The next task is to find real glasses to replace the model glasses. OSLO has a special command, accessible from the lens spreadsheet, that substitutes the nearest real glass for a model glass.

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At this point, the optimization has proceeded to the point of a default solution. Now, various trials can be conducted to improve the design, such as trying additional glasses. Other possibilities for additional optimization are to remove the edge contact solves to see whether the positive elements still want to get too thin, to change the weights on selected terms in the error function, or to re-enter the error function with different rays. The general approach to optimization using OSLO is illustrated by the steps of this example: you should approach the optimization cautiously and change only a few things at a time, working interactively, until you are confident that the combination of variables and operands can be trusted to produce a system of high quality. You should always maintain a way to restore your system to an earlier state, such as using the revert capability of OSLO spreadsheets.

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Tutorial example - Optimizing a double-Gauss objective The double-Gauss objective can serve as a starting system to illustrate the techniques for lens optimization with the GENII error function, specifically using the OSLO LT program (although the same system can be optimized similarly using any of the OSLO programs). It is assumed that the lens is to be optimized for f/2, 50mm focal length, 20 degrees field coverage, i.e. the same as the example system. The results will be different because the example system was created using a different optimization procedure. The example is presented as a series of explicit steps that you should duplicate on your computer. The OSLO user interface has been especially constructed to provide an easy-to-use interaction during optimization, if you follow the recommended procedure. As you progress through the design, you should save your lens so that you can recover to a given point rather than start over, if you make a mistake. The tutorial directory in public/len/demo contains lens files showing the design at various stages. You should expect your lens data to be similar, but not necessarily identical, to that in the tutorial files.

Final Solution (dblgauss5.len) The lens shown below (public/len/demo/tutorial/dblgauss5.len) is the result of about an hour's investigation of various options for improving the design using the GENII error function (with different weights on selected operands). It is not feasible to trace the course of this optimization explicitly. You should try to see if you can match, or improve on, the final design shown below. Note that checked apertures have been inserted to provide vignetting at the edge of the field.

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At this stage, we have only used curvatures as variables. This has the advantage that the system is still in the same general solution region as the starting system, and the disadvantage that the performance is not improved very much. The next stage of the design is to add the thicknesses as variables. When you add thicknesses, you must take extra care to provide boundary conditions to prevent the system from "blowing up", i.e. wandering off to a solution that is either non-physical or in a totally different solution region from the starting system.

For the design exercise in this tutorial, we will just vary the glasses in the inner doublets. After trying this, you can proceed on your own to make a final design by varying all the glasses. To vary glasses, it is necessary to first replace the catalog glasses with model glasses.