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 Airy’s disk
for the design f/number is shown in the extender diagrams. The circle
shown in the compressor diagrams has a diameter of 50 microns and
is aligned to the centroid of the rays and is not related to Airy’s
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.
LAYOUT
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. In this diagram, it is
placed 6.5 mm to the left of the image plane, which matches the popular
Phillips WEB camera specification. 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.
One site that I find especially worthy can be found at:
http://www.normankoren.com/Tutorials/MTF.html
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
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 a ½ 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.
