Glossary Of Performance Terms
- GEOMETRIC ENCIRCLED ENERGY: Usually, a CCD pixel used in astronomy will have a dimension of around 7 by 7 microns and are arranged in a regular
square pattern called an array. When imaging a star, this diagram shows approximately the amount of energy that would be captured by a square array of pixels with
the side dimension indicated on the x-axis. It is not necessary nor is it a good thing for a single pixel to capture 100% of the stellar energy profile. Excellent
astrometric results and good images are obtained if 95% of the stellar energy profile falls on a 5 x 5 pixel array. This also assumes that the stellar profile is
close to normal (standard normal distribution) and symmetrical.
- RAY TRACING: This diagram shows the cross sectional profile of the lens system with the image plane on the right. The second line a little
to the left of the image plane is the face plane of the camera lens mount. Ray bundles for the four field parameters are shown. Light travels from left to right.
- POLYCHROMATIC DIFFRACTION MODULATION TRANSFER FUNCTION:
Frequently referred to as the MTF of a system, this diagram most
represents the performance or resolution of a system. The modulus of the
OTF (y-axis) can be compared to contrast with 1.0 being the highest
contrast and 0.0 a plane gray view with no discernable image. The
Spatial Frequency (x-axis) can be referred to as the number of black and
white line pairs per millimeter. For a more complete understanding of
this diagram the reader is referred to Modern Optical Engineering by
Warren Smith. There are also many Internet web sites that discuss this
optical concept. Particularly helpful is Norman Koren's website.
The solid black line above the other lines is the optical diffraction limit for the system. A perfect optical system could match this line but can be no higher than this line.
The nature of light places limits on the resolution of an optical system. The MTF for each field parameter is shown for both the tangential (T) and sagittal (S) planes.
The T and S planes can be understood if one looks at the spot diagram. The T plane is for rays going up and down and the S plane are for rays going left and right
- SPOT DIAGRAM: Four blur spots are produced for four field parameters. The IMA: 0.000MM blur spot is for on-axis rays. The IMA: 2.500 MM
(the number may not be this value exactly but something close) is the blur spot for a point near the corner of the CCD chip. The other two blur spots are
for image points intermediate between on-axis and the corner blur spots. The bar on the left represents a distance scale of either 50 microns for the
compressor systems or 200 microns for the extenders. In addition to the bar scale, the size of
the Airy disk for the design f/number is shown in extender diagrams.
The circle shown in compressor diagrams is aligned to the centroid of the rays
rather than the Airy disk. For
astrometric work, it is important that the centroid of the stellar image align to the true position of the star, asteroid or comet. An unsymmetrical stellar profile
would make it difficult for an accurate position to be computed. Spots for the three design colors of 450, 550 and 650 nanometers are shown.