The parabolic mirror

Once the oslo command is issued several windows will appear on your computer screen if things are working well. One of them is a window with helpful tips. Click ``Close'' to make it go away, or use ``Next Tip'' to look through several hints.

Open an existing lens by selecting the push button in the Startup Option window, and ``OK''. Then select ``Private'', and ``parabola.len''. Click ``OK''. New graphics windows will appear, including one with a small diagram of the telescope optics. Click on the ``GW1'' window icon that seems to show two lenses. This button will create a ray trace window. You can expand the window by grabbing a lower corner with the mouse.

The ``Surface Data'' spreadsheet has three entries:

1. OBJ is the object. It's at infinity, or nearly so, $10^{20}$ mm to the left of an arbitrary reference point (plus is to the left, minus to the right).
2. AST is an aspherical reflecting surface, in this case a paraboloid. If you right click on the ``A'' you'll see that ``Conic/Toric'' is selected. You can change the conic constant from -1 (a parabola) to a positive number (a oblate spheroid), 0, a sphere, a number from 0 to -1 (an ellipsoid), or a number less than -1 (a hyperboloid). For now, leave it parabolic. The surface has a radius of of 500 mm, that is, a diameter of 1 meter. Its radius of curvature is -8000 mm, negative here because the surface is to the right (negative direction) from the center of curvature. A radius of curvature of -8000 mm causes the focal length of the mirror to be -4000 mm (again, negative because of the sign convention on directions). The f-ratio is the focal length divided by diameter, or 4 in this case. It is also given by $(1/2 \times \mathrm{NA})$ where ``NA'' is the numerical aperture. The ``Thickness'' entry controls where the surface is with respect to the last entry in the table. When there are multiple surfaces this entry is the distance from the previous surface to the new surface measured on the axis. Try changing it a small amount, and watch the ``Autodraw'' window to see what part of the system is visible.
3. IMS is the image surface. The ``Thickness'' is the distance to the place that the image is observed. In this example, the mirror surface is located at -5000 mm and the focal surface is set at +1000. The image is 4000 mm from the surface. You can ``Autofocus'' the image by right clicking on the box next to the Thickness for this surface and selecting the entry for Paraxial rays.

The ``Surface Data'' spreadsheet also has a ``Setup'' option. Under Setup you can alter the ``Entrance beam radius'' to define the effective diameter of the telescope. This will change the pattern of the Spot Diagram Analysis that you will use to evaluate optical performance. In this example it is set to 500 mm, the same as the aperture radius in the Surface Data spreadsheet. If you make it smaller, the Airy disk due to diffraction will get larger.