Lens entry - doublet

Lens entry - doublet

This section shows how to enter lens data in OSLO from first principles. The particular lens is an f/2 doublet working at 0.6328 microns, with a focal length of 60 mm, covering essentially zero field of view. This lens will be used as a starting design in a later example. Also, As an example of the use of tilted surfaces and mirrors, the lens will later be combined with a right-angle prism and turning mirror.

To aid the beginner, "cookbook" instructions are given as to what menus, spreadsheets, cells, etc. are to be used. Although several output screens are shown in the text, it is best for you to carry out the steps on your own computer as you read this section. The discussion assumes that you are familiar with the OSLO Windows interface, as described in Chapter 4. If you follow each step carefully, by the end of this section you should be comfortable enough with the program to start entering your own systems.

Close the surface data spreadsheet, if it is open. Click on File >> New. Enter lasrdb1 for the file name. Enter 4 for the number of surfaces (this will be an air-spaced doublet). Click OK, and the surface data spreadsheet will open automatically.
Move the highlighted block to the upper-left field that says "No name", and enter He-ne f/2 doublet focusing lens. This will be used as a title on drawings. It is good practice to enter a title, or lens id (lid) as it is called. You can, if you wish, also enter supplementary notes by clicking on the button at the end of this field.
In the View field, click Srf (optional, as no groups will be defined in this example).
In the Draw field, click On to enable Autodraw.
To set the Entrance beam radius field, note that the lens must have a focal length of 60, and a speed of f/2. The diameter of the entering beam must therefore be 30. Enter 15 for the entrance beam radius. Leave the field angle at its default value.
Click on the button with the asterisk in the Wavelengths field. This will pop up another spreadsheet showing the default wavelengths. Click on the row button for wavelength 2, then on the row button for wavelength 3, to select both rows. Then use the Cut tool to delete these wavelengths.
Click on the wavelength field for wavelength 1. Note that the box is black with a thin line around it. This means that the field is supported by an Options list. You can either type in a value, or click on the field again to pop up a list of values. Click on the field, and select 0.6328 micrometers from the list by double-clicking on it. Click 4 to close the spreadsheet and return to the surface data spreadsheet.

You have set up the basic operating conditions for the lens. The top portion of the spreadsheet should appear as follows.

Now you are ready to enter surface data. For historical reasons, the surface data is presented in the order, radius, thickness, aperture, glass. When you enter data for a new system, it is best to move from right to left: glass, aperture (if needed), thickness, finally radius. This will result in the setup process being easiest to understand as you progress.

To enter the starting system, some optical decisions must be made. Since the lens is for monochromatic light, there is no need for glasses of different dispersions. For this example, you will use the highest index glass available. The starting design will comprise a convex-plano singlet followed by a meniscus lens.

An axial-ray angle solve will constrain the focal length to 60 mm. Since the entrance beam radius is 15 mm, the emergent ray slope must be -0.25 to make the focal length 60 mm. This also constrains the f-number to f/2.
Click the options button in the GLASS field for surface 1. A pop-up menu will appear. Drag to Catalog, then drag right to Schott to select the Schott catalog.

A spreadsheet pops up that lists the glasses available in the Schott catalog. By default, the glasses are shown alphabetized by glass name. Click on the Index sort criterion to show the glasses in order of increasing refractive index. The first column gives the index at wavelength 1 of the first glass in that row. At the end of the Sort field at the top of the spreadsheet, there is a button labeled Recalculate n and v. This button only appears when you have changed the wavelengths so that the previous sort order might be incorrect. In the present case, you changed from the default d,F,C wavelengths to the He-ne 0.6328 wavelength. This change is not sufficient to have any effect on the sort order, so it is not necessary to do the recalculation.

Click on the last glass in the list, LASF35 (this is a new glass with an extremely high index). When you click on the glass, a summary of the data for the glass is shown in the message area of the main window (the complete data is only available for certain Schott glasses). Click on an OK button to accept the glass and close the spreadsheet.

To use the same glass on surface 3, you can use a pickup. This will cause the glass on surface 3 to track the glass on surface 1, i.e. if you change the glass on surface 1, the glass on surface 3 will change automatically in the same way. Although it is not strictly necessary to do this in the present system, it shows how to handle pickups. (Actually the quickest way to enter the glass on surface 3 is to type LASF35 in the glass field). To set the pickup, click on the Glass options button, then select Pickup from the pop-up menu. A dialog box will appear asking for the source surface. Enter 1 and click OK.

After you have completed this step, a light-gray line will appear in the spreadsheet between surfaces 2 and 3, indicating a glass-air interface. The Autodraw window will show the lens as a single vertical line with three horizontal lines, representing the trajectories of the axial and chief rays through the system.
It is not necessary to enter data for the apertures at this time. The default values are determined from the axial and chief ray data.
For the thicknesses of surfaces 1 and 3, enter 5.0. For surface 2, the intervening air space, enter 1.0. The Autodraw window will now show the system as two blocks of glass. For surface 4, click the button in the thickness SmartCell to pop up a menu. From the menu, select Solves >> Axial ray height solve. A prompt will appear to enter the value for the axial ray height. Enter 0 (the default value). This will locate surface 5 in the paraxial image plane (which is still located at infinity).

Set up the radius of surface 4 to be determined by an axial ray angle solve. Click on the button in the radius SmartCell, then enter a value of -0.25 in the prompt box in response to the prompt.

Now we must choose initial values for the radii. The guideline here is to make the lens look like the desired starting point. Set the radius of surface 1 to 100.0, and the radius of surface 3 to 50.0.
Click on the radius buttons for surfaces 1 - 3 in turn. Select Variable for each. This will set up the lens for the optimization problem considered later (page *).
12. Now, the starting design is entered. The Autodraw window and spreadsheet should now appear as follows.

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