# Telescope
**Introduction to Astronomy**

> “*For my part I know nothing with any
> certainty; but the sight of the stars
> makes me dream.*”
> -- Vincent van Gogh, Dutch painter

Astronomy is one of the gems of life that everyone should cherish. Gazing up at the stars is a wonder that never ceases to amaze. Astronomy is a hobby for any age, any race, and any class of wealth. If you can’t afford a telescope, pick up a pair of cheap binoculars or just gaze at the stars with your own eyes. In fact, there are several star clusters, a nebula, and some galaxies that can be seen with your own naked eyes, along with meteor showers and beautiful colors of stars to see without a telescope or binoculars.

The goal of this book is to educate the new astronomer and help the seasoned astronomer gain new knowledge and build on their skills. It will include sections on the Planets, the Moon, the Stars, and
deep sky objects (things like clusters, galaxies, and nebulae). The book will also expansively cover the constellations of each season, and what is best seen in those seasons. Graphics will be included to aid you in your endeavors.

The book will also include information about telescopes, and telescope accessories, that are available for you to buy. There will be a reference section at the end, where you will have tables of the planets, the moons of the planets, and various other important information and equations that the astronomer will need in the field.

Clear skies,
the Author

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**The
Telescope**
> “*Equipped with his five senses,
> man explores the universe around him
> and calls the adventures Science.*”
> -- Edwin Hubble, astrophysicist
> and father of extragalactic
> astronomy.

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The telescope was a wondrous invention of the early 17th century. Hans Lipperhey, a Dutch eyeglass maker, filed the first patent for a telescope in 1608, and is widely credited with its invention. Lipperhey said his instrument was “for seeing things far away as if they were nearby.” Lipperhey’s telescope has evolved into what they are today over centuries of tinkering with the original design.

There are three main types of telescope: refractor, reflector, and catadioptric. The catadioptric scopes are further divided into classes like Maksutovs and Schmidt-Cassegrains.

**Types of Telescope**
Lipperhey’s telescope was a small refractor, meaning that it “refracted” light. The telescope has a convex lens (known as the objective) at the end, bending light rays inward until they all converge at the end of the telescope. The convergence of light rays is called focusing. If the light rays do not converge on the back of the telescope, the focus may need adjusting. This is done by turning a knob on the telescope until the image in the scope becomes clear.

Refracting telescopes are typically made in smaller apertures. The aperture of a telescope is how wide the lens or mirror in it is. Larger apertures allow more light into the scope, meaning you can see dimmer things through the telescope.

The reflecting telescope, known by the astronomical community as “reflectors,” was invented by famed mathematician and astronomer Sir Isaac Newton in 1668. Reflecting telescopes are arguably the most widespread telescopes, being the scope of choice for large telescopes. A reflector, unlike a refractor, has an open end. At the back of the telescope, there is a large mirror called the primary. The primary reflects light back towards the open end where it hits the secondary mirror, which reflects the light into the eyepiece.


Reflectors need focusing, like refractors. A knob, typically near the eyepiece, will shift the position of said eyepiece. Turn the knob until stars come to a point. Reflectors have a small thorn in their side, however. Towards the edge of the image in the eyepiece, stars may appear to have a slight tail. This is called a coma. Coma is caused by light rays hitting the eyepiece edges at a slight angle, an effect of having a parabolic mirror (a mirror that is curved inward, see above).

Catadioptric telescopes are a special combination of reflecting and refracting glass. The word “catadioptric” comes from the combination of dioptrics (lenses) and catoptrics (mirrors). A catadioptric telescope, as you may have guessed from the origin of the word, is a telescope that makes use of both lenses and mirrors. There are several ways to do this, but we will cover the most common ones.

The telescope at left is a Schmidt-Cassegrain. The light goes in through a frontal lens, hits a mirror at the back, then reflects onto a flat secondary, then the eyepiece.

Gregory-Maksutovs, like at right, have a curved lens instead of a flat one like a Cassegrain. Otherwise, they are very similar.


This scope is a Rutten-Maksutov, a.k.a. a “Rumak.” It is nearly identical to the Gregory-Maksutov, except the secondary mirror isn’t directly touching the lens. Rather, there is a space between the secondary and the objective lens.

