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Equipment / Equipment Terms (Beginner's Equipment Topic #1)
« Last post by Thomas on February 20, 2014, 08:44:36 PM »The goal of this post is to give the reader a basic understanding of the terms and concepts surrounding amateur astronomy equipment so the reader can understand the purpose of each piece of equipment and how equipment choices affect one's observing and photography.
The terms covered here are focal length, aperture, focal ratio, eye relief, exit pupil, field of view, true field of view, and apparent field of view, telescope, eyepiece, barlow, diagonal, and filter. These definitions are meant to describe in a concise manner the most important aspect of the term being defined to a newcomer. In most cases there is a more formal definition which is strictly accurate, but which may be unnecessarily confusing if the goal is to convey a practical use of the term.
The next topic is a general discussion of telescopes.
The terms covered here are focal length, aperture, focal ratio, eye relief, exit pupil, field of view, true field of view, and apparent field of view, telescope, eyepiece, barlow, diagonal, and filter. These definitions are meant to describe in a concise manner the most important aspect of the term being defined to a newcomer. In most cases there is a more formal definition which is strictly accurate, but which may be unnecessarily confusing if the goal is to convey a practical use of the term.
- Telescope - An instrument that gathers light and focuses it. Its primary function is to make dim objects brighter, and its secondary function is to make details more visible.
- Eyepiece - Used to magnify the telescope image. Eyepieces are primarily used by visual astronomers, but they are sometimes used in astrophotography.
- Barlow - Increases the focal length of a telescope.
- Diagonal - Used with some telescopes to divert the optical path to a more convenient angle.
- Filter - Used to reduce or eliminate undesired frequencies of light.
- Mount - This is the mechanism that holds the telescope and allows you to aim the telescope at different parts of the sky.
- Objective - The main light collecting element of a telescope.
- Aperture - The diameter of a telescope's objective.
- Focal Length - Focal length is used to characterize both telescopes and eyepieces.
- Focal Ratio - This is the ratio of the telescope's aperture to its focal length. Telescopes will commonly be specified by the aperture and either the focal ratio or the focal length.
- Eye Relief - This is the distance of your eye from an eyepiece.
- Exit Pupil - This is diameter of the image that exits the eyepiece.
- Field of View - This term is a bit ambiguous. If applied to a telescope it would probably describe the maximum angle of sky viewable through the instrument. If applied to an eyepiece, it would probably refer to the apparent field of view described below, but the user of the term may be implicitly referring to a true field of view which would apply to a particular eyepiece and telescope combination (see true field of view below).
- True Field of View - This is the angle of sky viewable with a particular eyepiece in a particular telescope.
- Apparent Field of View - A given telescope and eyepiece will show the user a specific area of the sky, but the design of the eyepiece may spread that area out over, for example, 30 or 80 degrees.
The next topic is a general discussion of telescopes.
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Equipment / Telescopes (Beginner's Equipment Topic #2)
« Last post by Thomas on February 20, 2014, 08:44:22 PM »The objective of this post is to give the reader a sense for the different types of telescopes typically in use by amateurs and the areas of astronomy for which they are best suited. Like the other beginner topics in the forums, there may be some simplifications here which overlook some of the nuances that more experienced observers just love to discuss with the goal of keeping things simple for the newcomer while capturing the essence of the topic.
Telescopes are categorized in three general categories based on the composition of the optical components. This section will be be broken up into these categories. Telescopes that use only mirrors to gather and focus light are called reflectors (or rarely catoptrics). Telescopes that use only only lenses to gather and focus light are called refractors (or rarely dioptrics). The final category of telescopes are those that use both mirrors and lenses to gather and focus the light, and these telescopes are called catadioptrics.
When shopping for telescopes in the United States, telescopes are nearly always specified by aperture and focal ratio. In Europe, it seems that this is still the most common practice, but you may also see some telescopes specified by aperture and focal length. Examples of telescopes specified by aperture and focal length would be “90mm f/10” or “8” f/5”. An example of a telescope specified by aperture and focal length would be “100/800” where 100 is the aperture in mm and 800 is the focal length in mm.
