CityDeepSky - New pages [en]

Wiki Creation Guide

Komia — Mon, 12 Sep 2011 03:57:50 GMT

Summary:

Mediawiki post submission guide

Spam

Evilscientist — Tue, 19 Jul 2011 07:18:11 GMT

Summary: [[Spam]] moved to [[CityDeepSky:Spam]]: put into proper namespace

If you have insisted on spamming this site the following terms of use apply.

1. Your spam will be deleted and your account and IP blocked.

2. The spamvertised site may be billed for the spam you have created at the rate of $5000 Canadian Dollars per hour until the offending spam is deleted. By placing the advertisement on this site you agree to this.

3. You agree to have any legal action arising to this to be held in the city of Calgary in the Province of Alberta at your expense.

[[Category:Policy]]

Astrophotography

Evilscientist — Sun, 17 Jul 2011 16:20:20 GMT

Summary:

Astrophotography is taking photographs of objects in the night sky.

{{stub}}

NGC 6885

Evilscientist — Mon, 04 Oct 2010 04:18:33 GMT

Summary: Vulpecula spelling correction

{{object
|ob_name=NGC 6885
|ob_number=75
|ob_type=Open cluster
|ob_other=none
|ob_mag=8.1
|ob_ra=20:11:58.00
|ob_dec=26:29:00.00
|ob_const=Vulpecula
|ob_season=Summer
|ob_visibility=Binocular Telescope
|ob_mapfile=Ngc6885_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=02 Oct 2010
|ob_obstype=NBT
|ob_viscat=BT
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This cluster is spread out and appears sparse through a telescope from inside a city. Only one star was visible through binoculars.
}}

[[Category:Open Cluster]]
[[Category:Summer Objects]]
[[Category:NGC Object]]
[[Category:Binocular Objects]] [[Category:Telescope Objects]]

NGC 6883

Evilscientist — Mon, 04 Oct 2010 04:02:59 GMT

Summary:

{{object
|ob_name=NGC 6883
|ob_number=74
|ob_type=Open cluster
|ob_other=none
|ob_mag=8.0
|ob_ra=20:11:20.00
|ob_dec=35:49:55.00
|ob_const=Cygnus
|ob_season=Summer
|ob_visibility=Telescope
|ob_mapfile=Ngc6883_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=02 Oct 2010
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This cluster is very spread out but many stars are visible in a telescope from inside a city.
}}

[[Category:Open Cluster]]
[[Category:Summer Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

NGC 6866

Evilscientist — Mon, 04 Oct 2010 03:46:29 GMT

Summary:

{{object
|ob_name=NGC 6866
|ob_number=None
|ob_type=Open cluster
|ob_other=none
|ob_mag=7.6
|ob_ra=20:03:55.10
|ob_dec=44:09:33.00
|ob_const=Cygnus
|ob_season=summer
|ob_visibility=Not Visible
|ob_mapfile=Ngc6866_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=02 Oct 2010
|ob_obstype=NBT
|ob_viscat=X
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This object was not visible during observations.
}}

[[Category:Open Cluster]]
[[Category:Summer Objects]]
[[Category:NGC Object]]
[[Category:Not Visible]]

NGC 6834

Evilscientist — Mon, 04 Oct 2010 03:32:57 GMT

Summary:

{{object
|ob_name=NGC 6834
|ob_number=73
|ob_type=open cluster
|ob_other=none
|ob_mag=7.8
|ob_ra=19:52:12.50
|ob_dec=29:24:29.00
|ob_const=Cygnus
|ob_season=summer
|ob_visibility=Telescope
|ob_mapfile=Ngc6834_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=02 Oct 2010
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description= Not a particularly exciting object with only a couple of stars visible from a city.
}}

[[Category:Open Cluster]]
[[Category:Summer Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

M60

Evilscientist — Mon, 04 Oct 2010 03:03:37 GMT

Summary:

{{object
|ob_name=M60
|ob_number=72
|ob_type=Galaxy
|ob_other=NGC 4649
|ob_mag=8.8
|ob_ra=12:43:39.80
|ob_dec=11:33:11.00
|ob_const=Virgo
|ob_season=spring
|ob_visibility=Telescope
|ob_mapfile=M60_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=22 May 2010
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This elliptical galaxy is very challenging from light polluted skies. Use averted vision.
}}

[[Category:Galaxy]]
[[Category:Spring Objects]]
[[Category:Messier Object]] [[Category:NGC Object]]
[[Category:Telescope Objects]]

Hertzsprung-Russel diagram

Evilscientist — Wed, 16 Jun 2010 21:34:37 GMT

Summary:

The Hertzsprung-Russel diagram (H-R Diagram) is a way of relating the spectral type of a star to its luminosity.

