Choosing which telescope to buy?

• How do I choose an astronomy telescope? (Aperture)
• What are the 3 main factors that should be considered when choosing a telescope?
• A note from Derek about Dobsonian telescope.
• Size of objective lens for refractor telescopes
• How to read a telescope magnification? (focal length)
  …– 30mm eyepiece
  …– 25mm eyepiece
  …– 13mm eyepiece
• Comparison chart for your average magnification
  …– Visual Mode: NGC 6888 (Crescent Nebula in Cygnus)
  …– Visual Mode: Jupiter
  …– Visual Mode: Full Moon
• Astronomy Tools – Field of View Calculator
• How to read a Focal Ratio?
• Eyepiece calculator before you buy
• Telescope Shop (ENS Optical)

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How do I choose an astronomy telescope? (Aperture)

How do I choose an astronomy telescope? (Aperture)

Aperture

• The aperture is the most important feature as the larger.
  – this is the more light your telescope gathers therefore revealing fainter objects to your eye.
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• On the design of choice as most modern telescopes can be used for low and high power as well as imaging.
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• A telescope’s most important attribute is its size, meaning the diameter of its main mirror or lens.
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• The bigger the telescope, the more light it collects, which allows you to see more fainter objects.
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What are the 3 main factors that should be considered when choosing a telescope?

What are the 3 main factors that should be considered when choosing a telescope?

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Bigger is better, or is it?

• the main specification you want to consider when choosing a telescope is its aperture.
• the diameter of its main mirror or lens.
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Three main factors for telescope

• 1) Types of telescopes – the diameter of its main mirror or lens.
• 2) Magnification.
• 3) Cost.
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A note from Derek about Dobsonian telescope.

A note from Derek about Dobsonian telescope.

Dobsonian telescope
It could be a good-starting first telescope for anyone to start with.
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• From 8″ to 10″ Dobsonians, this one is a far better product for just a little bit more money or for above the beginners and amateur, from 12″ to 20″ Dobsonians for above the beginners but it’s cost you more.
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• Adjustable tensioners, improved mount, dual-speed Crayford focuser, thumbscrew adjustable secondary, and upgraded eyepieces make this’s good.
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• This is probably the best telescope for deep space as long as you have the room, don’t mind lifting a little weight.
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• The Dobsonian design of maximized objective diameter combined with portability makes the design ideal for observing dim star clusters, nebulae, and galaxies (deep sky objects), an activity that requires large objectives and travel to dark sky locations.
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• Under light polluted skies, the moon, the planets, double stars, star clusters, are better seen with a 10 inch than a 4 inch.
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• The moon, the planets and many double stars are not affected by light pollution but the larger scope helps resolve finer details.
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Size of objective lens for refractor telescopes

Size of objective lens for refractor telescopes

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70mm
– the minimum size for good views.
– starts to show more detail on the planets.
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80mm
– good size for detailed views of the planets.
– starts to show detail in the detail!
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90mm
– excellent detail level on the planets and good views of all objects.
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100mm
– bright planets like Jupiter, Saturn, and Mars shine bright in the 100mm reflector.
– even when viewing from urban locations with mediocre seeing conditions.
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110mm
– the 110mm refractor excels in both deep-sky imaging and wide-field observing.
– the 110mm aperture, combined with extra-low dispersion (ED) FPL-51 glass.
– it offers great colour correction, as well as crisp images and fantastic views.
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120mm
– all telescopes lose contrast, and refractors are no exception.
– the smaller the telescope, the more contrast is lost.
– however, the increase in contrast transfer between a 100mm and a 120mm scope is only about 20%.
– meaning 100mm refractor telescope which is far less than the gain in light gathering in 120mm scope.
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How to read a telescope magnification? (focal length)

How to read a telescope magnification? (focal length)

Formula for calculating the eyepiece

The math is easy division:

• The magnification is the telescope focal length divided by the eyepiece focal length, in millimetres.
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• If you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 33x.
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For example
Here further below to test all different focal length measurements on the 30mm, 25mm, 13mm eyepiece and all the same 203mm (8 inch) aperture.

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30mm eyepiece

30mm eyepiece

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Field of View: 30mm eyepiece
(telescope focal length divided by eyepiece = magnification)

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• 30mm eyepiece (Telescope focal length ÷ eyepiece focal length = magnification)

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25mm eyepiece

25mm eyepiece
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Field of View: 25mm eyepiece
(telescope focal length divided by eyepiece = magnification)

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telescope focal length		eyepiece focal length		magnification
400mm	divide by	25mm	=	16x magnification
1000mm	divide by	25mm	=	40x magnification
1500mm	divide by	25mm	=	60x magnification
2000mm	divide by	25mm	=	80x magnification
2500mm	divide by	25mm	=	100x magnification
.