The five primary aspects when considering a telescope are size, optical quality, aperture, focal length, and focal ratio.
Size
Size matters! For newcomers to the hobby, this attribute is really a combination of the size of the telescope and the mount required to use it because you will probably buy both together as part of a single combined package. Size may be more important than any of the optical attributes listed above because if you buy a telescope that is too large or cumbersome, you won't be using it. At that point it doesn't matter what the optical properties are. Fortunately, this isn't a complicated matter to understand. You just need to consider when looking at a telescope if it will fit in your car if you're planning to take it to dark skies outside the city or if the telescope is too heavy for you to move around easily. This is one of the main drivers behind the advice to seek out your local astronomy club. Many areas of the country no longer have shops where telescopes are setup to look at before you buy. Even if you're lucky enough to have such a shop, the local club is likely to have a wider variety of telescopes than would be on display at the shop.
Optical Quality
This needs to mentioned because a simplifying assumption in these beginner topics is that to describe the other metrics of your equipment the optical quality of the equipment being compared is similar. This is intuitively obvious. Telescopes are just tools, and just like, for example, wrenches, some may be manufactured to higher standards or made from better materials than others. With telescopes it can be that on paper one telescope could look better than another, but poor optical quality can override other factors. Unfortunately, manufacturer specifications can be difficult to find or vague when they are found. One can rely on price to some extent, because if something is super cheap its likely to be on the lower end of the quality scale, but there ways other than skimping on optical quality for manufacturers to things cheaper. For example, the mount quality could be lowered. On line reviews can be helpful, but many of those are perhaps too unforgiving at least with respect to the quality standards that one might be using for a beginner's telescope.
Aperture
The brightness of objects in your telescope is a function of the area of the primary objective. In other words, if you double your aperture, the diameter of your objective, objects will appear four times brighter not two times brighter.
The aperture also affects the maximum resolution of your telescope, but this relationship is linear. Thus, if you double the aperture of your telescope, you will get a theoretical doubling in resolution although in actual practice the atmosphere is going to put an upper limit on your resolving power. So what is resolution? Put at its most simple, resolving power is the ability of an optic to distinguish two points from each other. So imagine two stars that are very close to one another. If when viewed through a telescope they appear as two separate stars, they are said to be resolved. This is sufficient depth on this resolution for now. A more in depth discussion of resolution will be provided in a separate post.
Focal Length
This is the distance from the objective required for the image to come to focus. Simply put, telescopes with longer focal lengths magnify objects more than telescopes with shorter focal lengths. The corollary to this is that telescopes with longer focal lengths smaller areas of sky than telescopes with short focal lengths. The exact magnification produced depends on the eyepiece being used (which will be discussed separately with eyepieces), but when considering multiple telescopes against each other remember the longer focal lengths produce more magnification.
Focal Ratio
The focal ratio of the telescope is derived by dividing the focal length of the telescope by the telescope's aperture. Make sure that the focal length and aperture are in the same units (i.e both in inches or both in mm). A focal ratio of 10 would be expressed as “f/10”. This measure is identical to the focal ratio used in camera lenses if you're familiar with how that works.
The focal ratio is used to calculate the exit pupil of an eyepiece used in a particular scope. Telescopes with larger focal ratios (slow telescopes) are also more forgiving of eyepiece aberrations. Correcting for optical aberrations generally increases the cost of an optic, so in general cheaper eyepieces perform better in slow telescopes (telescopes with large focal ratios) than they do in fast telescopes (telescopes with small focal ratios).
See the following posts for a discussion of each telescope design.
Reflectors
Refractors
Catadioptrics
Telescopes are categorized in three general categories based on the composition of the optical components. This section will be be broken up into these categories. Telescopes that use only mirrors to gather and focus light are called reflectors (or rarely catoptrics). Telescopes that use only only lenses to gather and focus light are called refractors (or rarely dioptrics). The final category of telescopes are those that use both mirrors and lenses to gather and focus the light, and these telescopes are called catadioptrics.