{{stub}}

List of non-visible objects

Evilscientist — Wed, 16 Jun 2010 21:26:48 GMT

Summary:

This is a list of the objects that were surveyed, that is an observation was attempted, but were not visible from inside the light polluted environs of a city.

[[M63]]

[[M66]]

[[M101]]

[[NGC 1499]]

[[NGC 2903]]

[[NGC 4631]]

[[NGC 5053]]

[[NGC 6210]]

[[NGC 6539]]

[[NGC 6543]]

[[NGC 6572]]

[[NGC 6649]]

[[NGC 6712]]

[[NGC 6755]]

[[NGC 6760]]

[[NGC 6802]]

[[NGC 6819]]

[[NGC 6826]]

[[NGC 7000]]

[[NGC 7023]]

[[NGC 7027]]

[[NGC 7062]]

[[NGC 7234]]

[[NGC 7261]]

[[NGC 7789]]

[[Category:Not Visible]]

NGC 1545

Evilscientist — Sun, 27 Dec 2009 20:12:55 GMT

Summary:

{{object
|ob_name=NGC 1545
|ob_number=71
|ob_type=open cluster
|ob_other=None
|ob_mag=6.2
|ob_ra=04:20:56.20
|ob_dec=50:15:19.00
|ob_const=Perseus
|ob_season=winter
|ob_visibility=Telescope
|ob_mapfile=Ngc1545_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description= This sparse cluster is easy to find.
}}

[[Category:Open Cluster]]
[[Category:Winter Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

NGC 1528

Evilscientist — Sun, 27 Dec 2009 20:08:50 GMT

Summary:

{{object
|ob_name=NGC 1528
|ob_number=70
|ob_type=open cluster
|ob_other=none
|ob_mag=6.4
|ob_ra=04:15:23.00
|ob_dec=51:12:54.00
|ob_const=Perseus
|ob_season=winter
|ob_visibility=Telescope
|ob_mapfile=Ngc1528_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=27 Dec 2009
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This cluster contains many stars and is relatively easy to find in light polluted skies.
}}

[[Category:Open Cluster]]
[[Category:Winter Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

NGC 1444

Evilscientist — Sun, 27 Dec 2009 20:04:19 GMT

Summary:

{{object
|ob_name=NGC 1444
|ob_number=69
|ob_type=open cluster
|ob_other=None
|ob_mag=6.6
|ob_ra=03:49:25.00
|ob_dec=52:39:30.00
|ob_const=Perseus
|ob_season=winter
|ob_visibility=Telescope
|ob_mapfile=Ngc1444_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=27 Dec 2009
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=Though easy to find, this cluster seems to consist of basically one magnitude 6 star.
}}

[[Category:Open Cluster]]
[[Category:Winter Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

NGC 1342

Evilscientist — Sun, 27 Dec 2009 20:00:07 GMT

Summary:

{{object
|ob_name=NGC 1342
|ob_number=68
|ob_type=open cluster
|ob_other=None
|ob_mag=6.7
|ob_ra=03:31:38.00
|ob_dec=37:22:36.00
|ob_const=Perseus
|ob_season=winter
|ob_visibility=Telescope
|ob_mapfile=Ngc1342_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=27 Dec 2009
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=A somewhat sparse cluster that is tricky to locate as it is in a sparse field.
}}

[[Category:Open Cluster]]
[[Category:Winter Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

NGC 1502

Evilscientist — Sun, 27 Dec 2009 19:54:09 GMT

Summary:

