17.0 Bending Achromats

# 17.0 Bending Achromats - THE UNIVERSITY OF CENTRAL FLORIDA...

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FALL Semester 2010 THE UNIVERSITY OF CENTRAL FLORIDA OSE 6265 17.0 Bending Achromats James E. Harvey, Instructor 17.0

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17.1 Introduction The achromat designed in the homework for Chapter 16 on page 182 is based upon a thin lens approximation; i.e., this lens has no thickness (shown schematically in Figure 17.1 a). A real achromat, one that would be designed in ZEMAX, has finite thickness (Figure 17.1 b). 17.1 Figure 17.1 Difference between: (a) a thin lens achromat , and (b) a real achromat. In this chapter we will design a real achromat from the thin lens pre-design. We will also re-employ “lens bending” and see what happens to spherical aberration, coma and astigmatism. We will also introduce another chromatic effect, called transverse chromatic aberration or “lateral color”.
17.2 Achromats in ZEMAX Returning to the design example in Section 16.5.3 (page 181), and inserting the values into ZEMAX along with reasonable thicknesses for the lenses (say 5 mm and 2 mm ), a miniscule airspace (0.00001 mm ), and an entrance pupil diameter (EPD) of 40 mm . Inter F, d, and C wavelengths and put an M-Solve on the air thickness between the lens and the focal plane. We have the following lens editor. Clicking on the “Sys” button, we see that the focal length is not quite 400 (399.9521). Going to Analysis Misc. Chromatic Focal Shift , the plot indicates that our color is off because the F and C do not coincide. Under “Settings” the values for “MAX SHIFT” and “PUPIL ZONE” are zero. Our lens will have to be optimized to take into account the real thicknesses of the two elements. To do this set variables on the 1 st , 2 nd , and 4 th radii. Slave the 3 rd radii to the 2 nd (using Pickup). In the MFE, use operands EFFL and AXCL and set targets of 400 and 0 respectively, with weights of 1. CHRM1o1b

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17.2 Achromats in ZEMAX Running the optimization (No TRAC), we obtain the lens editor shown below. The focal shift curve now shows the proper correction as indicated at the right. The curve has a quadratic appearance , and the F and C foci are coincident. The separation between this common focus and the d -light focus is called “secondary spectrum” or “secondary color”. Returning to the “Sys” button, the focal length is now 400. The focal lengths of the component lenses are slightly different than they were for the thin lens design. To see what the new component focal lengths are we have added two EFLY operands, one for each element. Since this is for informational purposes only, we left the weight set at zero. CHRM1o1a Seconday Spectrum
Achromats in ZEMAX The improvement in axial color for this example is shown in Figure 17.4. Note that the change is dramatic. The axial separation between F and C light is ~ 0.2 mm . 17.4

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17.0 Bending Achromats - THE UNIVERSITY OF CENTRAL FLORIDA...

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