Telescope Optics

This section needs better diagrams for telescopes. Astronometry, photometry, spectroscopy should all have sample graphs. A picture or link to a timing experiment would also be helpful.

Telescopes mirrors or lenses (or both) to gather light and focus it. An eyepiece is put at the focal point to create an image that the observer (us) can see.

Main Function of Telescopes

A telescope’s main purpose is to gather more light, to make objects brighter. Astronomical objects are too faint for the naked eye, no matter how much they are magnified. In fact, magnifying a dim object just makes it harder to see. That’s why even microscopes come with bright lights to illuminate the objects being magnified.

Your eye is a natural lens system that gathers as much light as will fit in through your pupil and focuses it onto your retina. A telescope objective (either the primary mirror or lens) is like a giant pupil that gathers in lots of light which the telescope focuses down to the eyepiece and then into your eye.

The pupil of your eye dilates under low light conditions. This peaks at about age 15, typically around 7mm but can be up to 9mm. After about age 25, the maximum diameter decreases eventually dropping to about 3mm. A pair of 10×50 binoculars has a 50mm diameter objective lens. How does this affect the amount of light gathered?

The amount of light gathered depends on the area of the aperture (the opening). So it increases as the square of the diameter. So a 50mm diameter binocular aperture has a diameter 7 times bigger than a fully dilated pupil. But it gathers 7×7 times more light (because the area is proportional to the square of the diameter). That’s for a moderately sized binoculars. An 8-inch telescope is 200mm in diameter, so it gathers over 800 times as much light as your naked eye.

Telescope Types (Requirement 3b)

There are three main types of telescopes: refractors, reflectors, and catadioptric.

Refractors

Refractors use only lenses. Galileo’s original telescope was a refractor. This is the type of telescope that most people think of. The original refractor design involved a simple lens at the front and an eyepiece. However, a single lens suffers from something called chromatic aberration. That simply means that not all colors focus the same. Modern refractors will use two or three lenses made of special types of glass to better bring all colors to focus.

 

Relectors

Reflectors use only mirrors to bring the image to a focus. The first reflector design was by Isaac Newton and is still called a Newtonian reflector. Even though there are two mirrors in this design, it is considered a single mirror design. Only one mirror, the parabolic primary mirror in the rear of the telescope, actually does any focusing. The secondary flat mirror deflects the light beam out the side but does not actualy focusing.

There are two two-mirror designs, the Cassegrain and the Gregorian (and several variations on both). Both of these have a curved secondary which reflects light back toward the primary. The primary mirror has a hole in the center to allow the light to pass through to the focal plane. 

Catadioptrics

There are many different types of catadioptric telescope designs. The most popular is the Schmidt-Cassegrain which uses a lens in front, a large primary mirror in the back, and a small secondary mirror in front that combined bring light to a focus in the rear after it passes through a hole in the middle of the primary.

Each of the different designs is optimized for different purposes and is often also driven by the technology of the time and how difficult it is to make a mirror or lens of the required shape. There is no one perfect or ideal telescope design. Different designs are better for different things. Some work best for planetary observations, some better for deep sky objects (nebula, galaxies), 

Instruments (Requirement 3c)

The Astronomy merit badge book has dated information on instruments used by astronomer. No on uses, for example, a filar micrometer for measuring double star positions, or phototubes for photometry. Virtually all astronomical measurements are done using a CCD. Astrometry measures positions and uses a CCD. Photometry measures brightness and uses a CCD. Spectroscopy uses a diffraction grating to spread of the different wavelengths and then images the resulting spectrum using a CCD. There are, of course, other niche instruments used. For example, the International Occulatation Timing Association (IOTA) uses low-light video cameras and video timing injectors to measure the exact timing of when an asteroid passes in front of a star.

I prefer to focus on three different purposes and describe the instrument used.

Astrometry

Astrometry measures the positions of objects on the sky with high position. Although the positions of stars don’t seem to change, they actually do, albeit very slowly. Additionally, they’re apparent position can change due to parallax. This is the same effect you get when holding an upraised finger at arm’s length and looking first through one eye and then the other. Your finger will appear to shift positions compared to objects further away. High precision astrometry is now down by space telescopes (but not Hubble, it does other work) and can directly measure the distances to stars out to a few hundred light years by measuring their positions at different times of the year, when the Earth is in different positions in it’s orbit.

The principal instrument used is a CCD camera and computer software for data analysis.

Photometry

Photometry measures the brightness of objects. Astronomical objects are measured by their apparent brightness which is called their magnitude. Magnitudes are measured on a logarithmic scale, with each change by one magnitude being a change of about a factor if 2.5 in actual brightness. This might sound strange but is actually the way our eye’s work, i.e., they work over a logarithmic scale. And it’s a sliding scale. Our eyes can adjust to the general brightness level of an environment both by dilating or constricting the pupil and, under dark conditions, chemical changes that activate our night vision. 

For astronomy, CCD cameras are used to collect carefully calibrated images where the brightness scale is linear. This makes CCD cameras better for science purposes than our eyes, plus different cameras can be calibrated to produce the same brightness values. Different eyes can’t.

Photometry is used to not just measure a star’s apparent brightness, but also it’s change in brightness. These changes can be used to help determine what’s happening to a star (or even a galaxy) internally. Some stars are known to regularly change brightness. The American Association of Variable Star Observers (AAVSO) coordinates observations by thousands of amateur astronomers for use by professionals.

The Kepler mission uses a space telescope to do high precision brightness measurements to discover stars where a planet will pass in front of a star causing the star’s brightness to dip by a very small amount (typically less that 0.1%).

Again, the primary instrument is the CCD camera and computer software for the data analysis.

Spectroscopy

Spectroscopy uses a diffraction grating to spread out the different wavelengths of light. Think of how a prism works; a diffraction grating is essentially a high precision prism.

Stars emit light of different colors. We perceive the light from the sun as being white, the color you see reflected off the clouds. But not only is it composed of many different colors, but hidden within those many colors are certain wavelenghts that are exceptionally bright or dim due to the chemical composition of the Sun. At high temperatures like the surface of the Sun, the elements are all in a plasma state where the rules of quantum mechanics result in hydrogen, for example, emitting light at very specific wavelengths. When the light from the Sun is passed through a spectrograph, those bright wavelengths can be easily detected. The same is true for other stars, so astronomers can detect what elements are present. It’s mostly hydrogen and helium, but traces of other elements are also present. Stars are even categorized by the present (or absence) of these other elements.

The primary instruments used are the spectrometer, the CCD camera, and computer software to analyze the data.

Others

Sometimes timing information is important for an event. Usually, measurements with a time scale of a few seconds are fine, but for some events even higher timing precision is necessary. One such case is occulation timing. When an asteroid passes in front of a star (which happens quite often someplace on Earth), what is effectively happening is that a shadow is being cast. If we somehow had lots of telescopes all over the Earth all looking at the same star when this happened and could then have them all report at the same time which one do and don’t see the star (the latter because the asteroid is blocking it), then from the pattern of telescopes on the ground that can’t see the star, we could construct an image of the shadow and thus the shape of the asteroid, even though no telescope can actually see the asteroid. 

The problem is, we can never arrange for that many telescope to all be looking at the same time. But there are some mathematical tricks that can be done with a much smaller set of telescopes that can precisely measure where they are (via GPS) and can measure exactly when the shadow stared and stopped (again, via GPS timing). The International Occulation Timing Association (IOTA) coordinates these measurements.

The primary instruments uses are low-light video cameras, video timing inserters, and computer software to analyze the data.