Catadioptrics are by far the most expensive telescope, due to all the optics (lenses and mirrors) involved. They also have a very long focal length, which is the length the light must travel through a scope to focus. Because the light bounces back and forth in a catadioptric scope, it travels more space, giving the scope a higher focal length. To clarify, the focal length in a refractor or a catadioptric is the distance between the objective lens and the point of focus, and the focal length in a reflector is the distance between the primary mirror and the point of focus.

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**Moving a Telescope**
Moving a telescope across its axes is often called slewing. There are two major ways that telescope makers accomplish this, each with their own capabilities and faults. First, we talk about the sky itself: celestial coordinates.

The celestial coordinates are declination and right ascension. Think of these as latitude and longitude lines projected into the sky, with Polaris (the North star) and Octans (the Octant) as north and south poles, respectively. Declination serves as latitude, in degrees from +90 to -90, and 0 serving as the "celestial equator." Right ascension serves as longitude and is measured in hours, with 24 hours of R.A. encompassing the sky.

There are 24 hours to a circle, and 360 degrees, meaning that there are 15 degrees to an hour. Measurements smaller than degrees are often used to describe the separation between objects or the width of planets from Earth. These are minutes and seconds of arc, with 60 arcminutes to a degree and 60 arcseconds to an arcminute.

Now, back to mounting a telescope. The two types of mount are equatorial (moving along the celestial axes) and alt-azimuth (moving up and down, side to side). While alt-azimuth mounts require no setup besides attaching the telescope to it, equatorials require polar alignment. This means lining up your mount with the north or south celestial pole so that it can travel along the celestial coordinates. Although polar alignment can be a bit difficult to master, it makes finding celestial objects extremely easy.

Certain motorized equatorial mounts allow users to "track" objects, meaning the lens or mirror will essentially follow the movement of said object through the sky. This feature is useful to astrophotographers who want to take photos over long periods of time.

Alt-azimuth mounts come in two major types: the "forks" that sits on a tripod or other stand, and the Dobsonian. Forks are simple, with one or two prongs holding up the telescope. Dobsonians are quite simple as well, but have a certain following in astronomy circles, thus having a sort of sentimentality to them.

The Dobsonian was popularized by avid astronomer John Dobson in 1965. Dobson’s goal was to create a cheap but sturdy mount that would make larger telescopes more available to the masses by making them cheaper to produce. Large reflectors are almost always placed on Dobsonian mounts. A telescope placed on a Dobsonian is colloquially known as a “dob” or by the name of the mount. The slang term "dobbin'," coined by Discord user AbsentmindedEagle#4316, is now widely used among teenagers and young adults as a way to say they're heading outside with their Dobsonian.

**Telescope Accessories**
A multitude of telescope accessories are available for purchase on websites such as High Point Scientific, First Light Optics, and company websites like Orion and Celestron. The main accessories are eyepieces and filters, but there are other extras that are worth the buy too, like Barlow lenses and dew shields.

Eyepieces come in a wide variety of sizes and shapes. Eyepieces typically are either 1.25, 2, or 3 inches wide. 0.965 in eyepieces exist but are less common. Buy eyepieces that fit in your eyepiece slot, and buy an adapter that allows you to fit smaller ones in if needed. One important factor of an eyepiece is its magnification. This is calculated by dividing the focal length of your scope by the focal length of the eyepiece. An eyepiece’s focal length is generally given somewhere on it.
There are also zoom eyepieces that can change focal length, allowing you to change magnification without changing out eyepieces. These are generally lower quality, however.

There are several different ways to construct an eyepiece. Some notable ones are the Huygens, Ramsden, Kellner, Plossl, Nagler, and Erfle. The quality of these eyepieces are generally considered to be in this order as well, with Huygens eyepieces being cruddy department store ones and Plossls and Erfles being much more high-end. The included diagrams show the positions and shapes of the lenses in certain types of eyepiece.

Filters are pieces of glass, attached to the end of an eyepiece, that change the view seen. Color filters are simply stained pieces of glass that absorb certain colors, bringing out details in planets. UHC filters (ultra-high contrast) let through specific wavelengths of light (typically OIII, H-Beta, and H-Alpha), all while blocking wavelengths of mercury vapor and other light pollution sources. This increases the contrast of nebulae by darkening the background. Filters that only allow OIII and H-Beta lines also affect certain nebulae.

