Inside an IR Cassegrain objective
Follow the light through an open primary, onto a convex secondary, and back to a concave primary that focuses around it.
Open in the canvas →Background
A reflective microscope objective can focus infrared light without refractive glass. A common layout uses a large concave primary with a central opening and a small convex secondary. In the infinity-input focusing direction, light first passes through the primary opening, reflects from the secondary, returns to the primary, and finally converges past the secondary toward the sample.[1]
This example exposes each optic as an independent, editable element. The two golden curves are actual traced surfaces. The small absorber and slit select the entrance pupil; the camera's active face marks the sample plane.
A prescription you can understand
This is a Cassegrain-type teaching design, not a manufacturer prescription or the classical concentric spherical Schwarzschild design. The convex secondary is a paraboloid with radius 80 mm and conic constant −1. It turns collimated light into a diverging return bundle with a virtual focus at x = 410 mm. The ellipsoidal primary sends that bundle to the second focus at x = 530 mm.
The primary vertex is x = 250 mm. Its two focal distances are therefore 160 and 280 mm: ellipse semi-major axis a = 220 mm, focal half-separation c = 60 mm, vertex radius R = (a² − c²)/a = 203.636363… mm, and k = −c²/a² = −0.074380… . The secondary vertex is x = 370 mm. Both radii are positive in the scene's +x axis; the coated sides face each other.
These conics provide an exact on-axis geometrical focus. The 160 mm working distance and oversized dimensions make the path readable; they are illustrative dimensions, not a claimed commercial IR objective specification.
- Clip the entrance: select the primary and reduce Central opening from 36 to 20 mm. Fewer rays reach the focus. The opening is an absence of mirror, not a transmissive surface.
- Try spherical mirrors: set both conic constants to zero, leaving their radii and positions unchanged. The sensor spot span rises from numerical zero to about 0.152 mm; the camera inspector makes this small blur easier to inspect than the overview.
- Change wavelength: change the source from 3000 to 10000 nm. The ray focus stays put because the modeled reflection geometry is achromatic.
- Change loss: reduce primary reflectivity from 98% to 49%. The relative detected signal halves; at 0% the primary absorbs the beam.
Why this pairing, and not a classical Cassegrain. The choice is a directly derivable on-axis teaching construction, not a claim of better performance. A convex paraboloid creates a virtual source; an ellipsoid images that source to the chosen sample position. The generous spacing exists to make the folded path readable. Note also the order: here the collimated beam meets the convex paraboloid first, whereas a conventional classical Cassegrain sends it to the concave parabolic primary first and then to a convex hyperbolic secondary. Swapping the conic constants in this scene would not reproduce that arrangement.
On the reported signal. It is a relative sum over this 2D ray section, not transmitted power through a circular pupil, so central-obstruction losses here differ from a 3D area calculation. With the scene's 14.4 mm central stop and 32 mm illuminated width, a continuous 1D blocked fraction would be 14.4/32 = 45%, while the corresponding area fraction for a uniformly illuminated circular pupil would be (14.4/32)² = 20.25%. The figure the scene reports is the sampled result for these rays, and the two 98% mirror reflections reduce it further.
This is a 2D meridional ray trace, not a 3D objective design or electromagnetic calculation. The mirror is a zero-thickness, one-sided coated conic with an opaque back and absorptive coating losses. No substrate thickness, mounting spiders, diffraction, Airy rings, vector PSF, wavefront phase, coating dispersion, or IR detector responsivity is calculated. The geometric point focus is not a prediction of physical spot diameter. Commercial Schwarzschild objectives and their aberration correction are different prescriptions.[2]
The selected entrance pupil is represented by two ray bands in this section. Its area throughput cannot be inferred from the displayed 1D ray weights. Golden mirror strokes and red IR rays are display colors, not a material or visible-color claim. Source watts are metadata for power readouts; the existing tracer continues to draw normalized rays at zero watts.