Choosing a Mounting for your First Telescope
In the previous page we have understood various telescope designs. But these telescopes are big, and quite bulky. They need some form of a stable support so that observations can be largely hands free. Most medium to large telescopes are advertised as Optical Tube Assembly (OTA) only. This is because the user can then have a choice between various support systems available. The common term for these supports and Mounts. Mounts that cost £100 £2000 are typically supplied with a Tripod. Expensive ones have tripod optional. So, question is, what type of mounts exists and what do they do?
The user needs to find objects in the sky. If he/she wishes to use star charts to navigate the stars and find the object, perhaps the best way to do so is by moving the telescope up or down and left or right (Altitude-Azimuth). Such a system is very simple and large objects can be found with ease and small objects like galaxies, nebulae etc can be found by the method of star-hopping. If the manufacturer fits gears and electronic and computer controlled motors, such a system becomes very versatile for the visual observer. The computerization is called GOTO because the computer once set up will make the telescope "GOTO" any object of desire in the night sky.
A simple representation of an Alt-Az mount is shown above (left).
A simple representation of an Alt-Az mount is shown above (left).
What we also know thanks to popular documentaries and talks on TV is that Earth has a pole star. This star, for all practical purposes is considered stationary. Earth rotates West to East giving us the impression that the Sun, Moon, stars rise and set East to West. If the pole star is exactly in line with Earth's North pole, then these objects rotate around this star. What if we were to have one of the rotational axis of the mount point at the pole star? It will then make all stars rotate around this axis. This helps us because through the telescope, where we see for example the moon at 100x magnification, rotation of the Earth is also "magnified" in the eyepiece. That is, if the rotational axis is not motorised, then the object will quickly go out of the field of view of the eyepiece and the user need to move it slightly to bring it back. Imagine doing this in both axis if it were an Alt-Az mount.
However in the mount that looks at the pole star, one needs to move only one axis, thus keeping track of objects easier. This mount is commonly known as the Equatorial mount. It is shown in the photo below (courtesy Optical Vision Ltd). Notice how one of the axis is tilted? Other objects you will notice is a set of weights. This is simply to balance out the weight of the telescope. These are required for smooth motions. Otherwise, the it will be one side too heavy and the telescope can tip over.
A photo of a telescope on a equatorial mount can be found on the left and at the start of this page. One point to be noted is that all telescopes come fitted with a pair of rings and a metal bar underneath. This metal bar, called a dovetail bar is of standard size. Typically 45mm or 75mm width. The 45mm bar, supplied on telescopes up to 11 inch aperture fits on any mount.
Summarizing the above (as well as information given in the page about choosing your first telescope), Astronomy is a vast, diverse hobby. It can be practiced by anyone with reasonable interest. One does not have to invest in equipment worth thousands of pounds. Even a good pair of binoculars will suffice. A proper, well made telescope will last very long. I must advice the beginner against buying telescopes from camera stores. The camera stores, most of the time have little to no idea of what they sell. Below are the few points one may choose to follow for selecting the telescope.
a) Optical design
b)Bulk: Telescopes can be portable or extremely bulky. Consider logistics before a purchase. Visit your local telescope showrooms to understand actual size. Look at the links section for telescope dealers locally.
c)Optical Coating: If it i Multi-coated in case of a refractor, or the quality of the mirror in case of reflector or cassegrain.
d)Theoretical limit for magnification: From a city, approximately 40 times per inch of aperture.
e)Aperture: The size of the objective lens or mirror.
f) Focal Length: Distance (usually in mm) from the center of the objective to the point where all rays of light come to a sharp focus.
g)Focal ratio: The Focal Length divided by the Aperture. Smaller the number, wider the field of view.
h)Field of View: Area of sky covered by the objective. Larger the focal length, smaller the field of view.
i)Magnification: The Focal length of the objective divided by focal length of the eyepiece. Larger this number, lesser the field of view and better suited for planetary and Lunar observation. if magnification is small, more sky is covered, permitting the telescope to gather more light from faint objects. The Thumb rule is that faint fuzzies (galaxies, nebulae) become fuzzier at high magnification. Planets and the moon, being near and being solid objects sustain high magnification and show detail.
j)Eyepiece: can be made up of a single lens or a combination of several lenses. Thumb rule is that higher the magnification, better should be the optical quality. Mass manufactured telescopes are supplied with average quality eyeieces to get the user started off in the hobby.
