Choosing which telescope to buy?

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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?

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

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

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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?

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

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

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Aperture
203mm
(8 inch)

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Eyepiece
30mm
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Focal length
400mm

13x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
30mm
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Focal length

1000mm

33x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
30mm
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Focal length
1500mm

50x
magnification

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Aperture
203mm
(8 inch)
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Eyepiece
30mm
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Focal length
2000mm

67x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
30mm
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Focal length
2500mm

83x
magnification

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  • 30mm eyepiece (Telescope focal length ÷ eyepiece focal length = magnification)
telescope focal lengtheyepiece focal lengthmagnification
400mmdivide by30mm=13x magnification
1000mmdivide by30mm=33x magnification
1500mmdivide by30mm=50x magnification
2000mmdivide by30mm=67x magnification
2500mmdivide by30mm=83x magnification
.

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25mm eyepiece

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

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Aperture
203mm
(8 inch)

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Eyepiece
30mm
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Focal length
400mm

16x
magnification

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Aperture
203mm
(8 inch)
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Eyepiece
25mm
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Focal length
1000mm

40x
magnification

O

Aperture
203mm
(8 inch)

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Eyepiece
25mm
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Focal length
1500mm

60x
magnification

O

Aperture
203mm
(8 inch)

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Eyepiece
25mm
o
Focal length
2000mm

80x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
25mm
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Focal length
2500mm

100x
magnification

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telescope focal lengtheyepiece focal lengthmagnification
400mmdivide by25mm=16x magnification
1000mmdivide by25mm=40x magnification
1500mmdivide by25mm=60x magnification
2000mmdivide by25mm=80x magnification
2500mmdivide by25mm=100x magnification
.

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13mm eyepiece

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

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Aperture
203mm
(8 inch)

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Eyepiece
13mm
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Focal length
400mm

31x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
13mm
o
Focal length
1000mm

77x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
13mm
o
Focal length
1500mm

115x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
13mm
o
Focal length
2000mm

154x
magnification

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Aperture
203mm
(8 inch)

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Eyepiece
13mm
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Focal length
2500mm

192x
magnification

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telescope focal lengtheyepiece focal lengthmagnification
400mmdivide by13mm=31x magnification
1000mmdivide by13mm=77x magnification
1500mmdivide by13mm=115x magnification
2000mmdivide by13mm=154x magnification
2500mmdivide by13mm=192x magnification
.

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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
300mm8x10x12x15x20x30x60x
400mm10x13x16x20x27x40x80x
500mm13x17x20x25x33x50x100x
600mm15x20x24x30x40x60x120x
800mm20x27x32x40x53x80x160x
900mm23x30x36x45x60x90x180x
1000mm25x33x40x50x67x100x200x
1100mm28x37x44x55x73x110x220x
1200mm30x40x48x60x80x120x240x
1500mm38x50x60x75x100x150x300x
2000mm50x67x80x100x133x200x400x
2500mm63x83x100x125x167x250x500x


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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.

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

  • How can I choose an eyepiece to buy?
    First of all, three things that you have to revise your telescope;

  • 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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Telescope Shop

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BSL available

Most Deaf people who use sign language in the UK use BSL. It is a rich combination of hand gestures, facial expressions and body language. Facetime, Derek who can help with it.

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Website

Astronomy Telescope retailers
at Birmingham

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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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