MAS Frequently Asked Questions
Frequently Asked Questions about astronomy (see also MAS Beginners page)
This page answers many commonly asked questions about astronomy. If you have a question on a subject any not covered here, please help support this page by using our on-line enquiry form (requires Java Script to be enabled).
Much of the material here has been taken from topics presented at out monthly meetings. If any member has subject they would like to present for 15-20 miniutes as a 'second half' topic, please contact the Meetings Secretary (to contribute directly to this page, please contact the Webmaster
(+) 0001 How do I find a local Astronomical Society ?
(+) 0002 How is Star brightness measured ?
(+) 0003 What are the Constellations ?
(+) 0004 What is the shape of the Earth ?
(+) 0005 What equipment do I need to start astronomy ?
(+) 0007 What telescope should I buy ?
(-) 0008 How much Magnification can I get ?
Whilst the actual magnification of your telescope may be relevant for Bird Watchers, Astronomers rarely even bother to work it out. We are much more interested in 'quality seeing' and 'field of view', however this has not stopped 'chain stores' and supermarkets from stocking low end 'toy' telescopes that boast '300x magnification' in big text across the front of the box.
In the turbulent UK skies, an average telescope in average 'seeing' conditions is limited to about 30x of magnification per inch of telescope aperture. So in the example above, to realistically achieve 300x magnification you need a 10" diameter lens - needless to say, when you open the box you will be lucky your new toy has a lens exceeding 2.5" (about 65mm). Such an instrument will typically be capable of delivering a viewable image at 75x mag. or less, far short of the 'headline' 300x. Even in the very best seeing conditions the 'viewable mag. limit' is about 50x per inch (so the very best you could expect from a 2.5" telescope is 125x mag.)
For those who really insist on working it out, actual magnification is the focal length of your telescope (crudely, the length of the tube) divided by the focal length of your eyepiece.
In the example above, a typical 2.5" telescope might be 600mm long (i.e. 600mm focal length). To 'achieve' the 'specified' 300x, a 600/300 = 2mm eyepiece would be required. Even if that existed, it would be unusable (2mm is smaller than your eye pupil diameter). So, instead, they provide a virtually unusable 5mm eyepiece along with a 2.5x Barlow. Since they know such a combination will prove useless, they typically also provide a more usable 20 or 25mm eyepiece that will deliver 600/(20 or 25) = 30x or 24x mag. which actually delivers a reasonable image (and stops you taking it back for a refund).
In theory a 20/25 eyepiece could be used with a 2.5 Barlow to deliver 75x / 60x mag., however the mountings of low end telescopes are rarely stable enough to allow such a mag. to be used in practiceThe author has a 10" telescope with 1200mm focal length. Most observing sessions are started with a 42mm eyepiece (so 1200/42 = 28 mag). Once the 'target' has been acquired, a 25mm eyepiece is used (1200/25 = 48mag) to 'see the details'.
'In theory' a 10" telescope will support 300x mag, so 1200/300 = a 4mm eyepiece. Whilst the author has a 'reasonable' 7mm eyepiece and a x2 Barlow (so could achieve 3.5mm effective eyepiece focal length), in practice he has never bothered to observe with anything below a 10mm eyepiece (120x mag).
This note last modified: 5th Feb 2015 22:05.