University of Michigan - Department of Astronomy




Version: Intro

Daytime Observing at Angell Hall

You can see a lot just by observing

--Yogi Berra




A telescope is a device that collects light and concentrates, or focuses it. Thus, faint objects can be greatly brightened with a telescope. Objects can also appear magnified by the telescope. In this activity, you will have the opportunity to use astronomical telescopes at the Angell Hall Observatory to observe the Sun and the planet Venus. Astronomical telescopes can point in most directions on the sky, but their mounts may use different coordinate systems. The telescope can be pointed to a given position using setting circles the show the coordinate position on each axis. As you use the telescopes, take note of their optical elements and mounting configuration, to compare with the telescopes at the Detroit Observatory. See the Timekeeping and Telescopes at the Detroit Observatory activity.

Figure 1: Refractorrefracting telescope

Figure 2: ReflectorNewtonian reflector

Light is focused by manipulating the paths of the light rays with lenses, mirrors, or a combination of both. A telescope that has a lens as its primary optical element, or objective, is a refracting telescope (Figure 1). A telescope that has a mirror as its objective is a reflecting telescope (Figure 2). The eyepiece is a lens that restores the original direction of the light paths, which are now more concentrated or closely packed (best seen in Figure 2). This is necessary for our eye to view the image.

Astronomers often use filters that permit only some of the light to pass through them, and you will use filters in your solar observations. Filters can reduce the amount of light at all wavelengths (neutral density filters) or they can eliminate all light except a specific wavelength range (Figure 3). Astronomical telescopes also have clock drives to allow the pointing to counteract the Earth's rotation and thereby remain pointed at the same position on the background sky.

H-alpha filterFigure 3: Hα solar filter. Incoming sunlight of all colors approaches from the left. Its spectrum is in the left graph, showing that the received sunlight is predominantly yellow-green, which has a wavelength of 550 nm. After the light passes through the filter, only one wavelength remains: 656.3 nm. In the right graph, the new spectrum shows only this one wavelength, which corresponds to a deep red color.

The Sun


The Sun is our closest star, which makes it the easiest star to observe and study. It is a giant sphere of hydrogen gas, which is progressively denser toward the inside than the outside. It is transparent in the outermost layers, but at a certain radius, it is dense enough to be opaque. The photosphere is the "surface" where Sun is opaque, and its continuous, thermal spectrum is emitted at this layer. The layers above the photosphere are transparent and are considered to be in the Sun's atmosphere. If we look closely, we can see that the surface is not the uniform yellow-green of the thermal spectrum's brightest emission.  Chinese astronomers observing the Sun at sunrise noted there were dark spots, and when Galileo turned the telescope on the Sun, he too saw such sunspots. Galileo also went partially blind from looking at the Sun through the telescope, so be sure to always use the proper filters when observing the Sun!  Sunspots are slightly fainter than the surrounding area, and redder in color, so we know that they are cooler than the rest of the photosphere.  Sunspots show the location of magnetic storms on the photosphere. Surrounding the sunspots, faculae may be visible, which are slightly lighter, brighter regions. Additionally, magnified observation (with a safe filter) shows granulation, the tops of individual convection cells. Thus, just beneath the photosphere, energy moves through the Sun by means of convection, and the region beneath the photosphere is the convection zone.

You can observe the photosphere directly by using a neutral density solar filter attached to the telescope. This filter permits less than 0.1% of the light to pass through it, but it does allow all of the usual wavelengths. You should be able to hold a good solar filter up to a 100W light bulb and NOT see the light. In this activity, the GSI may set up the telescope to project the Sun's image on a screen. This is an even safer way to observe the Sun.

The Sun's chromosphere is its transparent lower atmosphere. Since it is a hot, thin gas, it emits radiation in an emission-line spectrum, but its light is overwhelmed by radiation from the photosphere. However, we can see the chromosphere if we use a filter which blocks all light except for a narrow wavelength range around a chromospheric emission line. The red nebular line at 656 nm is a hydrogen emission line that astronomers call Hα, which is emitted by electrons releasing energy to move from energy level 3 to 2. A filter that transmits only this wavelength is an Hα filter. This allows us to see the chromosphere, since through this filter, it is now brighter than the photosphere. You may see prominences or solar flares near the edge of the of the Sun's profile, which is called the limb.

The Sun’s upper atmosphere, the corona, is also very faint because it is so thin. It, too, is normally overwhelmed by light from the photosphere. However, the corona can be spectacularly visible during a total solar eclipse, when the moon exactly blocks the light from the photosphere (see figure 4). Some solar telescopes reproduce this effect with a device called a coronagraph, which is a disk placed in the telescope's field of view to block the light from the photosphere. Coronagraphs are also used to search for circumstellar disks and faint companions around other stars, where the brightness contrast and angular proximity are extreme.

Figure 4: the Solar Corona during the total eclipse of Aug. 11 1999. By combining several images, finer details are brought out, and the complex structure of the corona becomes visible. (This is image number T99comp1Dd-Bw in Fred Espenak's gallery:

Detailed solar corona from the 1999 eclipse


The planet Venus is bright enough to be visible during the day. You will use the Angell Hall Observatory 0.4-m telescope to observe Venus. It is the planet that is most similar to Earth, both in orbital properties, and properties of the planet itself, being about the same size and mass as the Earth. It has a dense atmosphere which shrouds its surface, thus we see entirely reflected light from the Sun.

Venus shows phases similar to those of the Moon. Be sure to note the phase when you observe Venus. The phases of Venus are explored in more detail in the Motions of the Planets activity.



updated: 7/9/10 by MSO and SAM

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