Why Astronomers Prefer Multiple Smaller Mirrors Over Larger Ones for Telescopes

Astronomers, when designing telescopes such as the renowned Hubble Telescope, often employ multiple smaller mirrors instead of a single large mirror. This approach offers significant advantages, including cost-effectiveness, ease of fabrication, temperature management, and ease of deployment. In this article, we will explore the reasons behind this choice.

Cost-Effectiveness and Fabrication

The fabrication of a single large mirror is a formidable engineering challenge. The necessity of achieving a perfectly smooth surface poses substantial difficulties. Smaller mirrors, on the other hand, are much easier to produce because they require variances on the order of thousands of a centimeter. Polishing such mirrors is an iterative process, but it is manageable in smaller segments.

By creating a mold for smaller mirrors, the process can be scaled, ensuring consistency across many mirrors. A large single mirror, however, demands a perfect first attempt, making it both expensive and risky. Cutting a small mirror and polishing it is significantly less challenging than cutting and polishing a large mirror to the same level of precision.

Temperature Management and Adjustability

Varying temperatures can affect the focal point and curvature of a mirror. This is a critical issue, especially in space where temperatures fluctuate significantly. Smaller mirrors can be individually realigned to counteract these changes, thereby maintaining optimal performance. A single large mirror would be much more difficult to adjust and maintain in such dynamic conditions.

Moreover, the thickness of a mirror affects its thermal properties. Smaller segments can be adjusted for temperature-related distortions, whereas a large single mirror would suffer from the thermal variations of its thicker sections more severely. This could lead to potential warping or even breaking of the mirror, a scenario that would be disastrous for any telescope.

Durability and Deployment

The weight and thickness of a large mirror present significant challenges in deployment. A large mirror is inherently heavier and thicker, which increases its overall mass. This mass, combined with the need to support itself, makes it exponentially more difficult to fabricate a single large mirror. Doubling the size of the mirror does not double its weight; instead, it could quadruple or even more.

Thermal expansion and contraction due to temperature fluctuations can cause a large mirror to warp, leading to potential structural damage. By using smaller segments, the thermal impact can be managed more effectively. This modularity also means that any individual segment can be replaced if necessary, whereas a large single mirror would require the entire unit to be repaired or replaced.

Furthermore, the logistics of fitting a large mirror into a rocket for deployment in space pose significant challenges. Rockets need to be aerodynamic for atmospheric flight, and a massive, heavy mirror would compromise this aspect. Deploying a single large mirror in space is virtually impossible with current technology, making smaller mirrors a practical solution.

In conclusion, while the Hubble Telescope employs a single large mirror, other telescopes often use multiple smaller mirrors for their inherent advantages in cost, fabrication, temperature management, and deployment. This approach ensures that telescopes can achieve high levels of precision while remaining practical and cost-effective.