Knowledge Base: How to use the Asphere Module General Contents The described add-on module is available for µshape 42 and higher (earlier versions may have slight different user interfaces or reduced functionality). This paper describes which possibilities the add-on module Aspheres offers, which requirements and restrictions have to be taken into consideration and how the individual analyses work. The Asphere module allows you to analyze analytically described rotational symmetric aspheres in an on-axis test setup. The module contains following analysis setups: aspheric/aspheric test setup with aspheric test wave front, i.e. the classical setup usually used with CGH spherical/aspheric test setup with spherical test wave front flat/aspheric test setup with flat test wave front All individual analyses are automatically enabled if the module is enabled in the dongle. A special aspheric alignment fit corresponding to the setup eliminates systematic errors due to alignment and setup. As result in all setups the deviation from the theoretical nominal aspheric shape is displayed. A zero result represents an error free sample. For other aspheric surfaces, like cylinders and tori, exist individual add-on modules in µshape. They are not described here. 1 Asphere description...2 1.1 Asphere formula...2 1.2 Surface type (convex/concave)...2 2 Entry of aspheres...2 1 Import of Aspheres...3 2 Edit Aspheres...3 3 Test setups...4 3.1 Flat asphere testing...4 3.2 Spherical asphere testing (without CGH)...4 3.1 Standard analysis...4 3.2 Base radius fit...5 3.3 Base radius fit with Cat s eye correction...6 3.3 Aspherical asphere testing (with CGH)...6 FISBA OPTIK 2009 Page 1 (6) Author: Thomas Blümel
1 Asphere description 1.1 Asphere formula The asphere has to be defined in an analytic way. We offer different descriptions that all basing on equation (1): 2 h N R0 2n z( h) = + A2nh (1) 2 n n h = 0 14243 1+ 1 ( 1+ k) even polynomial R 0 144424 44 3 conic section It defines a rotational symmetric aspheric surface, and consists of two terms: first is describing a conic section, the second a polynomial of even order. z(h) is the sagitta, R 0 the base radius of the basic sphere, k is the conic constant and A 2n the polynomial coefficients. Sometimes instead of the conic constant k the value for the eccentricity is given e = ( 1 + k) (2) Following asphere definitions are available: Conic section only (without additional polynomial) ISO (conic section + polynomial 10 th order (with n 0 =2, N=5)) Extended ISO (conic section + polynomial 20 th order (with n 0 =1, N=10)) Polynomial only (polynomial 20 th order (with n 0 =1, N=10) without conic section) The first two descriptions are special versions of the third one. The fourth is a slight different one because it is impossible to set the parameter in equation (1) so, that the conic section is equal 0 for all height values. 1.2 Surface type (convex/concave) Equation (1) just describes the shape of the asphere. It depends on where the aspheric surface is used in the optics, that means where the lens body is (left or right), whether it is a concave or convex surface. So the sign of the base radius R 0 does usually not define the surface type (ie convex or concave) as it is usually done in the optical metrology where a negative radius value indicates a concave and a positive value a convex surface. If the sagitta table defined by equation (1) does not fit to the metrology definition you have to flip all signs of the parameters in equation (1) except the conic constant k resp. e, to change the surface type from convex to concave or verse versa. If you only flip the sign of the base radius you change the asphere. It becomes steeper or weaker. The same happens if you only change the sign of polynomial coefficients. Changing the sign of the conic constant changes the base type of the conic section. 2 Entry of aspheres asphere Before you can analyze aspheric surfaces you have to enter the parameters of the desired asphere into the µshape program. There are two possibilities. Whether you import a file, that include the desired asphere or you have to enter the parameter manually. The asphere has to be enter before specifying the set-up. z h zc zs basic conic surface basic sphere R0 r R0 FISBA OPTIK 2009 Page 2 (6) Author: Thomas Blümel
1 Import of Aspheres Select Import objectives from the Extra menu. A dialog opens. Select aspheres as objective type to import and browse for the file. After file the selction the names of all available aspheres included in this file are displayed. By clicking on the corresponding entry you can select or deselect the aspheres for import. When pressing the Import selection button all selected aspheres are imported. It is automatically checked whether the selected aspheres already exist in µshape. To leave the dialog press Exit. 2 Edit Aspheres To change the current asphere list select Edit objective/asphere from the Extra menu. A dialog opens (see right) where all already stored aspheres in µshape are displayed. Each asphere can be modified or removed from the listinduvidually. Furthermore new aspheres can be added to the list. Pressing one of the three buttons Modify, Duplicate or Add new opens the Asphere parameter dialog with different presetting according to the selected button. The entered name identifies the asphere and has to be unique. Use the combo box Asphere type to select one of the asphere descriptions. The currently selected description is displayed in the box above. Enter the values for base radius R 0, conic constant k and polynomial coefficients A n. The conic constant defines the type of the conic section. The current type is displayed right. Click on the k= button to switch between conic constant k and eccentricity e (compare chapter 1.1). The sign of the base radius defines the surface type. A positive value defines a convex shape, a negative one a concave. Pressing the Flip button transforms a convex asphere into a concave one and vice versa (see chapter 1.2). For some descriptions the maximum polynomial order can be changed by the spin controls. Reset sets all parameters to zero. It is recommended to check the entries by calculating a sagitta table and comparing it with the design data. Press the Create saggitta table button, specify the maximum aperture height and the step width. Than the sagitta table is displayed and can be also stored into an ASCII file. In addition the corresponding radius R BF of the best fitting reference sphere is displayed. The best fit sphere has a minimum PV deviation to the asphere. FISBA OPTIK 2009 Page 3 (6) Author: Thomas Blümel
