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Blog de diseño óptico
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Rapid Rectilinear
In the following article we will review the fundamentals of a "Rapid Rectilinear", a design invented in 1886 by Dallmeyer.
It is called "rapid rectilinear" a type of photographic lens composed of two achromatic symmetrical doubles. Between these two doublets is the aperture diaphragm. In Figure 1 we can see an outline of what the classic design of a "rectilinear rapid" would look like. It is designed to cover a large field and usually fulfills the condition of flatness. The Dallmeyer design works with a f/6 or f/8.
Figure 1. Basic scheme of a Celor Lens and the limitations they have to have.
The next page shows a rectangular rapid design, which I will try to improve. In this case, although the doubles are separated, it is still a symmetrical design. As in other articles, I will show a strict or symmetrical design, followed by an asymmetrical one. The asymmetrical design will improve the result over the symmetrical design.
The easiest and fastest way to design a "rapid rectilinear" is to start from the rear. Our system will have a focal length of 50 mm, f/5.6 and will cover a semi-field of 24º.
First of all, the doublet of the rear system will have an EFL of 100 mm, since when we put the front systems the total EFL will become half. At the beginning of the design, the rear is like a Chevalier lens. This will allow us to control off-axis aberrations. As always, in Zemax it is best to go step by step, first optimizing small fields and, once the desired field is reached, adding the front part, adjusting the EFL of the total system and reoptimizing until the best possible system is reached.
During the first part of the process, the FCGT command can be used to force the field to flatten and decrease astigmatism, which will be problematic in the early stages of the design. Once we have the two parts together, the biggest problem I've had with the symmetrical system has been to decrease comma aberration, distortion and field curvature. Figure 2 shows the result of the complete symmetric system.
Figure 2. Symmetrical rectilinear rapid report
As we can see, I'm improving Professor Limmon's system. The system has less than 1% distortion, the field curvature is relatively small and the size of the Spot Size RMS is much smaller.
However, as we can see in the captures of Ray Fan, OPD and Spot Size RMS, the system shows a problem with comma aberration, especially noticeable in the 14º and 24º fields. Neither FFT nor Diffraction Encircled Energy captures are shown, because the system is not of sufficient quality.
If we break the symmetry of the system, as has already been done on other occasions, we will see how greatly improves the quality of the system.
Figure 3. Asymmetric rectilinear rapid report.
Breaking the symmetry, we see the improvements clearly. The Spot Size RMS is much smaller, becoming slightly higher than the diffraction limit. The low field curvature is relatively low and the distortion practically remains the same. The system is still achromatic and the color correction is quite good, as seen in the Ray Fan capture.
The OPD and Ray Fan curves now show that the system is compensated correctly, thanks to the quadratic and sinusoidal curves shown in the graphs respectively. The comma aberration problem has been satisfactorily solved, as can be confirmed by these curves and the Spot Size RMS pattern.
After the improvement of the system, now I can show the captures of FFT and Diffraction Encircled Energy. It is not a system that is limited by diffraction, but the improvement of the asymmetric system over the symmetric is very noticeable.
If you found this article interesting, let me know. Below you have how to contact me.
Thank you very much and I'll see you later.
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