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 ?
Equatorial (EQ, or GEM = German Equatorial Mount) and Alt-Az are two popular ways of mounting your telescope. Alt-Az is essentially the same as a camera tripod mount - one axis moves left-right, the other up and down. The main disadvantage of such a mount (for a telescope) is that, to 'track' the stars, you have to move in both axis. Also known as a 'fork mount', it is popular on both the cheaper refractors and heavier folded Newtonian.
To avoid the need to move in both left-right and up-down axis, the German Equatorial Mount was developed. This was achieved by 'tipping up' the axis of the Alt-Az mount so that it 'pointed at' the celestial North (Polaris). The angle needed to do this is equal to the latitude of the observing location (so, for London, that's 52.5 degrees). The 'left-right' movement then follows the plane of the Earths rotation - so to track the stars you only have to move one axis.
For normal viewing, the type of mount matters little - however when using your telescope for long-exposure astrophotography an Alt-Az mount introduces 'Field Rotation' (so stars are elongated into tiny arcs). It is possible to 'convert' an Alt-Az mount to EQ by fitting a 'wedge', however these are aimed at 'serious' astro-photographers and the prices are truly astronomical !
A good solid mount is a 'must' for astro-photography and a huge advantage at high magnifications. As a 'rule of thumb' you can expect to pay as much (or even more) for your mount as you do for your telescope 'tube' or 'OTA' (Optical Tube Assembly)
This note last modified: 5th Feb 2015 22:05.
(+) 0100 How do I use my first telescope ?
(+) 0109 What is a finder ?
(+) 0110 What is an eyepiece ?
(+) 0111 What is a Barlow ?
(+) 0112 What is a focal reducer ?
(+) 0114 What are Nebular filters ?
(+) 0116 How do I use Setting Circles ?
(+) 0120 What is collimation ?
(+) 0125 How can I safely observe the Sun ?
(+) 1030 How can I take photos of the stars ?
(+) 1033 How to take photos of the Aurora ? - (Northern Lights)
(+) 1035 How to calculate FOV for prime focus ?
(+) 1036 How do I calculate FOV for Eyepiece projection ?
(+) 1037 How do I use a Raspberry Pi camera for astrophotography ?
(+) 1038 What is Star trailing ?
(+) 1039 How can I take photos of Meteors ?
(+) 2100 What is Universal Time (UTC) ?
(+) 2114 What are AUs Parsecs and Light Years ?
(+) 2115 What is Bodes Law ?
(+) 3010 When was Neptune discovered ?
(+) 4000 How do I update Stellarium with new Comet data ?
(+) 5000 How To build the MAS (Raspberry Pi) photoframe ?