While some astronomers detest color filters, others claim they make real differences for planetary observations. Red and blue filters are said to bring out Jupiter's belts and Mars' ice caps. Green filters bring out detail in Saturn's rings, while deep green and blue help with lighter areas of the gas giants. Yellow and orange might tease details out of Uranus and Neptune in large scopes, as well as bring contrast to Mars' maria. Purple filters help add contrast Venus' light, uniform clouds.

Barlow lenses (designed by English mathematician Peter Barlow) are attachments to eyepieces that essentially increase the focal length of the telescope. The lens in a Barlow pushes the focus point of light farther out into the Barlow's tube, making the focal length of the telescope longer and the eyepiece focal length shorter. Since eyepiece focal length divided by telescope focal length equals magnification, a Barlow lens increases magnification. Typical Barlows come in 2x, 3x, and 4x magnification.

Finder scopes are miniature telescopes that are attached to a larger scope. Conventional finder scopes, also known as finders, are always refractors. They can either be straight or at a right angle for ease of use. Finders with a right angle at the end are known as RACI (Right Angle Correct Image) because they also show the image as right-side up and unmirrored, unlike telescopes.

Other finders include red dots (which contain no optics at all, and are typically not recommended) and Telrads, which are very special finders. They use a set of rings projected on a small window, making them appear to be on the sky. Centering the approximate location of a celestial object within the rings acts as the finding mechanism.

It is important for finderscopes to be aligned with the main telescope, because they assist in finding objects by acting as a “guide scope.” To align your finder, center a bright star in your telescope. Then turn the knobs on the finder until the bright star is in the center of the finder as well.

RACI finders make use of built-in star diagonals, which are small attachments to the end of a telescope that reflects the light at a 90 degree angle, allowing the user to look through the eyepiece from the side. These are made for the main scope, too. For telescopes that do not offer a way to view from the side, star diagonals are a must. A star diagonal can save much neck and back pain from constant bending over and craning.

There are various ways to combat dew in a telescope. One such way is a dew shield, which is an extension to the light-collecting end of a telescope that helps prevent dew from settling on the mirrors or lenses. Dew heaters are electric heaters that evaporate dew off of your telescope. You can also buy a combination of the two. There are also "dew strips" that can be bought to wrap around eyepieces, to prevent them from fogging up.

Acclimation is the term for allowing your telescope to reach an ambient temperature. When taking your scope outside, allow it to try and become the same temperature as the outside air. Two different temperatures can affect the quality of the optics temporarily, so a recommended buy is an acclimation fan, which is a small fan on your scope that helps it acclimate faster. Some scopes come with one already.

Collimation is aligning the mirrors of a telescope so light hits the eyepiece in the right spot. A telescope that is out of collimation will not focus very well, and in serious cases, the light may miss the secondary mirror entirely and not bounce into the eyepiece at all. A laser collimator is a laser that goes into the eyepiece slot. It will help you realign your telescope by firing a laser at the mirrors. Simply realign the mirrors until the laser hits the right spot of the mirrors. Most laser collimators have a “target face” with a hole in the center. If the laser hits that hole, your telescope is collimated. Consult your telescope’s manual for how to collimate it precisely.

**What Should I Buy?**
There are hundreds of telescopes for the market, with each suiting a set of needs. The high-magnification catadioptric scopes are excellent for planets and small deep sky objects. Reflectors are usually good for all objects, but have coma issues. Small refractors are usually low-quality, but larger ones, as well as small to medium reflectors, are excellent for astrophotography.

Specifically, if you are into observational astronomy, I highly recommend a six or eight inch Dobsonian. Most of these are made by the same optics company, so varying brands have nearly identical scopes. A typical dob comes with a right angle finder, two eyepieces (around 30mm and 9mm), a focus extender, a laser collimator, and likely a 1.25 inch eyepiece adapter. I personally own an Apertura AD8, their eight inch Dobsonian, and it serves me well.

Mount-wise, Dobsonians also trump all. They make large telescopes effortlessly transportable and cheap to boot. While forked mounts can carry large aperture Cassegrains and Maksutovs, these are highly expensive compared to dobs.

In regards to equipment, anti-dew gear, a right angle finder, star diagonal, UHC filter, eyepieces, and a laser collimator are all must-haves. Most trusty scopes come with Plossl eyepieces, which are fairly quality. While expensive, small focal length but wide field eyepieces are handy for observing large objects in a scope. Erfle, Nagler, and Abbe eyepieces are all higher priced but sturdy and well-built lenses for observation. On the other hand, Kellners, Huygenses, and Ramsdens are all pretty crappy pieces, due to their flimsy, primitive builds and low-quality glass.

