This video, "From Cooktop to Optical Part," is Part 3 of a series on glass-ceramics, demonstrating how to transform ordinary household glass-ceramic cooktop material (Ceran and Robax) into high-precision optical components. The creator focuses on fabricating two mirrors—a flat secondary and a parabolic primary—for a small Newtonian telescope.
The video details the entire fabrication process, starting with cutting and grinding the glass-ceramic disks to achieve precise flatness, followed by optical pitch polishing on specialized machinery. A key theme is leveraging the material's near-zero coefficient of thermal expansion (CTE), a property critical for stable optical shapes. Challenges are encountered, such as deformation during angled drilling for the secondary mirror, which is meticulously addressed through advanced polishing techniques and custom tooling. The primary mirror involves bonding two disks for increased stiffness and then parabolizing the surface using iterative polishing strategies, including the "W-stroke" method. The video concludes with a qualitative demonstration of the assembled, albeit uncoated, telescope optics, revealing sharp imaging capabilities.
Summarizing "From Cooktop to Optical Part"
0:01 Introduction to DIY Optics: The video, Part 3 of a glass-ceramics series, demonstrates making two small mirrors for a Newtonian telescope using scavenged Ceran cooktop material.
0:25 Zero-Expansion Glass-Ceramics: Ceran and Robax glass-ceramics are chosen for their extremely low (near-zero) coefficient of thermal expansion (CTE), a crucial property for maintaining optical precision under temperature changes, even surpassing quartz glass. The material's CTE can be tuned, and a previous video showed how baking can further reduce its tolerance.
2:23 Preparing Glass-Ceramic Disks: Round disks (85mm diameter, 4mm thick) are made by bonding raw material to sacrificial glass, hollow-diamond drilling, and then baking to achieve zero CTE. Initial bumps, prints, and scratches from the cooktop are removed.
3:41 Grinding for Flatness: Disks are ground flat using silicon carbide grit (from rough 220 to fine 1200 mesh) on a large glass or metal disk, ensuring complete removal of pits before proceeding to finer grains.
5:40 Optical Pitch Polishing: A custom-built angular polisher with a flat, grooved pitch lap and cerium oxide slurry is used for optical polishing. A custom granite ring and aluminum sheet holder with weights ensure uniform pressure, with 6-8 hours of continuous polishing required for a smooth, pit-free surface.
8:48 In-Process Flatness Checks: A sodium lamp and test glass are used to quickly check flatness by observing interference fringes, while white light interferometry is used for precise surface roughness measurements, achieving single-digit nanometer range.
10:08 Newtonian Telescope Configuration & Stiffness: The plan is to create a parabolic primary and a flat secondary mirror. Due to their thinness (21:1 diameter-to-thickness ratio), the primary mirror is formed by bonding two disks together to achieve a stiffer 1:10 ratio, essential for telescope mirrors.
12:18 Challenges in Secondary Mirror Fabrication: Angled drilling of the secondary mirror introduced severe astigmatism (3 lambda wavefront error) due to sub-surface damage and internal material stress from rapid cooling.
15:55 Resolving Secondary Mirror Deformation: Repolishing the angled secondary is challenging due to uneven pressure. A custom sample holder supporting the mirror on its minor axis is designed, improving flatness from 3 lambda to 0.5 lambda. The remaining elliptical astigmatism can be compensated by adjusting telescope focus.
18:37 Bonding for Primary Mirror Stiffness: Two disks are bonded together with a thin layer (<10 microns) of EPO-tek 301 epoxy, increasing stiffness by a factor of eight without affecting the overall CTE.
20:34 Milling and Grinding the Primary: A Loh Spheromatic machine is used to mill the correct 656mm radius of curvature for an f/4 primary mirror. This is followed by grinding with a glass counter tool to achieve a smooth concave spherical surface.
22:26 Parabolizing the Primary Mirror: An automated polishing machine is used to transform the spherical primary into a parabolic shape. The "mirror-on-top" method with a "W-stroke" strategy is explained, which selectively removes more material from the center to deepen the curve.
28:08 Iterative Parabolizing Process: Mirror shape is measured with an interferometer and DFTfringe software. The process involves iterative polishing sessions based on measurements, with a temporary setback when the creator impatiently dug a 400nm deep hole, requiring corrections.
30:00 Near-Perfect Primary Mirror: The final primary mirror achieves better than 1/4 wavelength deviation from a perfect parabola, with remaining errors (like trefoil astigmatism) being negligible for practical use.
30:40 First Light Test: Despite being uncoated (0.16% total reflection), the assembled mirrors demonstrate sharp imaging of a distant LED flashlight, proving their functionality as a telescope.
31:54 Future Plans: The next video (Part 4) will explore a mystery package from Nippon Electric Glass and use remaining Robax disks to build a special optical component leveraging glass flexibility.