6.1/6.2 Telescopes

Telescopes

1. Serve as a bucket to collect visible light or radiation 

• Larger telescopes gather more light

• Functions of a telescope

1. Collect the faint light from an astronomical source

2. Focus all the light into a point or an image

Apertures

• The diameter of the opening through which light travels or reflects

  • A=nd²/4  → d is the diameter of the circle 

Resolution

• The precision of detail present in an image 

  • Measured in arcseconds: 1arcsecond = 1/3600 degree 

    • 360 degrees in a full circle goddamn

  • Larger aperture = sharper images 

  • Not as sharp as it can be due to our atmosphere

Rayleigh Criterion

• how close two point sources of light can be and still be distinguished as separate.

• two point sources are just resolvable when the central maximum of one diffraction pattern coincides with the first minimum of the other.

  • It’s a fundamental physical limit caused by diffraction, not imperfections in the optics

• Mathematically, for a circular aperture (like a telescope lens or mirror):

  • delta =1.22x lamda/D

    • Delta is angular resolution 

    • D diameter of the aperture

• The larger the aperture (𝐷), the smaller the angular separation (𝜃) that can be resolved.

• The shorter the wavelength (𝜆), the finer the detail that can be seen.

Sorting attachment

1.  Sort through different radiations by wavelength

Detectors

1. Device that sense the radiation in the wavelength regions and permanently records the observation 

Refracting telescope (lens)

• Light passes through a lens to refract and then to the focus where the image is observed

• All parallel rays of light are refracted to coverage at the focus of the lens 

  • This is where the image appears → need to use an eyepiece to view this image

    • Helps change the magnification as well of the image

Focal length

• The distance from which the light rays focus behind the lens to see the image 

• Hard to make large flawless lenses

chromatic aberration

(different colors focus differently, blurring the image); large lenses sag under gravity since they can only be supported at the edges; both sides must be perfectly shaped

1. Reflecting telescopes (mirror)

• Use of concave mirror instead of lenses the reflect light 

  • Internal flaws don't affect it, surface needs to be perfect

• Can be thinner and a lot larger

• Do not absorb light

• No chromatic aberration: Since it can be supported from the back

• Different types of reflectors → have at least one focus

Site for Telescopes

1. Weather conditions 

• Must have clear weather up to 75% of the time 

2. Dry atmospheres at high altitudes

• Infrared radiation disturbance due to water vapour 

3. No light pollution from nearby cities 

• Light pollution causes the sky to be not dark 

4. No turbulent air: causes a blur in imaging and creates “bad seeing” 

Adaptive Optics

Help beat the effects of Earth’s atmosphere

• Makes use of small flexible mirrors placed in the beam of a telescope - sensors measure how much the atmosphere has distorted the image and then the software fixes it by changing the mirror shape

• Ground based telescopes can now reach 0.1 arcseconds of resolution 

  • Adaptive optics works best on infrared radiation