Lecture 4 - Telescopes

How do telescopes work to register an image?

Telescopes use lenses or mirrors to guide incoming light to a focus and register an image

What is the difference between refracting and reflecting telescopes? Which are more common today and why?

Refracting telescopes use lenses to bend light, while reflecting telescopes use mirrors to reflect light. Reflecting telescopes are more common today because refracting telescopes are hard to make large.

What are the two most important properties of telescopes?

Light collecting power (how faint you can see) and angular resolution (how clearly you can see)

How is magnification different from light collecting power and angular resolution?

Magnification just enlarges a blurry and faint image but does not provide new information, while light collecting power and angular resolution determine how much light a telescope gathers and how fine the details in the image can be seen.

How do light collecting power and angular resolution depend on the size of the mirror? Which increases faster?

Light collecting power depends on the area of the mirror and angular resolution depends only on the diameter. Thus light collecting power increases more quickly than angular resolution.

What other quantity does angular resolution depend on?

Wavelength of the light

What is diffraction limit?

Diffraction limit is the minimum detail you can resolve, measured in arcseconds. If you want to see two objects, their angular separation must be greater than or equal to the diffraction limit.

How does wavelength of light impact diffraction limit?

Increasing wavelength increases the diffraction limit, making it harder to take clearer images

How does the diameter of the telescope impact diffraction limit?

Increasing the diameter of the telescope decreases diffraction limit, making it possible to separate smaller objects that are closer together.

How does the physical size of an object and its distance from us affect its angular size?

The angle the object subtends on the sky increases with object size and decreases with distance to the object.

How do solar eclipses happen?

The sun and the moon are different true sizes and distances from us, but have the same angular size so one can cover the other.

What is the surface roughness criterion? How does this affect how telescopes are made?

The required smoothness of a telescope’s mirror become stricter with decreasing wavelength. This means that for radio wavelengths, telescopes are easier to make larger since they can be rougher. For infrared wavelengths, the mirrors of the telescopes need to be very smooth.

What is aperture synthesis/interferometry?

Aperture synthesis is the technique of electronically linking telescopes together to mimic larger telescopes.

What does aperture synthesis improve? What does it not greatly improve?

Aperture synthesis improves angular resolution by mimicking the angular resolution of the largest separation between dishes. It does not greatly improve light gathering power because the combined area of the individual dishes is still small.

Why do we put some telescopes in space?

The atmosphere blocks light at certain wavelengths and blurs the light that makes it through. Thus, placing telescopes in space allows us to observe ultraviolet, gamma, and X-rays, and to take clearer pictures with smaller telescopes because there is no atmosphere.

Why do stars twinkle but planets don’t?

Since stars are so far away, they appear as point sources, so the Earth’s atmosphere creates a twinkling and blurring effect. Planets are much closer and have a size, so they are unaffected by the atmosphere’s turbulence.

What is adaptive optics?

Adaptive optics is when the mirror of the telescope is deformed to compensate for the distortion of the atmosphere. It requires a reference star or a laser point source. This technique decreases the angular resolution of the telescope, thus improving image clarity.