# Units

## Positions and sizes

When dealing with positions, Lensed uses the pixel coordinate system as read from the input FITS image.

By default, the coordinate system is positioned and oriented as follows.

• The centre of each pixel is at an integer coordinate.
• The centre of the bottom-left pixel has coordinates (1, 1).
• The left and bottom borders of the image are at x = 0.5 and y = 0.5, respectively. Similarly, the right and top borders of the image are at x = IMAGE_WIDTH + 0.5 and y = IMAGE_HEIGHT + 0.5, respectively.

These are the same coordinates that one would get, for example, by looking at the FITS file in SAOImage DS9.

In case the FITS file was cropped using the extended filename syntax (e.g. image = observation.fits[400:500,600:700]), the coordinate system is adapted accordingly.