{{object
|ob_name=NGC 1502
|ob_number=67
|ob_type=open cluster
|ob_other=None
|ob_mag=6.9
|ob_ra=04:07:49.20
|ob_dec=62:19:54.00
|ob_const=Camelopardalis
|ob_season=winter
|ob_visibility=Telescope
|ob_mapfile=Ngc1502_map.png
|ob_username=evilscientist
|ob_altname=Jason Nishiyama
|ob_location=Calgary
|ob_date=27 Dec 2009
|ob_obstype=NBT
|ob_viscat=T
|ob_bintype=10x50
|ob_teltype=[[MAK127]]
|ob_description=This tight cluster is easily located at the end of Kemble's Cascasde.
}}

[[Category:Open Cluster]]
[[Category:Winter Objects]]
[[Category:NGC Object]]
[[Category:Telescope Objects]]

Eyepiece

Evilscientist — Wed, 09 Dec 2009 20:53:56 GMT

Summary:

An eyepiece is a lens or series of lenses put at the end of the light path of a telescope to allow the human eye to view the image.

{{stub}}
[[Category:Astronomical concept]]
[[Category:Observing Concepts]]

Red giant

Evilscientist — Wed, 09 Dec 2009 20:52:03 GMT

Summary:

A red giant is a star nearing the end of its life. The star has swollen in size and thus has cooled, looking red.

{{stub}}
[[Category:Astronomical concept]]

Star

Evilscientist — Wed, 09 Dec 2009 20:40:48 GMT

Summary:

A star is a large ball of hot plasma that glows due to it's high temperature. The temperature is maintained by nuclear fusion in the star's core.

{{stub}}
[[Category:Astronomical concept]]

Stellar evolution

Evilscientist — Wed, 09 Dec 2009 20:39:03 GMT

Summary: add 2.2 solar mass and greater section - remove stub tag

Stellar evolution refers to the life cycle of stars from their formation in [[nebula|nebulae]] to their final demise. The exact path that evolution takes is dependent on the initial mass of the star.

==Formation==

In general, stars start out forming from the gas and dust found in [[Nebula|nebulae]]. Some event such as a [[supernova]]<ref name="Kumar">Kumar, P. and Johnson, J.L., 2010, ''Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disc'', Monthly Notices of the RAS, V404, pp 2170-2176</ref><ref name="boss">Boss, A.P. And Keiser, S.A., 2010, ''Who pulled the trigger: A supernova or an asymptotic giant branch star?'', Astrophysical Jounal Letters, L1-L5</ref>, [[galaxy]] collision<ref name="Karl">Karl, S.J., Naab, T., Johansson, P.H., Kotarba, H., Boily, C.M., Renaud, F., Theis, C., 2009, ''One moment in time – Modelling star formation in the Antennae'', Astrophysical Journal Letters, V715, ppL88-L93</ref>, or the passage of a density wave in a spiral galaxy<ref name="Martinez">Martinez-Garcia, E.E., Gonzalez-Lopezlira, R.A., Bruzual-A, G., 2009, ''Spiral density wave triggering of star formation in Sa and Sab galaxies'', Astrophysical Journal, V694, pp512-545</ref> causes the gas to compress. This causes regions of higher density which will cause a gravitational collapse of the gas towards these higher density regions.

As the gas collapses, it becomes hotter near the centre due to the conversion of gravitational potential energy into kinetic energy causing the gas at the centre to want to expand. Gravity prevents this expansion and more and more gas is attracted, causing the temperature and pressure at the centre to increase to the point where it becomes hot enough for hydrogen to fuse into helium <ref name="iben">Iben, I., 1991, ''Single and binary star evolution'', Astrophysical Journal Supplement Series, V76, pp 55-114</ref>. At this point the star turns on and moves onto the main sequence of the [[Hertzsprung-Russel diagram|H-R Diagram]].

