Using a Telescope for VSO

Do not worry about what telescope works best for variable-star observing. Use what you have or what you can afford. If you are new to amateur astronomy, I will tell you something that will take years to discover on your own. You will never be satisfied with the telescope that you have. It will be too small, too big, too heavy, too light, lacking a computer system, without motor drive ... the list is perpetual. You must, at some point, just say "enough is enough," and use what you have to its full potential. A good telescope, regardless of size, will provide you access to more variable stars than you can observe and study within your lifetime.

However, if you haven't reached that point where "enough is enough," and you're thinking about purchasing a new telescope, wishing to specialize in variable-star observing, then this section may provide some ideas that could be valuable. If you already have a telescope, I think that you'll find some value within this section too. The most important thing is to understand how your telescope works. I mean how it really works! It's crucial for you to use your telescope's full capability and observing variable stars will require you to use all of the capabilities of your telescope. Your understanding of a few important terms is essential.

The single most important factor to a variable-star observer is aperture. The primary function of all telescopes is to collect light and at any given magnification, the larger the aperture, the better the image will be. The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified in either inches, centimeters (cm) or millimeters (mm). With all things being equal (this is of course never the case), the larger the aperture, the more light the telescope collects and the brighter and better the image will be for the observer. Greater detail and image clarity will be apparent as aperture increases. Considering your budget and portability requirements, select a telescope with as large an aperture as possible. This is the one physical characteristic of your telescope with which you will never be satisfied. All amateur astronomers suffer from aperture fever and it's chronic. You will always want a bigger telescope. It's not just a "guy" thing either.

Focal length (FL) is the distance in an optical system from the lens, or primary mirror, to the point where the image is considered to be in focus. This point is called the focal point. The longer the focal length of the telescope, generally the more magnifying power, the larger the image and the smaller the field of view. For example, a telescope with a focal length of 2000 mm has twice the power and half the field of view of a 1000 mm telescope when using the same eyepiece. Most manufacturers specify the focal length of their various instruments; but if it is unknown and you know the focal ratio you can use the following formula to calculate the focal length: focal length is the aperture (in mm) times the focal ratio. For example, the focal length of an 8 inch (203 mm) aperture with a focal ratio of fl 10 would be 203 x 10 = 2030 mm. This would normally be rounded to 2000 mm.

Resolution is the ability of a telescope to render detail. The higher the resolution, the finer the detail. With all other things being equal, the larger the aperture of a telescope, the better the resolution. Resolution is not the most important criterion with which to judge a telescope for variable star observing since you are not going to see any detail on any star except the Sun (with proper filters, of course!). What this means is, you don't need to buy a really expensive refractor when a reflector, with a smidgen less reflector is usually bigger too.

Light gathering capability (light grasp) is the telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture. You can calculate this by first dividing the aperture of the telescope (in mm) by 7 mm (average size of a dilated eye) and then squaring this result. For example, an 8 inch telescope has a light gathering power of 843 (check the math: (203/7)2 = 843). This means that an 8 inch telescope will collect 843 time more light than your naked eye. It should be obvious that even a small telescope is going to collect much more light than your eye. If you understand this, you also understand that you do not need a huge telescope to observe variable stars.

Magnification is the least important factor and magnification is a relationship between two independent optical systems: the telescope itself, and the eyepiece that you are using.

To determine magnification power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the magnification power of your telescope. For example, a 25 mm eyepiece used on an //10, 8 inch (FL = 2000 mm) telescope will yield a power of 80 x (check the math: 2000/25 = 80) and a 12.5 mm eyepiece used on the same instrument would yield a power of 160 x (2000/12.5 = 160). Since eyepieces are interchangeable, a telescope can be used with a variety of powers for different applications by simply changing eyepieces.

There are practical upper and lower limits of magnification for telescopes. These limits are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable magnification is approximately 50 times the aperture of the telescope (in inches) under ideal conditions. Magnification higher than this usually results in a dim, low-contrast image. For example, the maximum magnification possible for a 60 mm telescope (2.4 inch aperture) is approximately 140x. As magnification increases beyond a certain point, the sharpness and detail will be diminished. This is why the large magnification advertised for some small telescopes is not possible. I've seen 1200x listed for some small telescopes. This is absolutely impossible!

Anyway, most of your observing will be done with low powers. With lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. Another advantage to using lower power is that your greater FOV will make comparison stars easier to find.

There is also a lower limit of magnification, usually between three and four times the aperture of the telescope when using your telescope at night. Magnification lower than these limits is not useful with most telescopes and a dark spot may appear in the center of the eyepiece in a catadioptric or Newtonian telescope because of the secondary or diagonal mirror's shadow.

If you're new to astronomy, you may be wondering how far you can see with a telescope; in other words, can you see far enough to observe a variable star. Your friends will eventually ask this question too. Astronomers use a system of magnitudes to indicate the brightness of a star. A star is said to have a certain numerical magnitude. The larger the magnitude number, the fainter the star. Each star with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye (no telescope) is about sixth magnitude (from dark skies) whereas the brightest stars are magnitude zero (or even a negative number). So you see, it's not really a question of "how far" but "how bright."

