About Astronomical Telescope AberrationsChromatic aberration and spherical aberration in relation to astronomical telescopes should be heard offten.While I'll leave the deep technical details to the lens designers, I believe understanding their general meaning is valuable for appreciating astronomical telescopes.This explanation will be a bit lengthy, but I'd appreciate your patience.・About the Wavelength of LightVarious electromagnetic waves surround us, and those with wavelengths ranging from approximately 1 nanometer to 1 millimeter are what we call “light.”Among these, light with wavelengths ranging from around 400nm to 700nm—the range detectable by the human eye—is called "visible light".The human eye does not perceive the entire visible spectrum with equal brightness, exhibiting the sensitivity distribution shown by the blue and green curves in Figure 1.Furthermore, the human retina contains two types of photoreceptor cells. The photoreceptor cells with high resolution and strong color discrimination ability but low sensitivity exhibit a sensitivity distribution with a peak around 555nm, represented by the green curve. This is called "photopic vision".The other photoreceptor cells, which have lower resolution but higher sensitivity, exhibit a sensitivity distribution with a peak around 507nm, represented by the blue curve. This is called "scotopic vision".The human eye is quite insensitive to red light at the C-line (656nm) in photopic vision, and similarly, it is insensitive to brue light at the g-line (436nm) in scotopic vision. Therefore the human eye is insensitive to color, rather tolerant.On the other hand, the human eye's ability to distinguish subtle gradations of brightness is exceptionally high. Therefore, for visual telescopes, optical performance within the wavelength range from the F-line (486nm) to the d-line (588nm) in visible light is generally sufficient.But within this wavelength range, high contrast will be needed essentially.Previous photographic telescopes also prioritized optical performance in visible light because the film used to record images had a sensitivity distribution matching human visual sensitivity.However, imaging sensors like CCD and CMOS have high sensitivity across a broad wavelength range, as indicated by the red dashed line in Figure 1, and their pixel sizes have been miniaturized to levels unimaginable with film.The explosive proliferation of digital cameras using such imaging sensors in the 2000s led to significantly higher demands on the optical performance of astronomical telescopes.・Chromatic Aberration and Spherical AberrationFigure 2-a shows how light enters a single lens.Light entering from the left side of the figure converges at the focal point after passing through the lens. However, as the height of the incident ray (y) deviates from the optical axis, the point where the ray intersects the optical axis moves away from the focal point and closer to the lens.This is called spherical aberration and is represented by a spherical aberration diagram like Figure 2-b.The focal point shifts as the wavelength (i.e., color) changes. Since spherical aberration occurs at each wavelength, the spherical aberration diagram shows different curves for each wavelength.At this point, the base of the aberration curve shifts for each color. This indicates the shift in the focal point due to wavelength, called longitudinal chromatic aberration.A single lens composed of a spherical surface cannot eliminate spherical aberration or longitudinal chromatic aberration.However, an achromatic lens combining a convex and concave lens can cancel out both spherical aberration and longitudinal chromatic aberration.Figure 2-b shows the spherical aberration diagram for a fluorite single lens with a focal length of 400mm and an aperture of F6.Figure 2-d shows the spherical aberration diagram for the DF66 objective lens.As indicated by the scale (the numerical values at the bottom), the effect of the achromatic lens is extremely significant.On the other hand, Figure 2-d shows residual axial chromatic aberration despite correction; this residual aberration is called the secondary spectrum of longitudinal chromatic aberration.Additionally, the spherical aberration curve also varies with wavelength; this variation is called the chromatic gap of spherical aberration.The chromatic gap of spherical aberration increases as the wavelength becomes shorter; this is the cause of the blue halos that are so disliked in astrophotography.・Chromatic Aberration and Spherical Aberration in Astronomical TelescopesSince the human eye has difficulty perceiving colors with shorter wavelengths, traditional visual telescopes employed aberration correction that significantly diverged spherical aberration at the g-line.This was done to minimize variations in longitudinal chromatic aberration at wavelengths other than the g-line.On the other hand, for photographic telescopes, the advent of digital cameras made spherical aberration at the g-line a major issue, known as blue halo or purple fringe.Reducing spherical aberration at the g-line improves photographic performance, but this increases longitudinal chromatic aberration, creating a dilemma where achieving both photographic capability and visual quality becomes difficult.Recent astronomical telescopes have a trend toward over 4 elements.Increasing the elements makes it easier to have minimize both longitudinal chromatic aberration and spherical aberration of g-line.However, in developing the DF66 objective lens, which demands high performance for both visual and photograph, there was non-negotiable factor.It was minimizing manufacturing errors to maintain high optical performance.Therefore, the DF66 system have aberration balance suitable for visual in the doublet objective lens itself.And the corrector lens has the function to transform this aberration balance, the combination of objective and corrector lens achieve aberration balance suitable for photograph.This allows DF66 system had to get higher optical performance for both visual and photograph.Manufacturing Tolerances in Astronomical TelescopesActual optical systems inevitably exhibit manufacturing tolerances, and the optical performance of a completed optical system will always deteriorate from its designed performance.And minimizing this deterioration is a critically important and challenging problem.Porishing processes introduce shape errors on lens surfaces.The leftmost portion of Figure 4 shows the actual shape with errors (red solid line) superimposed on the designed spherical surface (blue dashed line).While this shape is not always identical, experience suggests it often exhibits changes such as protruding outward and then recessing inward as one moves from the optical axis toward the periphery.In this example, the difference between the protrusion and indentation of the surface was set to 0.2λ (±0.1λ). In reality, a shape error of 0.2λ on an actual lens surface is considered quite high precision.The right side of Figure 4 shows the simulation results when this shape error is applied to all four surfaces of the DF66 objective lens.Although the design value yields an excellent point spread function when evaluated on e-line, applying errors to all four surfaces reduces the point spread function.As the number of elements increases, surface shape errors accumulate, further decreasing the point spread function.Another significant error factor affecting the optical system is decenter of lens.When mounting a lens into its frame, a gap is required between the lens and the frame. This gap allows the lens to shift, causing decenter.The left side of Figure 5 shows simulation results for the DF66 objective lens when its first element is decentered by 0.015 mm perpendicular to the optical axis.The diffraction image biased significantly.By designing the shape of the spacer inserted between the first and second lens so that its end face contacts the lens surface, the first lens decentered along its second surface rather than being decentered perpendicular to the optical axis. (This called a face contact structure and it used commonly in high-precision optical systems.)The simulation results for this state are shown on the right side of Figure 5. The face contact reduces the bias in the intensity distribution of the diffraction image.The DF66 objective lens employs a face contact structure for lens support, minimizing degradation of optical performance due to lens decenter.No matter how excellent the optical performance is in design, as the number of lens elements increases, manufacturing errors accumulate, leading to significant degradation in optical performance.This is why the DF66 objective lens insists on doublet.The two lenses comprising the objective lens were manufactured by Canon Optron, renowned for its high-precision lens processing.From lens polishing to coating, all processing was handled by Canon Optron, ensuring you can get this high-precision optical system for years to come.
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