagesimal format.
• DATEOBS The start date and time of the observation.
• DATEEND The end date and time of the observation.
• NINT The number of integrations.
• BMAJ The size of the major axis of the restoring beam in arcsec.
• BMIN The size of the minor axis of the restoring beam in arcsec.
• BPA The position angle of the restoring beam in degrees.

epoch_8.fields

Fields.direction
An astropy SkyCoord instance containing the centres of all the beams in the epoch.

epoch_8.direction

<SkyCoord (ICRS): (ra, dec) in deg
    [(260.56342917, -30.86106111), (261.78657083, -30.86106111),
     (260.55655   , -31.911     ), ..., (329.39785   , -62.90618889),
     (329.59647083, -61.86028056), (329.78020833, -60.81385   )]>

Image Class¶

Code reference.

The Image class provides a template of representing an individual ASKAP image, allowing for easy access to properties and common tasks.

Initialising an Image Instance¶

Code reference.

The initialisation of an Image instance requires four inputs:

• field The field name of the image as a string.
• epoch The epoch of the image as a string.
• stokes The stokes value of the image.
• base_folder The base directory of the VAST Pilot Survey.

See the code reference for further options.

from vasttools.survey import Image

my_image = Image('VAST_1724-31A', '8', 'I', '/path/to/VAST/release')

my_custom_image = Image('field1', '1', 'I', 'None', path='/path/to/the/image.fits)

my_custom_image = Image(..., rmspath='/path/to/the/rmsimage.fits)

Image Attributes¶

Once initialised, an Image instance has access to a number of attributes, such as the image data, header, field and epoch. Please refer to the Code reference section for a full list of attributes available.

Image Methods¶

There are two methods available with the Image class.

Image.get_rms_img

This method checks for the presense of the rms image, and if it is available the rms image information, including the data and header are attached to the instance.

my_image.get_rms_img()

Image.measure_coord_pixel_values
A method to measure the pixel values of the image at the provided sky coordinates. The coordinates should be provided in the form of an astropy SkyCoord instance. By default the pixel values are taken from the image data. This can be changed to the rms data by setting rms=True. The values are returned in the form of a numpy array and will be in the units of the image (very commonly Jy). Be aware that NaN values can be returned if the coordinate falls within the null area of the image.

import astropy.units as u

from astrpy.coordinates import SkyCoord

my_coords = SkyCoord(
  [206.058966, 207.711250, 206.438710], [-57.278384, -57.645140, -57.486640], unit=(u.deg, u.deg)
)

pixel_values = my_image.measure_coord_pixel_values(my_coords)

pixel_values = my_image.measure_coord_pixel_values(my_coords, rms=True)