As stated earlier, RACI finders and star diagonals are necessary. Bending, twisting, and craning your neck to view a straight finder or eyepiece can cause discomfort and takes away from the experience. Dobsonian owner do not have this problem, because the eyepiece slot is loated on the side of the scope.

A UHC filter is an excellent accessory for avid nebula viewers. It helps block light caused by street lamps, dampening the effects of light pollution and adding contrast to nebula from the sky. Look for brands such as Baader, OptoLong, Orion, Celestron, and Astronomik. Although it may be tempting to buy cheaper brands like SVBONY, these filters are often flimsy and let in far too much light to fully serve its purpose. Color filters are a tossup: some astronomers swear by them, others don't. I suggest doing your own research on them, and buying the colors that suit your needs best. Refer to the previous section for information on color filters.

Finally, the need for dew gear and acclimation fans varies between regions. In dry areas, dew gear may not be as necessary than the tropics, where it is humid. Acclimation fans are more necessary in cold areas, where the temperature difference between inside and outside is greater than around the equator. Buy this gear based on your own location and climate.

In addition to telescope gear, flashlights, headlamps, and some good reading is recommended. Red flashlights are easiest on the eyes, messing with the human night vision the least. These are a must have, with headlamps having the perk of being hands-free. There are a bounty of excellent books to supplement this one out in the field. While The Stargazer's Guide to Dobbin' (and other Astronomical Hijinks) is an excellent indoor book, besides its limited star maps and tables, there are many other reads that are much better outside.

This includes Turn Left at Orion by Guy Consolmagno and Dan M. Davis, specializing in the deep sky; Michael E. Bakich's Complete Star Atlas, for navigation of the sky; and NightWatch by Terence Dickinson, for general information. The computer software and cell phone app Stellarium is another excellent navigation service, with an interactive map depicting stars far dimmer than Bakich's atlas.

**The Scale of Magnitude**
The final topic to be covered in this section is the scale of apparent magnitude. Although it isn't strictly telescope related, it is a vital concept of observational astronomy, telescope or no. Apparent magnitude, or apmag, is a precise way of quantifying an object’s brightness in the sky. In astronomy, apparent magnitude is almost always referred to as just magnitude. The scale was first developed by the Ancient Roman astronomer Claudius Ptolemy, who listed stars from magnitude 1 (brightest) to magnitude 6 (dimmest).

The math behind the current magnitude scale closely matches Claudius Ptolemy’s. An increase of 1 in magnitude is 100 to the fifth root brighter, or approximately 2.512. For example, a magnitude 4 star is 2.512 times brighter than a magnitude 5 star, and 2.5122 times brighter than a magnitude 6 star. USing the same math, a magnitude 1 star is exactly 100 times brighter than a magnitude 7 star.

The brightest stars in the sky are no brighter than about -1.5, but planets can run in the -2 to -4 range. The Sun has a magnitude of -26.7, the brightest object in the sky by far. In contrast, some of the dimmest and farther objects seen by the Hubble Space Telescope can reach past magnitude 30.

When considering wide objects like galaxies and nebulae, take apparent magnitude with a grain of salt. Wide and spacious objects tend to appear dimmer than a star of the same magnitude. This is because their light is not consolidated into one point, making them harder to see. Another factor in this is albedo and surface reflectivity.

Some objects are harder to see, not because they are spaced out, but because they reflect less light than similar objects. I cannot stress enough how important this is: a spread out object of magnitude 4 appears dimmer than a magnitude 4 star, but if you balled the nebula's light up into a point, it would be just as bright. To summarize, be careful when seeing magnitude statistics, especially with spread out (diffuse) objects.

The human eye can see down to magnitude 6.5 in optimal conditions. This means there is peak seeing, transparency, and dark skies. Seeing is a measurement of how still the air in the sky is. If it’s windy outside, chances are the seeing will be bad. Poor seeing causes stars and planets to appear wobbly, but will have less effect on nebulae and galaxies. Transparency is a measure of how clear the sky is. If the sky is foggy, smoky, or has thin clouds, this is considered poor transparency. Smoke and fog both lighten the sky and dampen the brightness of objects, making it difficult to see nebulae and galaxies, while having a minimal effect on stars.