How to rotate and slice a cube for pV diagrams: Difference between revisions
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A position-velocity diagram is typically along slice along a direction in the RA/DEC plane with a given position angle. The y-axis consist of the velocities along that slice. How to do this? Currently there's no task but one can use the tools in CASA. | A position-velocity diagram is typically along slice along a direction in the RA/DEC plane with a given position angle. The y-axis consist of the velocities along that slice. How to do this? Currently there's no task but one can use the tools in CASA. | ||
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1) rotate the input cube by a given position angle (to align the slice in the vertical direction): | 1) rotate the input cube by a given position angle (to align the slice in the vertical direction): | ||
<source lang="python"> | |||
#open the image | |||
ia.open('original.image') | |||
#rotate by the position angle and write out a new, rotated cube | |||
ia.rotate(pa='45deg',outfile='rotated.image') | |||
# close the image | |||
ia.close('original.image') | |||
</source> | |||
You now have a rotated cube 'rotated.image' | |||
2) now create a box region by finding the bottom left corner (blc) and top right corner (trc) pixel coordinates. Attach this to a variable, here 'mybox' | |||
<source lang="python"> | |||
mybox=rg.box(blc=[65,152],trc=[215,187]) | |||
</source> | |||
3) bin along one of the axes within that region, drop the degenerate axis, write to a new file 'slice.image' | |||
<source lang="python"> | |||
ia.open('rotated.image') | |||
ia.rebin(region=mybox,outfile='slice.image',bin=[1,35,1,1],dropdeg=True) | |||
ia.close('rotated.image') | |||
</source> | |||
4) load the slice into the viewer - don't forget that the axes need to be swapped in the control panel (in 'Display Axes' e.g. Declination vs Frequency). Unfortunately, the coordinates are now a bit scrambled as they mix RA and Dec. Safest is to display the x-axis as pixels via 'Axis Label Properties' -> 'World or Pixel coordinates' -> 'Pixel' |
Revision as of 17:14, 9 November 2010
A position-velocity diagram is typically along slice along a direction in the RA/DEC plane with a given position angle. The y-axis consist of the velocities along that slice. How to do this? Currently there's no task but one can use the tools in CASA.
Here's a recipe:
1) rotate the input cube by a given position angle (to align the slice in the vertical direction):
#open the image
ia.open('original.image')
#rotate by the position angle and write out a new, rotated cube
ia.rotate(pa='45deg',outfile='rotated.image')
# close the image
ia.close('original.image')
You now have a rotated cube 'rotated.image'
2) now create a box region by finding the bottom left corner (blc) and top right corner (trc) pixel coordinates. Attach this to a variable, here 'mybox'
mybox=rg.box(blc=[65,152],trc=[215,187])
3) bin along one of the axes within that region, drop the degenerate axis, write to a new file 'slice.image'
ia.open('rotated.image')
ia.rebin(region=mybox,outfile='slice.image',bin=[1,35,1,1],dropdeg=True)
ia.close('rotated.image')
4) load the slice into the viewer - don't forget that the axes need to be swapped in the control panel (in 'Display Axes' e.g. Declination vs Frequency). Unfortunately, the coordinates are now a bit scrambled as they mix RA and Dec. Safest is to display the x-axis as pixels via 'Axis Label Properties' -> 'World or Pixel coordinates' -> 'Pixel'