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 Airys 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 Airys 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 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.
Optec, Inc.