NOTE: If the user purchases a telescope from "X" brand, he does not have to use eyepieces from the same brand. One may use eyepieces from Y brand as the eyepieces and several accessories are made for two standard sizes 1.25" or 2". Ask before you buy.
k)True field of view: This is the "actual" area of sky covered and varies with the eyepiece used at that point of time. It is the apparent field of view of the eyepiece divided by the Magnification of the telescope .
l)Apparent field of view: Depending on the optical design of the eyepiece, its own field of view can vary between 35 degrees to 82 degrees. Eyepieces are available today which have about 100 degrees too, but are very expensive and seldom necessary. a good quality plossl (almost pronounced Plow-sell) typically has 52 degrees apparent field. Therefore, when a 25mm focal length plossl is used with a telescope of 1000mm focal length;
Magnification = 1000/25 = 40x
True Field of view: 52/40 = 1.3 degrees.
The full moon is half a degree in diameter. therefore the above calculation shows that the telescope will cover two full moons and more room to spare.
The book I was suggesting in the above article is "Telescope, Eyepieces and Astrographs" published by Willmann-Bell of USA and available at leading telescope retailers in UK for a good price.
A photo of a telescope on a equatorial mount can be found on the left and at the start of this page. One point to be noted is that all telescopes come fitted with a pair of rings and a metal bar underneath. This metal bar, called a dovetail bar is of standard size. Typically 45mm or 75mm width. The 45mm bar, supplied on telescopes up to 11 inch aperture fits on any mount.
Summarizing the above (as well as information given in the page about choosing your first telescope), Astronomy is a vast, diverse hobby. It can be practiced by anyone with reasonable interest. One does not have to invest in equipment worth thousands of pounds. Even a good pair of binoculars will suffice. A proper, well made telescope will last very long. I must advice the beginner against buying telescopes from camera stores. The camera stores, most of the time have little to no idea of what they sell. Below are the few points one may choose to follow for selecting the telescope.
a) Optical design
b)Bulk: Telescopes can be portable or extremely bulky. Consider logistics before a purchase. Visit your local telescope showrooms to understand actual size. Look at the links section for telescope dealers locally.
c)Optical Coating: If it i Multi-coated in case of a refractor, or the quality of the mirror in case of reflector or cassegrain.
d)Theoretical limit for magnification: From a city, approximately 40 times per inch of aperture.
e)Aperture: The size of the objective lens or mirror.
f) Focal Length: Distance (usually in mm) from the center of the objective to the point where all rays of light come to a sharp focus.
g)Focal ratio: The Focal Length divided by the Aperture. Smaller the number, wider the field of view.
h)Field of View: Area of sky covered by the objective. Larger the focal length, smaller the field of view.
i)Magnification: The Focal length of the objective divided by focal length of the eyepiece. Larger this number, lesser the field of view and better suited for planetary and Lunar observation. if magnification is small, more sky is covered, permitting the telescope to gather more light from faint objects. The Thumb rule is that faint fuzzies (galaxies, nebulae) become fuzzier at high magnification. Planets and the moon, being near and being solid objects sustain high magnification and show detail.
j)Eyepiece: can be made up of a single lens or a combination of several lenses. Thumb rule is that higher the magnification, better should be the optical quality. Mass manufactured telescopes are supplied with average quality eyeieces to get the user started off in the hobby.
NOTE: If the user purchases a telescope from "X" brand, he does not have to use eyepieces from the same brand. One may use eyepieces from Y brand as the eyepieces and several accessories are made for two standard sizes 1.25" or 2". Ask before you buy.
k)True field of view: This is the "actual" area of sky covered and varies with the eyepiece used at that point of time. It is the apparent field of view of the eyepiece divided by the Magnification of the telescope .
l)Apparent field of view: Depending on the optical design of the eyepiece, its own field of view can vary between 35 degrees to 82 degrees. Eyepieces are available today which have about 100 degrees too, but are very expensive and seldom necessary. a good quality plossl (almost pronounced Plow-sell) typically has 52 degrees apparent field. Therefore, when a 25mm focal length plossl is used with a telescope of 1000mm focal length;
Magnification = 1000/25 = 40x
True Field of view: 52/40 = 1.3 degrees.
The full moon is half a degree in diameter. therefore the above calculation shows that the telescope will cover two full moons and more room to spare.
The book I was suggesting in the above article is "Telescope, Eyepieces and Astrographs" published by Willmann-Bell of USA and available at leading telescope retailers in UK for a good price.