==Main Sequence==
The main sequence of the [[Hertzsprung-Russel diagram|H-R Diagram]] is defined as the time in a star's evolution where it is fusing hydrogen into helium in its core<ref name="iben1967">Iben, I., 1967, ''Stellar evolution within and off the main sequence'', Annual Review of Astronomy & Astrophysics. V5, p 571</ref>. During this phase there is a balance between the star's gravity, wanting to collapse it even further and the pressure generated by the fusion occurring in its core. This maintains the star's size. Since the amount of pressure in the star's core is dependant on it's mass, more mass means greater pressure, the rate at which a star consumes its nuclear fuel is also dependant on its mass. This means that more massive stars consume their fuel at a faster rate, and hence are more luminous on the main sequence. It also means that the more massive a star is, the shorter its stay on the main sequence<ref name="iben1967"> </ref>. Once a star has used up the hydrogen in its core, it moves off the main sequence.

==Post Main Sequence==
What happens to a star once it leaves the main sequence depends on its initial mass with stars more massive than around 2.2 times the mass of the Sun evolving differently than those smaller than that.
===Less than 2.2 solar masses===

Once a star less than 2.2 solar masses has used up the hydrogen in its core, it again begins to collapse. This collapse causes an increase in pressure throughout the star causing the hydrogen in a shell around the now mostly helium core to begin to fuse. This generates more energy than the hydrogen core burning phase, but instead of making the star brighter, it dims because the energy from this shell is used to expand the outer regions of the star, making it cooler as it moves onto the subgiant branch (SGB)<ref name="iben1967a">Iben, I., 1967, ''Stellar evolution. IV. Evolution from the main sequence to the red-giant branch for stars of mass 1, 1.25 and 1.5 solar masses'', Astrophysical Journal, V147, p624</ref><ref name="ostlie">Ostlie, D.A. & Carroll, B.W.,1996, An Introduction to Modern Stellar Astrophysics</ref>. As time progresses, the fusing hydrogen shell becomes thinner and thinner and the core denser and denser. Eventually the pressure on the core is so great that electron [[degeneracy]] pressure is needed to keep it from further collapse.

Throughout this phase, the hydrogen shell generates a large amount of energy, causing the outer layers of the star (the envelope) to expand greatly. This causes the apparent temperature of the surface of the star to drop and the diameter of the star to increase. This is when the star moves onto the red giant branch (RGB) of the H-R Diagram<ref name="iben1967"> </ref><ref name="ostlie"> </ref>.

As the hydrogen shell is burning, the temperature and pressure in the core increases to the point where helium begins to fuse. In stars less than 2.2 solar masses this happens quite rapidly, releasing <math>10^{11}</math> solar luminosities of energy in a few seconds<ref name="iben1967"> </ref>. This helium core flash does not blow the star apart however, as the vast majority of this energy is used to end the electron degeneracy. Once the degeneracy is lifted, the core expands slightly, slowing down the reaction rate and allowing helium core burning<ref name="ostlie"> </ref>.

Once the helium in the core has been exhausted, and the hydrogen in the shell exhausted, the star now has a carbon-oxygen (C-O) core with a shell of fusing helium surrounding it. The star has now entered the Asymptotic Giant Branch. The early stages of this phase may have the star pulsing, causing it's [[magnitude]] to vary. Later stages of this phase have the star lose most of it's outer envelope of gas due to strong stellar winds forming a [[planetary nebula]]. Eventually only the C-O core remains as a white dwarf<ref name="iben"> </ref>.

===More than 2.2 solar masses===
As with stars less than 2.2 solar masses, stars greater than 2.2 solar masses begin to contract once the hydrogen fuel is used up in the core. This also causes a shell of hydrogen to start to fuse around the now quiescent core as with smaller stars. Again this creates more energy than the hydrogen burning in the core, but this energy is used to expand the outer layers of the star and the star moves onto the subgiant branch <ref name="iben1967"> </ref>. As the hydrogen shell burns, it becomes thinner and thinner, causing more and more helium to exist at the core. As temperatures and density at the centre of the core increase, the helium in the core will begin to fuse. Unlike in a star smaller than 2.2 solar masses, this doesn't happen throughout the whole core of a larger star. In this case, the temperature and density increase first only at the centre of the core, the resulting increase in energy production pushes out the outer parts of the core, making them less dense. This throttles that reaction and the helium flash of smaller stars does not occur in larger stars <ref name="iben1967"> </ref>.

