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 ?
(+) 0013 What are Equatorial and Alt Az mounts ?
(+) 0100 How do I use my first telescope ?
(+) 0109 What is a finder ?
(-) 0110 What is an eyepiece ?
An eyepiece is used to magnify the image formed by the telescope's lens or mirror.
A telescope consists of two optical parts:
1) The 'Optical Tube Assembly' (OTA) containing the 'objective' lens (or 'primary' mirror), which gathers the light
2) An eyepiece, which collects and magnifies the light to produce an image in your eye
Telescopes are usually sold with two eyepieces, one for low magnification the other for high. Typical eyepieces supplied are a 25mm and 10mm focal lengths (sometimes a 2x Barlow is also part of the kit, see What is a Barlow ?)
The magnification achieved is calculated by dividing the focal length of the OTA (typically 1m or more) by the focal length of the eyepiece.
There are many different eyepiece designs, and the best cost as much as the telescope tube. For this reason many 'low end' telescopes (which typically have a perfectly good objective lens or mirror) are sold with very cheap eyepieces leading to poor performance and disappointment
Worst, because 'the public' associates 'magnification' with 'quality', many low end telescopes are sold with tiny glass 'bead' eyepieces (5 mm is not uncommon). The tiny 'exit' size of a typical poor 5mm eyepiece makes it virtually useless (generally, for a given focal length eg 10mm, the larger the 'eye' (exit) lens, the better the eyepiece (and for sure, the easier it will be to use)).
Advice:
If your telescope comes with an 'A' or 'H' type eyepiece (eg A10 or H20) the best way to improve you observing experience is to replace that with a 'Plossl' type - a decent 15mm 1.25" 'Super Plossl' should cost no more than £20 (eBay). The next step is to improve the mount = one trick to reduce the 'wobble' of a small tripod is to hang a weight off the center :-)
NB. Always keep the optical parts covered when not in use. Dust will spoil your equipment. If you don't have plastic storage 'tubes' for your eyepieces, you could use (various coloured) babies socks instead :-)
This note last modified: 15th Jan 2016 13:08.