Reflecting and Refracting Telescopes Module 2

Definition: Devices that collect and focus any type of electromagnetic radiation.
Types of Optical Telescopes:
- Refracting: Collect and focus visible light using lenses.
- Reflecting: Utilize mirrors to collect and focus light.

Mechanism: Use lenses to refract (bend) light toward a focal point.
Limitations:
- Lenses absorb some light, particularly problematic for ultraviolet (U.V.) & infrared (I.R.).
- Large lenses are heavy and can deform easily.
- Manufacturing and supporting large lenses is challenging.
- Two precision surfaces are required to maintain.
Light Speed in Glass: Light slows down in glass, reducing speed to 100,000 km/s and changing direction.
Chromatic Aberration: Resulting distortion where different colors focus at different points.

Capture Mechanism:
- Light from distant objects is reflected toward the focus.
- A secondary mirror often reroutes the light toward an eyepiece or image collector.
- Eyepiece contains a lens for image magnification.

Prime Focus: a
Newtonian Focus: b
- Features a small secondary mirror; eyepiece is positioned to the side.
Cassegrain Focus: c
- Light is bounced through a hole in the primary mirror to an eyepiece at the end.
Nasmyth/Coude Focus: d
- Similar to Newtonian design but includes an additional mirror.

Components:
- Large primary mirror to gather and reflect light.
- A small secondary mirror to direct the image into the eyepiece.

Light Path: Involves a primary and a secondary mirror to focus light toward equipment bays.
Instruments: Includes STIS, COS, ACS, NICMOS.

Influence of Size:
- Larger telescopes collect more light, making faint objects observable.
- More extensive electromagnetic spectrum can be gathered.
Mechanisms for Gathering Light:
- Longer exposure times and larger exposure areas increase light collection.
- Observed brightness increases with the square of diameter of the mirror area.
- E.g., a 5 m mirror provides 25 times the brightness of a 1 m mirror in 2.4 minutes.

Examples:
- Mauna Kea, Hawaii: Keck telescope combines 36 mirrors for a 10-meter effective mirror.
- Paranal Observatory, Chile: Four 8.2-meter mirrors function equivalently to a 16.4-meter mirror.

Definition: The ability to distinguish separate images of closely located celestial objects.
Relation to Size: A larger telescope provides better angular resolving power.

Old Technology: Photographic plates not efficient (only 5% light efficiency).
Modern Technology:
- Charge-coupled devices (CCD) are used, achieving 75-90% light efficiency.
- Charge builds up in pixels, recorded electronically for processing.
Example: Large Synoptic Survey Telescope (2022) with 3200 MP capabilities.

Challenges for Earth-bound Telescopes:
- Light refraction through varying air densities leads to distortion.
- Atmosphere turbulence causes stars to twinkle and affects clarity (known as "seeing").

Ideal Locations: Stable, dry air helps reduce atmospheric distortion.
- Southwest U.S. (Kitt Peak, Arizona)
- Mauna Kea, Hawaii
- Andes Mountains, Chile

Advantages:
- No interference from Earth's atmosphere (no heat, dust, light pollution).
- Examples include Hubble and Webb telescopes.

Pros:
- High-altitude orbit reduces atmospheric effects.
- Able to view wavelengths absorbed by the atmosphere.
Cons:
- Difficult to operate, with limited fix capabilities.
- Fast orbit restricts observation time; half of the sky is obscured by Earth.

Active Optics:
- Analyze images taken and adjust for mirror distortions and atmospheric conditions.
Adaptive Optics:
- Compensate for atmospheric turbulence by changing mirror shape using laser measurements.

The evolution of telescope design—from traditional refractors and reflectors to advanced active and adaptive optics—enhances our ability to gather light, resolve images, and obtain clearer astronomical observations.