This video details the process of fabricating high-precision optical components, specifically mirrors for a Newtonian telescope, using Ceran glass-ceramic, a material chosen for its exceptionally low and tunable Coefficient of Thermal Expansion (CTE). The fabrication journey begins with preparing Ceran disks, including heat treatment to achieve near-zero CTE, followed by meticulous grinding and fine-grinding stages to establish initial flatness. The core of the process involves optical pitch polishing on a Loh PM-500 machine, where the presenter demonstrates techniques for achieving both flat and parabolic surfaces. Challenges such as surface deformation caused by cutting angled pieces and the intricacies of parabolizing a primary mirror are discussed, along with methods for correction. The video concludes with the successful creation of a bonded, parabolic primary mirror and a flat secondary mirror, demonstrating their optical quality in a "first light" test, proving that high-quality optics can be produced from readily available, albeit uncoventional, materials with diligent effort.
Making High-Precision Optical Parts from Ceran Glass-Ceramic
0:01 Ceran Glass-Ceramic: The video introduces Ceran glass-ceramic as the material for optical parts due to its exceptionally low and tunable Coefficient of Thermal Expansion (CTE), which can be adjusted to near zero, making it ideal for stable optical components in varying temperatures.
2:23 Disk Preparation: Round disks of Ceran (4mm thick, 85mm diameter) are prepared by drilling through sacrificial glass using blocking wax, followed by a high-temperature baking step to achieve the desired zero CTE.
3:10 Initial Grinding: The rough, printed, and bumpy surfaces of the raw Ceran disks are removed by grinding them flat against a granite plate using P120 sandpaper, a process that takes only minutes for the bumpy side.
4:10 Fine Grinding: A multi-stage fine grinding process follows, progressing from coarse Silicon carbide powders (220, 320, 400 grit) to finer abrasive films (600, 800, 1000, 1200 grit). Microscopic inspection is crucial after each stage to ensure complete removal of previous grinding pits.
5:38 Optical Pitch Polishing: The smoothed matte disks are then polished to optical flatness using an old Loh PM-500 polisher with a pitch lap and Cerium Oxide abrasive. The pitch lap is prepared by pressing it flat against a granite plate.
7:07 Polishing Setup & Technique: An aluminum disk with recesses is used to hold multiple glass-ceramic parts, allowing uniform pressure distribution during polishing. Guide wheels are employed to control the rotational speed and position of the holder on the pitch ring.
8:40 Flatness Check: Flatness is periodically checked using a test glass, observing interference fringes to ensure they are as uniform and straight as possible. Microscopic inspection verifies the removal of all grinding pits.
10:08 Newtonian Telescope Components: The goal is to create primary and secondary mirrors for a Newtonian telescope. The 4mm thickness of the individual Ceran disks is noted as being too thin for a primary mirror of 85mm diameter due to gravity deformation.
11:23 Increasing Primary Mirror Stiffness: To overcome flexibility issues, two 85mm disks will be bonded together to create a thicker, stiffer primary mirror. Stiffness increases significantly with thickness (factor of 8 for bending, 16 for gravity sag).
12:18 Secondary Mirror Challenge: Making the elliptical secondary mirror by drilling the Ceran disk at a 45-degree angle introduces severe astigmatism (3 lambda Peak-to-Valley) due to sub-surface damage and potentially pre-existing stress in the material.
15:50 Correcting Secondary Mirror: The astigmatism on the secondary mirror is reduced through careful re-polishing with an adapted sample holder and added weights. While improved (0.5 lambda), a slight residual astigmatism remains, which can be compensated by adjusting telescope focus.
19:02 Bonding Primary Disks: Two Ceran disks are bonded together for the primary mirror using EPO-tek 301-2FL epoxy, applied in a very thin layer (less than 100nm) and cured at 80Β°C to minimize CTE impact.
20:28 Grinding Primary Concave Radius: The bonded primary mirror is then ground to a concave spherical radius (656mm for an f/4 mirror) using a Loh Spheromatic machine and a glass counter tool.
23:36 Theory of Parabolizing: The video explains the mathematical difference between spherical and parabolic surfaces and introduces the concept of asphericity, which involves selectively removing more material from the center or edges of a spherical mirror to achieve a parabolic shape.
25:50 Polishing Strokes for Parabola: A "long linear stroke" with the mirror off-center on the pitch tool, combined with specific tool and mirror speeds, is demonstrated as a method to create a smooth parabolic surface by preferentially removing material from the center or edges.
28:20 Parabolizing Primary Mirror: The primary mirror undergoes an iterative parabolizing process, involving polishing sessions and interferometer measurements. An initial "big mistake" resulted in over-deepening the center, requiring extensive correction.
29:55 Final Primary Mirror Quality: The final primary mirror achieves a near-perfect parabolic shape (conic constant virtually -1) with minimal remaining surface error (around +/- 0.07 lambda) after accounting for mounting-induced astigmatism and effective light path.
30:39 First Light Test: A "first light" test is performed with the uncoated (and thus very low reflectivity) primary and secondary mirrors, demonstrating that the image is "pretty sharp," despite the challenging setup.
31:48 Future Plans: The project will continue with mirror coating, telescope tube construction, and exploring other optical components using the remaining Robax material.