When a 2.2 solar mass (or larger) star runs out of helium in the core and moves onto the Asymptotic Giant Branch, it not only has a hydrogen shell burning, but also a helium shell about a carbon-oxygen core. This again causes heating in the core and eventually permits the carbon in the core to begin to fuse. At this point there is a fusing carbon core surrounded by a fusing shell of helium which itself is surrounded by a fusing shell of hydrogen<ref name="iben1967"> </ref>. Eventually depending on the mass of the star, the interior begins to look like an onion, with layers of heavier and heavier elements fusing as one heads towards the core. The more massive the star, the more layers. This all stops once the core begins to fill with iron. Iron cannot be fused with iron to produce energy, such a reaction is endothermic and requires energy from the environment so only elements up to iron can be produced in the cores of large stars.

Just before the carbon core burning phase while the hydrogen and helium shells are burning, the stars less than 8 solar masses becomes thermally unstable. This causes it to pulse at a rate in the range of a thousand years or so for stars near 8 solar masses to hundreds of thousands of years for lighter stars <ref name="iben1967"> </ref>.

Once fusion stops in a larger star, the core collapses rapidly. This rapid collapse sends a shock wave back up through the star causing it to explode in a supernova explosion. The ultimate fate of the star depends on it's initial mass and mass loss rate during it's final days. Stars with an initial mass less than 20 solar masses become neutron stars whereas those larger than 20 solar masses become black holes <ref name="freyer2002">Freyer, C.L., Heger, A., Langer, N., Wellstein, S. 2002, "The limiting stellar initial mass for black hole formation in close binary systems", Astrophysical Journal V578, pp 335-347</ref>.

==References==

<references/>

[[Category:Astronomical concept]]

Electro-magnetic spectrum

Evilscientist — Wed, 09 Dec 2009 20:37:09 GMT

Summary: add stub tag

The electro-magnetic (EM) spectrum is a form of radiative energy. Our eyes perceive a small part of this spectrum as light. Going from longest wavelength to shortest wavelength the spectrum is:
* Radio
* Microwave
* Infra-red
* Visible light
* Ultra-violet
* X-ray
* Gamma-ray

{{stub}}
[[Category:Astronomical concept]]

Parallax

Evilscientist — Wed, 09 Dec 2009 20:34:17 GMT

Summary:

Parallax is the apparent shift of a foreground object against a background object due to a change in location of the observer.

{{stub}}
[[Category:Astronomical concept]]

T:Foo

Evilscientist — Tue, 08 Dec 2009 23:24:25 GMT

Summary:

सदस्य:Foo

Evilscientist — Tue, 08 Dec 2009 23:24:09 GMT

Summary:

Участник:Foo

Evilscientist — Tue, 08 Dec 2009 23:23:42 GMT

Summary:

Utilisateur:Foo

Evilscientist — Tue, 08 Dec 2009 23:23:27 GMT

Summary:

Foo:Namespaces

Evilscientist — Tue, 08 Dec 2009 23:21:51 GMT

Summary:

Right ascension

Evilscientist — Tue, 08 Dec 2009 21:14:10 GMT

Summary:

Right ascension is a coordinate in the equatorial coordinate system. It is a measurement along the celestial equator measured in hours moving eastward from the first point of Aries. It is analogous to the geographic coordinate of longitude. [[Declination]] is the other coordinate in the equatorial system.

{{stub}}
[[Category:Astronomical concept]]
[[Category:Observing Concepts]]

Declination

Evilscientist — Tue, 08 Dec 2009 21:08:53 GMT

Summary: add stub tag

Declination is a measurement in the equatorial coordinate system. It is measured in degrees north (+) or south (-) from the celestial equator to the poles. Its equivalent in geographic coordinates is latitude.

{{stub}}
[[Category:Astronomical concept]]
[[Category:Observing Concepts]]

Supernova

Evilscientist — Tue, 08 Dec 2009 17:01:11 GMT

Summary:

A supernova is the explosion of a large star near the end of it's life. The result of a supernova is either a neutron star or black hole depending on the size of the progenitor star.

