v3.3.0

This release computes the plate scale of a sequential system. It has the same requirements as 3.2.

The plate scale of a sequential system (#259)

AbstractSequentialSystem.plate_scale(wavelength=None) returns the field of view divided by the number of pixels it spans, along each axis of the sensor, at each wavelength. It is a Cartesian2dVectorArray in field coordinates per pixel, as field_boundary reports them. That is an angle for an object at infinity and a position for an object at a finite distance.

• How it's computed:
  • The chief rays of the four ends of the two lines through the center of the field, −1 and 1 in normalized field coordinates, are traced in one call.
  • The spans of the field along those lines make the columns of a matrix F, and the spans of their images on the sensor, in pixels, make the columns of a matrix P.
  • Each column of F P⁻¹ is the step in the field which moves the image one pixel along that axis of the sensor, and the plate scale is its length.
• Rolled sensors: with matrices, the result stays along the axes of the sensor when the sensor is rolled against the field. The two axes can differ, as on a tilted sensor or behind a grating. Dividing the spans component by component would mix the axes: on the four channels of ESIS, rolled 22.5° to 157.5° against the sky, it gives 0.794 or 1.917″/pix where the answer is 0.734.
• What it averages: the span of the image is the integral of its derivative, so the result is the plate scale of the mean derivative along each line. It agrees with the mean of the local plate scale over an 11 × 11 grid of the field to 4×10⁻⁵ on the test systems and to 6×10⁻⁶ on ESIS.
• Off the sensor: the image is traced whether or not it lands on the sensor, so the plate scale is defined at wavelengths whose image misses the sensor.
• No sensor: a system without a sensor raises a ValueError.

Limitations:

• The ends of the lines are on the box bounding the field of view. For a field of view that isn't symmetric about its center, such as a rotated rectangle, they run past it.
• Across the lines: it doesn't average how the plate scale along one axis varies along the other.
• NaN where the chief rays don't reach the sensor, such as an order of diffraction that doesn't propagate.
• Sky angles: for an object at infinity, the azimuth and elevation of the field are angles on the sky only near the axis.

Full Changelog: v3.2.0...v3.3.0