This video documents the complete fabrication of a functional set of telescope optics—a parabolic primary mirror and a flat secondary mirror—using household glass ceramic (Saran) scavenged from a broken cooktop. The process leverages the material's extremely low coefficient of thermal expansion (CTE), which provides the necessary thermal stability for precision optics. The detailed guide covers every stage, from cutting the initial discs to achieving optical flatness through multi-stage grinding with silicon carbide and polishing with cerium oxide on an annular pitch lap. Key challenges and solutions are explored, including the significant stress-induced deformation (astigmatism) encountered when cutting the secondary mirror at a 45-degree angle and the meticulous, iterative process of repolishing to correct it. The creation of the primary mirror involves bonding two discs for increased stiffness, milling a spherical curve, and then carefully shaping it into a near-perfect parabola through a controlled polishing process guided by interferometric measurements. The video concludes with a successful benchtop demonstration of the uncoated optics.
From Cooktop to Telescope: Crafting Zero-Expansion Mirrors
0:00:03 Using Household Materials for High-End Optics: The project's goal is to create mirrors for a Newtonian telescope using Saran glass ceramic from a cooktop. This material is chosen for its extremely low coefficient of thermal expansion (CTE), making it highly stable against temperature changes, a critical property for precision optics.
0:02:33 Disc Preparation: Grinding for Flatness: The raw, 85mm diameter glass ceramic discs are initially imperfect and curved. They undergo a multi-stage grinding process using progressively finer silicon carbide grits (from 220 to 1200 mesh) on a flat plate to remove all surface patterns, scratches, and bumps, resulting in a smooth, matte finish.
0:05:38 Polishing to Optical Precision: To achieve a transparent, optically flat surface, the ground discs are polished on a custom-built annular (ring-shaped) pitch polisher. Using a cerium oxide slurry, this 6-8 hour process removes all subsurface damage from grinding, resulting in a surface flatness of a few hundred nanometers.
10:45 Overcoming Flexibility by Bonding Discs: The original 4mm thick discs have a high diameter-to-thickness ratio (21:1), making them too flexible to maintain a precise shape. To create a stiff primary mirror blank, two polished discs are bonded together with a very thin (<10 micron) layer of high-performance epoxy, achieving a much stiffer 10:1 ratio.
12:18 Cutting the Secondary Mirror and a Major Setback: The elliptical secondary mirror is cut from a flat disc at a 45-degree angle. Key Issue: This process introduces significant stress and deformation into the mirror, destroying its optical flatness and introducing severe astigmatism. This is attributed to the release of internal stress from a prior baking step.
15:55 Correcting the Deformed Mirror: The warped secondary mirror is carefully repolished to restore its flatness. A custom holder is built to push the mirror along its minor axis, ensuring even pressure despite the angled edges. After repolishing, interferometry confirms the surface is corrected to a usable accuracy (better than 1/4 wavelength).
22:26 The Art of Parabolizing: The process of converting the primary mirror's spherical surface into a precise parabola is detailed. This is achieved through an iterative cycle of polishing with a specific "W stroke" to remove more material from the center, followed by measurement with an interferometer to check the progress.
29:17 A Polishing Mistake and Recovery: The creator highlights the sensitivity of parabolizing by showing how an aggressive polishing stroke accidentally dug a 400nm deep hole in the mirror's center. This required carefully polishing the mirror back toward a spherical shape before successfully attempting the parabolic correction again.
30:39 Successful Demonstration: The finished primary and secondary mirrors are assembled in a simple benchtop setup. Despite being uncoated (reflecting only 0.16% of light), the optics successfully form a sharp, detailed image of a distant LED, proving the viability of using household glass ceramics for amateur telescope making.