{{stub}}
[[Category:Astronomical object]]

Maksutov

Evilscientist — Thu, 19 Nov 2009 02:24:23 GMT

Summary: add comma

A Maksutov is a type of [[Telescope#Catadioptric Telescopes|catadioptric telescope]] developed between 1929 and 1941 by Dmitri Maksutov<ref name="telweb">Telescope Engineering, ''Dmitri Maksutov'', http://www.telescopengineering.com/history/DmitriMaksutov.html</ref>. The Maksutov differs from other catadioptric and [[Telescope#Reflecting Telescopes|reflecting telescopes]] in that it uses spherical and not parabolic surfaces. Normally spherical surfaces suffer from spherical aberration which causes unclear images, but the aberrations of the spherical front meniscus lens are counteracted by the aberrations of the back spherical mirrors.

[[Image:Maksutov.svg.png |thumb|right|400px|Maksutov telescope light path]]

Maksutov telescopes come in Newtonian and Cassegrain configurations, the latter being the most common and most rugged. Since the surfaces are spherical, they are cheap to make as they are easy to grind. Further, in the Maksutov-Cassegrain, the secondary mirror is simply an aluminized spot on the meniscus corrector plate. This makes the Mak-Cas a very rugged optical system as it is difficult to cause the system to become mis-aligned.

[[Image:Jnishiyama telescope.JPG |thumb|right|200px|Maksutov Telescope]]

The advantages of the Maksutov telescope are good contrast and lack of coma, making them excellent for both planetary and deep sky observing. In this sense they provide the best of both [[Telescope#Refracting Telescopes|refracting telescopes]] and reflecting telescopes.

The Maksutov does have two disadvantages over other designs. First, in order to avoid some forms of aberration, the focal ratio has to be quite high, at least f11, making Maksutovs quite slow photographically. Second, as the aperture of a Maksutov increases, so does the thickness of the front meniscus lens. There is a point where it will take an unreasonable amount of time for the meniscus lens to cool down for observing as such Maksutov telescopes don't scale up well. This means that it is unusual to see commercially produced Maksutov telescopes with apertures over 7" to 8".

<references/>

[[Category:Telescopes]]

Telescope

Evilscientist — Thu, 19 Nov 2009 01:22:39 GMT

Summary: /* Catadioptric Telescopes */ correct typo

The word telescope comes from the the ancient Greek words for "far seeing", which is a pretty good description of what a telescope does. Various types of telescopes are used throughout the electromagnetic spectrum though this article will discuss the basic types of optical telescopes.

==Telescope Basics==

Optical telescopes come in many shapes, sizes and designs. In general there are three parameters used to describe all telescopes:
*aperture - the diameter of the primary lens or mirror
*focal length - the focal length of the optical system, in other words the apparent distance from the primary lens or mirror to the point where an image is in focus.
*f ratio - the focal length divided by the aperture, this is a measure of how photographically "fast" the telescope is. Smaller f ratios require shorter exposures with a camera to create an image.

Regardless of design or parameters, in astronomy the telescope performs three basic functions, collecting light, resolving images and magnification of images.

===Collecting Light===

First it collects light. The larger the aperture of the telescope, the more light it collects and hence the [[Magnitude|dimmer]] the objects it can see. The dimmest a particular telescope can see is called the [[Limiting Magnitude|limiting magnitude]]. Though the aperture of the telescope is the major factor in limiting magnitude, there are other factors that can affect this such as sky brightness.

===Resolving Images===

The second thing a telescope allows the astronomer to do is resolve images. That is, it allows the astronomer to see fine detail in an object or to see two objects that are close together as separate objects. Again the larger the aperture the better the resolving power of the telescope or the closer together objects can be. The theoretical resolution limit of a telescope is called the Dawes limit, after the British astronomer who discovered it. This empirical relationship describes how far apart two stars have to be to be seen with a particular aperture:

<math>R=\frac{4.56}{D}</math>

Where R is the resolution in arc seconds and D is the aperture of the telescope in inches.

===Magnifying Images===

The third thing a telescope does is the thing that most people think of when they think of telescopes, that is magnifying an image or making an object look closer. The magnification of a telescope is related to the focal length of the telescope and the focal length of the [[Eyepiece|eyepiece]] used to view the object. By changing the eyepiece of a telescope one can change the magnification. Magnification is given by:

<math>M=\frac{F}{f}</math>

Where M is the magnification, F is the focal length of the telescope and f is the focal length of the eyepiece. So for a telescope with a 1500mm focal length, using a 25mm eyepiece will produce a magnification of 60x.