When shopping for telescopes in the United States, telescopes are nearly always specified by aperture and focal ratio. In Europe, it seems that this is still the most common practice, but you may also see some telescopes specified by aperture and focal length. Examples of telescopes specified by aperture and focal length would be “90mm f/10” or “8” f/5”. An example of a telescope specified by aperture and focal length would be “100/800” where 100 is the aperture in mm and 800 is the focal length in mm.
The five primary aspects when considering a telescope are size, optical quality, aperture, focal length, and focal ratio.
Size
Size matters! For newcomers to the hobby, this attribute is really a combination of the size of the telescope and the mount required to use it because you will probably buy both together as part of a single combined package. Size may be more important than any of the optical attributes listed above because if you buy a telescope that is too large or cumbersome, you won't be using it. At that point it doesn't matter what the optical properties are. Fortunately, this isn't a complicated matter to understand. You just need to consider when looking at a telescope if it will fit in your car if you're planning to take it to dark skies outside the city or if the telescope is too heavy for you to move around easily. This is one of the main drivers behind the advice to seek out your local astronomy club. Many areas of the country no longer have shops where telescopes are setup to look at before you buy. Even if you're lucky enough to have such a shop, the local club is likely to have a wider variety of telescopes than would be on display at the shop.
Optical Quality
This needs to mentioned because a simplifying assumption in these beginner topics is that to describe the other metrics of your equipment the optical quality of the equipment being compared is similar. This is intuitively obvious. Telescopes are just tools, and just like, for example, wrenches, some may be manufactured to higher standards or made from better materials than others. With telescopes it can be that on paper one telescope could look better than another, but poor optical quality can override other factors. Unfortunately, manufacturer specifications can be difficult to find or vague when they are found. One can rely on price to some extent, because if something is super cheap its likely to be on the lower end of the quality scale, but there ways other than skimping on optical quality for manufacturers to things cheaper. For example, the mount quality could be lowered. On line reviews can be helpful, but many of those are perhaps too unforgiving at least with respect to the quality standards that one might be using for a beginner's telescope.
Aperture
The brightness of objects in your telescope is a function of the area of the primary objective. In other words, if you double your aperture, the diameter of your objective, objects will appear four times brighter not two times brighter.
The aperture also affects the maximum resolution of your telescope, but this relationship is linear. Thus, if you double the aperture of your telescope, you will get a theoretical doubling in resolution although in actual practice the atmosphere is going to put an upper limit on your resolving power. So what is resolution? Put at its most simple, resolving power is the ability of an optic to distinguish two points from each other. So imagine two stars that are very close to one another. If when viewed through a telescope they appear as two separate stars, they are said to be resolved. This is sufficient depth on this resolution for now. A more in depth discussion of resolution will be provided in a separate post.
Focal Length
This is the distance from the objective required for the image to come to focus. Simply put, telescopes with longer focal lengths magnify objects more than telescopes with shorter focal lengths. The corollary to this is that telescopes with longer focal lengths smaller areas of sky than telescopes with short focal lengths. The exact magnification produced depends on the eyepiece being used (which will be discussed separately with eyepieces), but when considering multiple telescopes against each other remember the longer focal lengths produce more magnification.
Focal Ratio
The focal ratio of the telescope is derived by dividing the focal length of the telescope by the telescope's aperture. Make sure that the focal length and aperture are in the same units (i.e both in inches or both in mm). A focal ratio of 10 would be expressed as “f/10”. This measure is identical to the focal ratio used in camera lenses if you're familiar with how that works.
The focal ratio is used to calculate the exit pupil of an eyepiece used in a particular scope. Telescopes with larger focal ratios (slow telescopes) are also more forgiving of eyepiece aberrations. Correcting for optical aberrations generally increases the cost of an optic, so in general cheaper eyepieces perform better in slow telescopes (telescopes with large focal ratios) than they do in fast telescopes (telescopes with small focal ratios).
