Chapter 2: Selecting an Observing Target Selection Criteria There are several factors that must be considered when selecting a target to observe: Is the target visible from Winnipeg? For what dates is the target visible? For what times? Is the target bright enough to be detected by the equipment? What is the size of the target? Is it too large or too small to be detected with the equipment? These questions can be broken down into three main criteria: Position Brightness Size Position Background The Earth is constantly in motion, spinning on its axis and revolving around the sun, thus so the stars are in constant motion in our sky. Since Winnipeg has a latitude of 50 N of the Earth s equator, the axis of rotation in the sky is located 50 above our northern horizon, near the star Polaris. The North Celestial Pole is the one point in our sky that does not move. It is the projection of the Earth s rotational axis on to the sky. All of the stars revolve in circles around this point; the nearest stars move in very small circles, while the stars that are 90 from this point, on the Celestial Equator, move the greatest distance. The stars in a 50 region around the pole star do not rise or set as observed from Winnipeg and thus these stars, located in the northern part of our sky, are called circumpolar. The stars in the Southern part of the sky are called the seasonal constellations because they change with the season.
Equatorial Coordinate System Astronomers need a system to describe the position of stars in the sky. This system is called the equatorial coordinate system. It is exactly analogous to the latitude/longitude system that is used to describe positions on Earth. The pole star, Polaris, is very near to the North Celestial Pole, which has a latitude of 90 N. Instead of using the term latitude, astronomers use the term Declination, which is often abbreviated as Dec and is sometimes given the symbol δ. So Polaris s declination is ~90 N. The equivalent of the Earth s equator is the celestial equator, which has a declination, δ = 0. To measure the position around the sky, astronomers use a coordinate called Right Ascension, which is analogous to longitude on Earth. But instead of measuring this coordinate in degrees like longitude, Right Ascension is measured in units of time. This is very natural because the sky (well, really the Earth!) rotates once on its axis every 24 hrs and so 24 hrs is the equivalent of 360. Right Ascension, or RA is sometimes given the symbol α. Every position on the sky can be uniquely described using only these two coordinates. For example, the position of the star called Betelgeuse is given as 5h 55.5m, 7.4 N.
Figure 2.1: Equatorial Coordinate System Effect of the atmosphere When observing from the Earth s surface, we are forced to look through the atmosphere, which can seriously degrade the quality of the image. The effect of the atmosphere on an image is variable and can change dramatically from night to night depending on the amount of turbulence present. Turbulence is caused from temperature and wind fluctuations in the different layers of the atmosphere. This effect is generally termed seeing. When observing, one generally attempts to observe on nights with the best seeing and to reduce the atmospheric effects as much as possible. One way of doing this is to avoid observing objects that have a very low altitude. I.e. we try as much as possible to only observe objects with an altitude > 30. The reason for this is that as you observe closer to the horizon, the amount of atmosphere that one must observe through actually increases (see Figure 2.2). Figure 2.2: Equatorial Coordinate System Selecting the target Using Starry Night, locate your proposed target and set the date and time to when you would like the observations to take place. What is the altitude of the target for this date and time? If the altitude is low (> 30 ),
you will need to find out if the target rising or setting; if it is rising, then the observing time should be made a bit later (either later in the evening or a later date), but if it rising, then the observing time should be made for an earlier time (either earlier in the evening or an earlier date). It is possible that your target may not ever get above 30 altitude for Winnipeg, it might not even come above the horizon! In this case, you will have to select a different target. It is also possible that your proposed target may need to be observed at a completely different time of the year than what you are proposing. In this case, you will either have to wait until the appropriate time of the year or select a different target. Brightness Background The brightness of astronomical objects is generally measured using a quantity called magnitude. The magnitude system uses our own eyes as the basis for the measurement. The Greek astronomer, Hipparchus, was the inventor of this system. He