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13mm eyepiece

13mm eyepiece
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Field of View: 13mm eyepiece
(telescope focal length divided by eyepiece = magnification)

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telescope focal length		eyepiece focal length		magnification
400mm	divide by	13mm	=	31x magnification
1000mm	divide by	13mm	=	77x magnification
1500mm	divide by	13mm	=	115x magnification
2000mm	divide by	13mm	=	154x magnification
2500mm	divide by	13mm	=	192x magnification
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Comparison chart for your average magnification

Comparison chart for your average magnification

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(Telescope focal length ÷ eyepiece focal length = magnification)

Focal length for telescope	Eyepiece 40mm	Eyepiece 30mm	Eyepiece 25mm	Eyepiece 20mm	Eyepiece 15mm	Eyepiece 10mm	Eyepiece 5mm
300mm	8x	10x	12x	15x	20x	30x	60x
400mm	10x	13x	16x	20x	27x	40x	80x
500mm	13x	17x	20x	25x	33x	50x	100x
600mm	15x	20x	24x	30x	40x	60x	120x
800mm	20x	27x	32x	40x	53x	80x	160x
900mm	23x	30x	36x	45x	60x	90x	180x
1000mm	25x	33x	40x	50x	67x	100x	200x
1100mm	28x	37x	44x	55x	73x	110x	220x
1200mm	30x	40x	48x	60x	80x	120x	240x
1500mm	38x	50x	60x	75x	100x	150x	300x
2000mm	50x	67x	80x	100x	133x	200x	400x
2500mm	63x	83x	100x	125x	167x	250x	500x

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Visual Mode: NGC 6888 (Crescent Nebula in Cygnus)

Visual Mode: NGC 6888 (Crescent Nebula in Cygnus)

(Telescope focal length ÷ eyepiece focal length = magnification)

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(Source by Astronomytools.com)

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Visual Mode: Jupiter

Visual Mode: Jupiter

(Telescope focal length ÷ eyepiece focal length = magnification)

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(Source by Astronomytools.com)

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Visual Mode: Full Moon

Visual Mode: Full Moon

(Telescope focal length ÷ eyepiece focal length = magnification)

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(Source by Astronomytools.com)

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Astronomy Tools – Field of View Calculator

Astronomy Tools – Field of View Calculator
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Here the link below to help you
testing different combinations of
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telescope
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camera
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eyepiece
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by astronomy tools,
before you decide to buy one.
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How to read a Focal Ratio

How to read a Focal Ratio

• The focal length divided by the aperture of a mirror or lens gives you the number known as the focal ratio. (f)
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• It is usually written, for example, as f/4, where the focal length is four times longer, f/10, where the focal length is ten times than the aperture.

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This is the ‘speed‘ of a telescope’s optics, found by dividing the focal length by the aperture.

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Focal ratio of f/1 >> f/3

• Generally, anything f/2.8 or lower with a focal length of 20mm or less (the lower the better) is ideal for astrophotography.
• Astro lenses also tend to be bigger and heavier than their standard counterparts due to the size and quality of the glass, which is something to bear in mind if you want to take it on the road.
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Focal ratio of f/3 >> f/5

• A lower focal ratio (f/5 or below) is often referred to as ‘fast’.
• These telescopes have a wide field of view and produce bright images, making them ideal for deep-sky observations, where you’d want to capture as much light as possible in a shorter amount of time.
• An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large.
• Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.
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Focal ratio of f/6 >> f/10

• For such objects, a focal ratio of f/10 or more is ideal – but if you want to see wide views of star clusters, galaxies, and Milky Way, a lower focal ratio is better.
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Focal ratio of f/11 >> f/15

• Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography.
• Medium f/6 to f/10 focal ratios work well with either.
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Eyepiece calculator before you buy

Eyepiece calculator before you buy

• How can I choose an eyepiece to buy?
  First of all, three things that you have to revise your telescope;
  
  (1) aperture size
  (2) focal length
  (3) focal ratio
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• Find out in our beginners’ guide
  How do eyepieces work, what are the different types and which are the best eyepieces for your telescope?
  
  …..– Find out in our beginners’ guide:- Before you buy eyepieces

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Telescope Shop

Telescope Shop

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A note from Derek

• Steve at ENS Optical, he’s a friendly experts for astronomy telescopes, binoculars, cameras, astrophotography, bags or cases, accessories, and many more items.
• great for astronomers to start with it at low budgets.
• he has also deaf awareness too.
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Website: ENS optical
Email: Contact them
Telephone: 0121 382 0928
Text to Steve Asbury: 07593 081684
Facetime / Text: Derek on 07845 319228
Opening Times: Monday to Friday 10am – 3pm
Location: ENT Optical, 802 Kingsbury Road, Erdington, Birmingham, West Midlands, B24 9PS
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Return to

• How do I choose an astronomy telescope? (Aperture)
• What are the 3 main factors that should be considered when choosing a telescope?
• A note from Derek about Dobsonian telescope.
• Size of objective lens for refractor telescopes
• How to read a telescope magnification? (focal length)
• 30mm eyepiece
• 25mm eyepiece
• 13mm eyepiece
• Comparison chart for your average magnification
• Visual Mode: NGC 6888 (Crescent Nebula in Cygnus)
• Visual Mode: Jupiter
• Visual Mode: Full Moon
• Astronomy Tools – Field of View Calculator
• How to read a Focal Ratio
• Eyepiece calculator before you buy
• Telescope Shop

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Go to Derekscope’s basic information

• In viewing between methods – Naked-Eye/Binoculars/Telescopes
• Choosing which binocular to buy?
• Choosing which telescope to buy?
• Types of telescopes
• Before you buy eyepieces
• How to read a telescope magnification?
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