The magnitude of the faintest star that you can see with a telescope (under excellent seeing conditions) is referred to as the limiting magnitude of your telescope. The limiting magnitude is directly related to aperture, so that larger apertures allow you to see fainter stars. A rough formula for calculating visual limiting magnitude is: 7.5 + 5 LOG(aperture in cm). For example, the limiting magnitude of an 8 inch aperture telescope is about 14.0 (check the math: 7.5 + 5 LOG 20.32 = 7.5 + (5 x 1.3) = 14.0). Atmospheric conditions and your visual acuity will often reduce the limiting magnitude a bit; however, it's not unusual to see stars a bit fainter than the limiting magnitude of your telescope on exceptional nights. The more time you spend looking through your telescope, the better you will be able to see faint stars. It takes time.

Using a CCD, you can extend the limiting magnitude by three, or four more magnitudes. With the proper application of colorful language, even another two magnitudes are possible.

Focal ratio is the ratio of the focal length of the telescope to its aperture. To calculate, divide the focal length (in mm) by the aperture (in mm). For example, a telescope with a 2030 mm focal length and an aperture of 8 inches (203 mm) has a focal ratio of 10 (check the math: 2030/203 = 10). This is normally specified as //10.

Some astronomers equate focal ratios with image brightness but strictly speaking this is only true when a telescope is used photographically and then only when taking pictures of so-called extended objects like the Moon and nebulae. Telescopes with small focal ratios are sometimes called fast and will produce brighter images of extended objects on film and thus require shorter exposure times. Generally speaking, the main advantage of having a fast focal ratio with a telescope used visually is that it will deliver a wider field of view. Generally, fast focal ratio telescopes are considered to be //3.5 to //6, medium focal ratios are //7 to //ll, and slow focal ratios are //12 and longer. Of course, these boundaries are soft and some dispute should be expected.

The amount of sky that you can view through a telescope is called the true field-of-view and is measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. Angular field of view is calculated by dividing the power being used into the apparent field of view, using degrees, of the eyepiece being used. For example, if you were using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece was lOOx, then the field of view would be 0.5 degrees (check the math: 50/100 = 0.5).

Manufacturers will normally specify the apparent field (in degrees) of their eyepiece designs. The larger the apparent field of the eyepiece (in general), the larger the real field of view and thus the more sky you can see. Lower powers used on a telescope allow much wider fields of view than do higher powers. As I said before, you'll use low power more than high power when observing variable stars.

There are several optical designs used for telescopes. Remember that a telescope is designed to collect light and when designing optical systems the optical engineer must make tradeoffs in controlling aberrations to achieve the desired result of the design.

Aberrations are any errors that result in the imperfection of an image. Such errors can result from design or fabrication or both. It is impossible to design an absolutely perfect optical system. The various aberrations due to a particular design are noted in the discussion on types of telescopes. In general, various aberrations that you will discover are discussed next.

Chromatic aberration is usually associated with objective lenses of refractor telescopes. It is the failure of a lens to bring light of different wavelengths (colors) to a common focus. This results in a faint colored halo (usually violet) around bright stars, the planets and the Moon. Achromatic doublets (a lens arrangement) in refractors help reduce this aberration and more expensive, sophisticated designs like apochromats and those using fluorite lenses can virtually eliminate it. Mild chromatic aberration should not effect your ability to observe variable stars.

Spherical aberration causes light rays passing through a lens, or reflected from a mirror, at different distances from the optical center to come to focus at different points on the axis. This causes a star to be seen as a blurred disk rather than a sharp point. Most telescopes are designed to eliminate this aberration but it's best to purchase telescopes with hyperbolic mirrors rather than spherical mirrors (of course, this pertains to reflectors only).

Coma is associated mainly with parabolic reflector telescopes that affect the off-axis images and is more pronounced near the edges of the field of view. The images seen produce a V-shaped appearance. The faster the focal ratio, the more coma that will be seen near the edge although the center of the field (approximately a circle, that is defined (in mm) as the square of the focal ratio) will still be coma-free in well-designed and manufactured instruments.

A lens aberration that elongates your images causing them to change from a horizontal to a vertical position on opposite sides of best focus is called astigmatism. It is generally found in poorly made optics or when collimation errors are present.

Field curvature is caused by the light rays not all coming to a sharp focus in the same plane. The center of the field may be sharp and in focus but the edges are out of focus and vice versa.

Collimation is a word used to mean the proper alignment of the optical elements in a telescope and proper collimation is critical for achieving optimum results. Poor collimation will result in optical aberrations and distorted images. The alignment of the optical elements is important but more critical is the alignment of the optics with the mechanical tube. This is called optical/mechanical alignment. Time must be taken to check this occasionally.

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