See the following posts for a discussion of each telescope design.
Reflectors
Refractors
Catadioptrics
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Equipment / Reflectors (Beginner's Telescopes Subtopic #1)
« Last post by Thomas on February 20, 2014, 08:38:10 PM »While there are other optical designs in use by amateurs, by far most amateur instruments in this category are Newtonian reflectors. This design was created by none other than Sir Issac Newton nearly 350 years ago. The design is composed of a large primary mirror (in the shape of a paraboloid) and a smaller elliptical flat mirror. The key attributes of the Newtonian telescope are:
Learn about refractors
Learn about catadioptrics
- Lowest cost for a given aperture
The Newtonian optical design is very simple. There are just two optical surfaces. The cheapest models in the < 10" aperture range will utilize either a cardboard or simple metal tube. Many telescopes are sold with a mount that does not track objects being viewed. This all results in the cheapest cost for a given aperture. - Easiest telescope to make
Because of the simple design, most amateur telescope makers will start with a Newtonian. - Typically lowest focal length for a given aperture
Focal ratios are almost always less than f/6 with ratios below f/5 being common. Other optical designs are typically f/6 or greater with f/5 being uncommon. Normally, this would mean that for a given aperture the Newtonian would offer you the greatest field of view, but for reasons that are probably best left to an intermediate topic, beginner Newtonians will typically have a smaller maximum field of view somewhat less than a refractor with a similar aperture. - Secondary Mirror
- Obstructed optical path
Newtonians have a small mirror that redirects the light path out the side of the telescope for viewing. This has a couple of effects optically.- The first, and probably least significant, is that some of the light entering the optical tube does not make it into the final image. So compared to a design with an unobstructed optical path of the same aperture, the Newtonian's image will be somewhat dimmer.
- The more significant effect, is that anytime something is in the optical path it degrades the image. Conventional wisdom is that if the secondary mirror is 20% or less (by diameter) than the primary mirror the effect is negligible. There are surely some that would argue with that position (just as there are probably some that would say 25% is OK). Just keep in mind that as far as optical quality is concerned the smaller the secondary obstruction the better.
- Collimation
In order for the telescope to form images the secondary and primary mirrors must be aligned. This process is called collimation. Some newtonians will need to be collimated every time they are used and others will not. Many people find the process intimidating, but it can be learned without too much difficulty. It does require special tools that will most likely not come with your telescope.
- Obstructed optical path
- Open optical tube
All optical surfaces are exposed to the environment. Designs with long tubes open only on the end can be more susceptible to distortion due to air currents caused by temperature differences between the telescope and the environment (tube currents). - Medium size for a given aperture
The typically small focal ratio and bent optical path means that for a given aperture, Newtonians tend to have fairly short tubes. They aren't the most compact design, but they're not bad. - Coma aberration
Coma is an aberration that causes a loss of sharpness that increases as the distance from the center of the field of view increases. Stars at the edge may appear like little comets.- Telescopes will low focal ratios are affected by the aberration to a larger degree than telescopes with large focal ratios.
- Coma correcting lenses available to reduce the aberration.
Learn about refractors
Learn about catadioptrics
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Equipment / Refractors (Beginner's Telescopes Subtopic #2)
« Last post by Thomas on February 20, 2014, 08:37:03 PM »6
Equipment / Catadioptrics (Beginner's Telescopes Subtopic #3)
« Last post by Thomas on February 20, 2014, 08:35:01 PM »Catadioptrics are very popular amateur instruments. The design was popularized by Celestron in the late 1960's in the form of the Schmidt-cassegrain. Catadioptrics include at least one lens and one mirror. The two most popular designs are the Schmidt-cassegrain and Maksutov-cassegrain. A cassegrain telescope is a purely reflective telescope composed of two mirrors where the image emerges through a hole in the primary mirror at the opposite end from the entrance aperture. The Schmidt and Maksutov modifications to the cassegrain design add a lens at the entrance aperture. Because these two types of catadioptric are by far the most popular and for this level of discussion are essentially identical, the rest of the discussion will focus on the attributes of these two types of catadioptrics which are:
Learn about reflectors
Learn about refractors
- Medium cost for a given aperture
With regard to the optical assembly only, these telescopes have a middle of the range cost. Typical amateur models will be sold with a "goto" mount, however, which pushes up the cost. They're still pretty good deals. - Very compact size for a given aperture.