3 Test setups How to use the Asphere Module The following chapters describe how the software has to be set to measure the three different test setups correctly. The process steps for the individual test setups may be quite similar nevertheless they are listed for each setup separately. General configurations like calibration and measurement parameter settings have to be done before and are not described here. When the software is once configured for a certain test setup it is recommended to save the file as measurement file or as template. Than you can easily reload the configuration and immediately start measuring. 3.1 Flat asphere testing 1. Enter the asphere parameter (see chapter 2) Specify the hardware setup on the hardware setup dialog: - set the objective type to plano and select objective 3. Press the More button to open the Aspheric fit dialog 4. Select the desired asphere from list and specify the test diameter as 2h value Press Calculate button; µshape calculates the theoretical setup parameters (expected aperture size in pixels, best fit radius) and displays them beside the corresponding sagitta value 6. Quit the dialog with OK 7. Quit the hardware setup dialog with OK 8. Start a measurement 9. The displayed aberration shows the deviation from the theoretical asphere shape. 3.2 Spherical asphere testing (without CGH) 4. 6. 3. 3.1 Standard analysis 1. Enter the asphere parameter (see chapter 2) Specify the hardware setup on the hardware setup dialog: - set objective type to spherical and select objective 1 3. Press the More button to open the Aspheric fit dialog 4. Select the desired asphere from list and specify the test diameter as 2h value Press Calculate button; µshape calculates the theoretical setup parameters (expected aperture size in pixels, best fit radius) and displays them beside the corresponding sagitta value 7. 3. 1 GenPack customers have first to enter their objectives to the objective list FISBA OPTIK 2009 Page 4 (6) Author: Thomas Blümel
6. Quit the dialog with OK 7. The software automatically activates Use ref radius so that the theoretical best fit position is automatically used as R_scale value for the scaling, e.g. in graphical views. 8. Quit the hardware setup dialog with OK 9. Start a measurement 10. The displayed aberration shows the deviation from the theoretical asphere shape. In addition the radius of the reference sphere used for the elimination of the setup error is displayed in the left down corner of the graphical view. 3.2 Base radius fit The base radius fit is a special option of the standard analysis as described in chapter 3.1. For that the knowledge of the actual test position (according to the cat s eye position) is required. If a position measurement device is used you only have to take care that it is zeroed in the cat s eye position (@ power = 0). Otherwise you have to enter the corresponding z-value at appropriate fields (explained below). If a position measurement device is active the corresponding fields are marked with auto and cannot be modified. The first five steps are similar to the standard analysis but are listed here again. 1. Enter asphere parameter (see chapter 2) Specify the hardware setup on the hardware setup dialog: - set the objective type to spherical and select objective 1 3. Press the More button to open the Aspheric fit dialog 4. Select the desired asphere from list and specify the test diameter as 2h value Press Calculate button; µshape calculates the theoretical setup parameters (expected aperture size in pixels, best fit radius) and displays them beside the corresponding sagitta value 6. Check Radius fit and enter the test position as distance focus vertex, i.e. the sample position according to the test beam focus (cat s eye position). The software automatically enters a first proposal calculated from the theoretical values. 7. Quit the dialog with OK 8. The software automatically activates Use ref radius and take over the test position value from the Aspheric fit dialog as z- pos value. It is also possible to change this value directly in the hardware setup dialog. 9. Quit the hardware setup dialog with OK 10. Start a measurement 10. 11. The displayed aberration shows the deviation from the theoretical asphere shape with the fitted base radius R 0. 1 The calculated base radius value can be displayed in the statistics window of the aberration. Press the right mouse button FISBA OPTIK 2009 Page 5 (6) Author: Thomas Blümel 4. 6. 4. 7. 1 6. 8. 3.
inside the statistics window, select Options and switch to Additional parameters. Select Base radius and quit with ok. 3.3 Base radius fit with Cat s eye correction As described in chapter 3.2 for the base radius fit the knowledge of the test position according to the cat s eye position is required. It can be easily detected by an interferometric measurement quite similar to the absolute radius measurement. Therefore the distance between the cat s eye and the test position has to be determined. In order to reduce the effort in finding the correct cat s eye position for referencing the position measuring device, the software offers the possibility of a so-called Cat s eye correction. If activated you are guided by the software through a measurement cycle. During the measurement cycle the software collects the required measurement positions, calculates the correct test position and finally analyzes the measurement data. 1. Execute step 1. 8. of chapter 3.2 to configure the base radius fit. Check Cat s eye correction; the z-pos value becomes disabled 3. Quit the hardware setup dialog with OK 4. To display the calculated base radius value press the right mouse button inside the statistics window, select Options and switch to Additional parameters. Select Base radius and quit with Ok (compare step 12 in previous chapter). Move to cat s eye position and start a measurement. The first guideline dialog pops up and asks you how to continue (immediately or after adjusting). During the measurements in the appropriate test positions the position measurement device is read out respectively you are requested to enter the corresponding position values. The second guideline dialog reports the detected measurement positions and asks how to continue (repeat measurement or continue with next step). 6. After finishing the measurement cycle with OK, the displayed aberration shows the deviation from the theoretical asphere shape with the fitted base radius R 0. The new base radius can be seen in the aberration statistics window. 3.3 Aspherical asphere testing (with CGH) 1. Enter the asphere parameter (see chapter 2) Specify the hardware setup on the hardware setup dialog: - set the objective type to aspheric and select an asphere 3. Quit the hardware setup dialog with OK 4. Adjust the asphere and scale the system manually Start a measurement The displayed aberration shows the deviation from the theoretical asphere shape. If you need more information don t hesitate to contact us at software@fisba.de. FISBA OPTIK 2009 Page 6 (6) Author: Thomas Blümel