BSL – Choosing which telescope to buy?

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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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BSL Version

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Choosing a telescope – bigger is better, or is it?
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• What are the 3 main factors that should be considered when choosing a telescope?
  
  
  1) Diameter of its main mirror or lens.
  
  2) Magnification
  
  3) Costs
  

BSL Version

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Dobsonian telescope
It could be a good-starting first telescope for anyone to start with.

• 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.

BSL Version

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.

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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The math is easy division:
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• 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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30mm eyepiece

25mm eyepiece

13mm eyepiece

BSL Version

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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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
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Aperture
203mm
(8 inch)
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Eyepiece
25mm
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Focal length
1500mm

60x
magnification
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Aperture
203mm
(8 inch)
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Eyepiece
25mm
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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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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
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Focal length
1000mm

77x
magnification
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Aperture
203mm
(8 inch)
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Eyepiece
13mm
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Focal length
1500mm

115x
magnification
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Aperture
203mm
(8 inch)
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Eyepiece
13mm
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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: 400mm
Eyepieces: 25mm
Magnification: 16x

Telescope: 1000mm
Eyepieces: 20mm
Magnification: 50x

Telescope: 2000mm
Eyepieces: 15mm
Magnification: 133x

Telescope: 400mm
Eyepieces: 25mm
Magnification: 16x

Telescope: 1000mm
Eyepieces: 20mm
Magnification: 50x

Telescope: 2000mm
Eyepieces: 15mm
Magnification: 133x

Telescope: 400mm
Eyepieces: 25mm
Magnification: 16x

Telescope: 1000mm
Eyepieces: 20mm
Magnification: 50x

Telescope: 2000mm
Eyepieces: 15mm
Magnification: 133x

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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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BSL Vision

Focal ratio of f/11 to 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.
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• Medium f/6 to f/10 focal ratios work well with either.

Focal ratio of 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.

Focal ratio of f/5 or below

• A lower focal ratio (f/5 or below) is often referred to as ‘fast’.
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• 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.
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• 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.
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• 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.

Focal ratio of f/3 or below

• Generally, anything f/2.8 or lower with a focal length of 20mm or less (the lower the better) is ideal for astrophotography.
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• 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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BSL Version

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• How can I choose an eyepiece to buy?
  
  First of all, three things that you have to revise your telescope;
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  (1) aperture size
  (2) focal length
  (3) focal ratio

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• Go to
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  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

BSL Version

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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

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
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Email:
Contact them
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Telephone:
0121 382 0928
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Text to Steve Asbury:
07593 081684
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Facetime / Text to Derek:
07845 319228
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Opening Times:
Monday to Friday 10am – 3pm
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Location:
ENT Optical, 802 Kingsbury Road, Erdington, Birmingham, West Midlands, B24 9PS.