Probably the most obvious feature of these designs are the shortness of the optical tube. This is the result of the fact that the optical path is folded on itself. Light enters the telescope on one end and hits the primary mirror on the opposite end of the tube. The primary mirror focuses the light at a point that is beyond the front edge of the optical tube, but before the light can escape it is intercepted by a secondary mirror that reflects the light back to the primary. At the center of the primary is a hole that allows the focused light to escape the tube. - Long focal ratios
These telescopes have focal ratios in the range of f/10 - f/15 which is quite long. This results in narrow fields of view and high magnifications. It also means that lower quality eyepieces produce more acceptable images than they would in a faster telescope. - Obstructed optical path
Because of the secondary mirror discussed above, the optical paths in these cassegrains are obstructed. This has the same deleterious effect that it has in the Newtonian reflector discussed previously. Additionally, the secondary obstruction in cassegrains tends to be larger than in Newtonians which further degrades the image to a greater degree. - Secondary Mirror
- Obstructed optical path
Catadioptrics have a small mirror that redirects the light path out the back of the telescope for viewing. This has a couple of effects optically.- The first, and probably least significant, is that some of the light entering the optical tube does not make it into the final image. So compared to a design with an unobstructed optical path of the same aperture, the Newtonian's image will be somewhat dimmer.
- The more significant effect, is that anytime something is in the optical path it degrades the image. The obstruction in these catadioptrics is typically between 30-40%. Conventional wisdom is that if the secondary mirror is 20% or less (by diameter) than the primary mirror the effect is negligible. There are surely some that would argue with that position (just as there are probably some that would say 25% is OK). Just keep in mind that as far as optical quality is concerned the smaller the secondary obstruction the better.
- Collimation
In order for the telescope to form images the secondary and primary mirrors must be aligned. This process is called collimation. Some newtonians will need to be collimated every time they are used and others will not. Many people find the process intimidating, but it can be learned without too much difficulty. It does require special tools that will most likely not come with your telescope.
- Obstructed optical path
- Closed optical tube
While temperature differentials between a telescope and its environment can cause air currents within the closed tube of a telescope, the fact that the air cannot flow freely between the two environments typically results in a greatly reduced effect on the image when the temperature of the telescope is equalizing with the outside air. - Balanced aberrations
While these designs can be quite good, the examples typically available are designed prioritizing cost above optical quality. This results in a scope with high coma distortion, but at a level that's equal to or less than a typical Newtonian of the same aperture. These scopes will also possess a minimal amount of chromatic aberration, but not to the degree of an achromatic refractor. Other aberrations are present, but not necessarily to a greater or lesser degree than found in refracting or reflecting telescopes.
Learn about reflectors
Learn about refractors
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Equipment / Eyepieces (Beginner's Equipment Topic #3)
« Last post by Thomas on February 20, 2014, 08:30:07 PM »Placeholder
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Equipment / Mounts (Beginner's Equippment Topic #4)
« Last post by Thomas on February 20, 2014, 08:29:50 PM »Placeholder
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Equipment / Filters (Beginner's Equippment Topic #5)
« Last post by Thomas on February 20, 2014, 08:27:54 PM »Placeholder
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Equipment / Barlows (Beginner's Equippment Topic #6)
« Last post by Thomas on February 20, 2014, 08:25:54 PM »placeholder
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