Optics Software for Layout and Optimization
by Rick Scott
Written on 26 August 1999
Has anyone out there tried to raytrace a telescope design? How about analyzing
the performance of their scope or maybe one they would like to build. Raytracing
software works by analyzing the path that light takes to traverse the optical
system. Many paths are traced and these are referred to as rays. As each ray
passes through or is reflected by an optical surface, the program changes the
direction the ray travels using optical principles such as Snell's law. Rays
can be blocked by an obstruction or even pass outside the boundaries of the
optical surface. If the optical system focuses the light, the rays that make it
through the system will converge at the focus. The pattern these rays make is
called the spot diagram and can show the effects of aberrations such as coma and
astigmatism. By counting the number of rays that make it to the focus and
dividing by the number of total rays used in the analysis, the percent
transmission can be calculated. This is useful to plot the vignetting of the
Recently, I've been working on the design of a telescope and during the initial
stages I spent some time perusing the Amateur Telescope Making Archive web site. I was looking for
information on the telescope type I've chosen and found discussions on various
raytracing programs. The one that seemed to have the best performance (i.e.
accuracy) was OSLO LT from Sinclair
Optics. I downloaded the software, which is a free version of their high-end
optics design tool. The limitations in the LT version are, the number of optical
surfaces is limited to 11 plus the image plane and only 50 variables may be used
in the optimization engine. For most telescope designs, these are not a concern.
I was able to do some analysis after a couple of days, but it took me about a
month of reading the user guide and help file to figure out most of the program.
The learning curve was worth every minute. OSLO LT can analyze just about
anything you could want besides performing raytracing. You can generate the
following analysis both on and off axis:
o Spot Diagrams (with or without a diffraction limit circle)
o Point Spread Function
o Optical (Modulation) Transfer Function
o Wavefront Analysis (peak-to-valley and rms)
o Diffraction and Geometric Energy Distribution
o Diffraction and Geometric Line Spread/Knife Edge Distribution
o Beam Footprint
o Ray Intercept Curves
o Longitudinal Spherical Aberration
o Chromatic Focal Shift
o Lateral Color
o 2-D and 3-D (from any angle) drawings of the optical system
o And much more!
OSLO LT keeps track of the optical path length each ray travels, so it knows the
phase relationship of the light as it travels through the telescope. It uses
this information to calculate diffraction related effects and aberrations. It
can overlay a circle on the spot diagram that shows the diffraction limit (or
Airy disk) for the particular optical system. If all of the rays fall within the
circle, the performance of the scope is considered to be diffraction limited. One
thing I learned in studying various telescope configurations is that fast
Newtonian scopes, even with textbook perfect paraboloidal mirrors, are
diffraction limited for only about a 4 arc-minute diameter field of view!!
The data for each surface of the design is entered in a spreadsheet format for
radius of the aperture, radius of curvature of the surface, thickness (or
spacing), surface type (air, type of glass or reflection), type of aperture,
(round, square, transmit or obstruct) and special attributes such as ideal lens,
aspheric coefficients, diffractive surface or gradient index of refraction. All
of the surfaces can also be tilted or decentered, so making a Newtonian or
Schiefspiegler configuration is easy. High order surfaces can be entered which
allows for the analysis of designs using Schmidt correctors. Besides surfaces of
revolution, OSLO LT also has cylindrical, toroidal, spline, fresnel and other
surfaces, for those with a different bent! The image surface can also have a
radius of curvature to investigate the effects of field curvature.
The analysis can be performed for any single or combination of wavelengths of
light by entering the wavelength in microns or selecting from a pull-down table.
The table contains standard wavelengths that are commonly used in optical design
along with their light source type if applicable, such as He-Ne laser, Argon,
Hydrogen, etc. OSLO LT contains an extensive library of glass types to choose
from for lens elements, or you can make up a custom glass type. It also contains
a large library of common lenses available from some commercial optical
companies. Sample files that come with the program cover a wide range of optical
systems, including a Schmidt camera and the Hubble Space Telescope.
In addition to generating all sorts of graphical output, various textual analysis
can also be generated. Some of the useful data from these include the actual
working focal ratio, percent ray transmission (great for making vignetting
curves), Strehl ratio, and the Seidel aberration coefficients.
I was able to use OSLO LT to replicate the graphs and data in the telescope
Telescope Optics, Evaluation and Design by Harrie Rutten and
Martin van Venrooij. The software runs on Windows 95 and up, and is very well
behaved. Many of the functions are written in the SCP (Star Command Programming)
language, the OSLO macro programming language, which is very much like C. This
allows advanced users to modify existing functions or to write their own routines
to add functions that currently don't exist. Any of the text or graphics output
can be printed or captured to the clipboard to paste into other applications.
Through the use of OSLO LT, I've been able to refine the optical design of the
telescope I'm working on. Many what-if's can be checked by varying the
curvatures, spacing, tilts and centering of the surfaces to determine how
sensitive the design is to variations. After extensive analysis and just fooling
around, I now feel very confident that when my new telescope is completed, it
will work very well.
I highly recommend that you download and learn OSLO LT if you're interested in
some truly graphic demonstrations (pun intended) of optics or just want to learn
more about optical design through computer based experiments. Best of all, this
software is extremely useful if you're designing or modifying a telescope.