There is a limit to how much magnification you can get out of a telescope. The smaller the diameter of the telescope the lower this limit is due to the resolution of the telescope. There comes a point where more magnification just magnifies the lack of resolution and doesn't produce a better image. The maximum useful magnification for a telescope is between 40 and 50 times the aperture of the telescope in inches<ref name="astroform">Astronomy formulas, http://www.xmission.com/~alanne/AstronomyFormulas.html</ref>. So a 5" telescope has a maximum useful magnification of between 200x and 250x. Anything more than this does not provide a better image since the resolution is not there.

==Telescope Types==

There are many optical designs for telescopes but there are only three basic types:
*refracting telescopes
*reflecting telescopes
*catadioptric telescopes.

===Refracting Telescopes===

Refracting telescopes, often called refractors, use lenses to form an images. In a basic refractor, the primary objective is a large lens at the front of the telescope. The light from the object comes through the lens, is refracted (bent) and comes to a focus somewhere behind the lens where the eyepiece or imager is located.

[[Image:Refractor.png|thumb|right|400px|Refracting Telescope]]

Refractors have the disadvantage in that they suffer from chromatic aberration, that is light of different colours focus at different places. Modern refractors are designed to reduce this problem and a good modern refractor can provide very good views.

Refractors also tend to be quite expensive per unit of aperture as many surfaces of optically clear glass must be ground to provide aberration limited views. This means that most refractors owned by amateurs are small, 4" or less in diameter. Despite these problems refractors are prized by planetary observers as the refracting telescope provides good contrast which is needed for planetary observation. Refractors also don't have a central obstruction as reflectors do as the optical system is straight through, which limits diffraction around objects caused by the secondary mirror of the reflector.

===Reflecting Telescopes===

Reflecting telescopes avoid the problem of chromatic aberration by avoiding the use of a lens as a primary objective. In it's place, a parabolic mirror is placed at one end of the telescope and the light is then brought to a focus at some point outside of the telescope. There are many types of reflecting telescopes, the type depending on where the light is brought to a focus. Two common types are the Newtonian and the Cassegrain.

[[Image:Newtonian.png |thumb|right|400px|Newtonian Reflecting Telescope]]

Apart from having no chromatic aberration, reflecting telescopes also have the advantage of being cheaper per unit aperture than refracting telescopes as there are fewer surfaces that need to be polished. In fact, many amateur astronomers routinely build Newtonian telescopes from scratch. There was a time when amateurs would even grind their own mirrors, but with the advent of relatively inexpensive, commercially available mirrors, there is less of this.

Reflecting telescopes do suffer from coma, that is objects near the edge of the field of view tend to be drawn out into teardrop shapes. This causes images of objects that cover most of the field of view to degrade towards the edge of the field. Reflecting telescopes also have a central obstruction in the form of the secondary mirror. This mirror and its support cause diffraction in the image which doesn't exist in the refractor.

===Catadioptric Telescopes===

Catadioptric telescopes are a mix between refracting and reflecting telescopes. A corrector plate is placed in front of the primary mirror to eliminate the problem of coma. Since the mirror still is responsible for focusing the light, there is no chromatic aberration. Because of this catadioptric telescopes are popular among amateur astronomers. There are two basic types of catadioptirc telescope, the Schmidt and the [[Maksutov]].

[[Image:Schmidt.png |thumb|right|400px|Schmidt Catadioptric Telescope]]

Catadioptric telescopes combine the best qualities of both refracting and reflecting telescopes and have few of their problems. In terms of price per unit aperture, Catadioptric telescopes tend to fall between refractors and reflectors. This is due to the fact that the corrector plate has to be ground.

Like reflecting telescopes, catadioptric telescopes also have a secondary mirror and hence a central obstruction. This also causes diffraction, though the corrector plate supports the mirror and there is no diffraction pattern caused by a mirror support.

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[[Category:Astronomical concept]]
[[Category:Observing Concepts]]
[[Category:Telescopes]]