Retrieving data from the current widget’s view

So far, we’ve seen how to send information (tables, MOCs, …) into the widget. The other way also works! This notebook contains a list of methods to extract diverse information about the current view. However, here are several information about retrieving data from the widget:

• when the view is modified programmatically (i.e. not by clicking on the buttons), the update of its properties is done between cell execution. This means that you’ll see a WidgetCommunicationError when you try to modify the view and retrieve information about it in the same cell. Simply switch the property-reading to the next cell and everything will work as intended!

• if you generate different views of the same Aladin() instances – either by calling display multiple times, or by using the Create new view for cell output button in Jupyter, the information contained in wcs and fov_xy will always correspond to the most recently created view.

from ipyaladin import Aladin

aladin = Aladin(fov=5, height=600, target="M31")
aladin

Getting the current WCS

The World Coordinates System (WCS) describes the field of view, the projection, and it’s rotation. It is returned as an astropy.coordinates.WCS object.

aladin.wcs  # Recover the current WCS

WCS Keywords

Number of WCS axes: 2
CTYPE : 'RA---SIN' 'DEC--SIN'
CRVAL : 10.6847083 41.26875
CRPIX : 843.0 300.5
PC1_1 PC1_2  : 1.0 0.0
PC2_1 PC2_2  : 0.0 1.0
CDELT : -0.0029673590504451 0.002967359050445104
NAXIS : 1685  600

If you edit the view either by modifiing the widget through its interface, or programmatically:

aladin.height = 800
aladin.survey = "CDS/P/PLANCK/R2/HFI/color"
aladin.target = "LMC"
aladin.frame = "Galactic"
aladin.fov = 50

The wcs is updated and you can print its new value in the next cell:

aladin.wcs

WCS Keywords

Number of WCS axes: 2
CTYPE : 'RA---SIN' 'DEC--SIN'
CRVAL : 80.89416999999995 -69.75611
CRPIX : 843.0 400.5
PC1_1 PC1_2  : 1.0 0.0
PC2_1 PC2_2  : 0.0 1.0
CDELT : -0.0029673590504451 0.002967359050445104
NAXIS : 1685  800

If you try to recover the value in the same cell, you’ll get a WidgetCommunicationError error. This is because the calculation of the WCS is done by Aladin Lite between cell executions.

Getting the field of view

The field of view is printed in the bottom left corner of the view. You can grab the two values with:

aladin.fov_xy  # Recover the current field of view for the x and y axis

(<Angle 5. deg>, <Angle 2.37388724 deg>)