called the brightest stars first-magnitude. Stars about half as bright were second-magnitude stars. The dimmest stars visible were sixth-magnitude stars. There are a couple tricky things about the magnitude scale. The first is that it is based on our eyes. When measured, the difference in brightness between a first-magnitude star and a fifth-magnitude star is actually 100 times even though it appears less than this to our eyes! Our eyes have adapted to our world, which contains very bright things (like the sun and moon) and very dim things (like very faint stars). Our eyes make bright things look fainter than they really are so that we can see them at the same time as the fainter things. Digital cameras however, do not behave this way; they see things as they really are. If you can detect a 10 th magnitude star in 1 second, it would take 2.5 seconds to detect a 11 th magnitude star, and 6 seconds to detect a 12 th magnitude star. So how long would it take to detect a 20 th magnitude star? Well, it would take 2.5 x 2.5 x 2.5 x 2.5 x 2.5 x 2.5 x 2.5 x 2.5 x 2.5 x 2.5 = 9536 seconds or 158 hours or about 6.6 days! The other tricky thing about the magnitude scale is that to some it seems a bit backwards; brighter things have smaller magnitudes and faint things have larger magnitudes. Observing limits In general, the brighter a target is, the easier it will be to detect and you will require less time for the observations. Our equipment can detect 16 th magnitude stars in about 20 minutes of observing time. So stars brighter than this are acceptable for observations. When observing galaxies or other extended objects (i.e. big objects), you also have to consider the size. The magnitude of a galaxy is given
as if all of the light was concentrated into a single point. In reality the light is spread out over the entire region of the galaxy. So a 10 th magnitude galaxy that is 30 in diameter will be much fainter than another 10 th magnitude galaxy that is only 5 in diameter. (see the next section on Size for a definition of 30 and 5 ). Size Background The size of astronomical object can be described in two ways: one can give the physical size, which is how big that object actually is (i.e. measured in kms or light years or another such unit) or one can give an object s angular size, which is how large that object appears in our sky. The angular size is most useful when observing, since objects that are the same physical size can have very different angular sizes if they are located at different distances. An object that is twice as far away as another object of the same physical size will appear to have half of the angular size. An angular size is given in units of degrees. One degree can be further broken up into arcminutes ( ) and arcseconds ( ); there are 60 arcminutes in 1 degree, and 60 arcseconds in 1 arcminute. A simple and handy method for measuring angles on the sky is to use your hand and fingers as a cross-staff. Hold your arm fully extended outward from your body. Sight from one eye to the edges of your clenched fist. The angle that the edges of your fist make at your eye held this way is about 10 degrees. You are to determine the angular size of width of a) the first knuckle on your index finger; b) your fist; and, c) the tip of your thumb to the tip of your little finger. Begin by measuring the following dimensions: a) Width of the first knuckle of your index finger, b) Width of your fist from edge to edge (from the index finger to the little finger), c) Length of the span of your hand from the tip of your thumb to the tip of your little finger when you spread your hand as widely as possible, and, d) Distance from your eye to your hand held outstretched from your body. You compute the angular size of the first knuckle by dividing the width from measurement a) by the distance from your eye measured in d) and multiplying that quantity by 57 degrees. This gives you the angular size in degrees. That is, the angular size in degrees is given by: Width of the first knuckle of index finger --------------------------------------------------- x 57. distance from eye to the first knuckle
The sizes you get will be about 2.5, 10 and 20 (Note the sizes may be as small as 1, 5 and 10 depending on the size of your hand and length of arm). Be sure to record all your measurements in your logbook along with your calculations. Observing limits When selecting your target, it is important to keep the size of the detector in mind. The images that you will receive are about 12 on a side (for reference, the size of the full moon is about 30 ). It would be very difficult to observe an extended object that is smaller than 1 or 2 ; the object would appear to look like a star in this case and very little detail would be visible. It is also difficult to observe an object that is much larger than 12, unless one is only interested in a portion of the object.