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MAS Frequently Asked Questions


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 ?

Star Brightness (Planets at maximum) The star brightness (magnitude) system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as "size". Since they knew there must be many stars 'too small to see' (with the unaided eye), they based their scale on the 'biggest' (brightest) star know at the time = Vega, set that to 'value' 0, and counted up from there. As a result, 'smaller' stars have higher values (so, rather then 'brightness' we should actually be talking about the 'dimness' value, since higher values = dimmer stars :-) ). Of course we now know that brighter stars are not necessarily bigger (some are just closer) nor are dimmer necessarily smaller (since the light from the star may be obscured by interstellar dust and gas) These days we talk of 'apparent magnitude' (thus avoiding the whole 'size' & 'brightness / dimness' issue), however this is still measured 'relative' to Vega (at 0), which means that Sirius (which is brighter than Vega) has to be given a value 'smaller than 0' (as, indeed, do all brighter objects, such as the planets  and our Sun !). The 'apparent magnitude' of Sirius is thus -1 = for other object see below :- From Wikipedia (note that the Handbook of the BAA has a lower value for the Sun (-26) & thus for the full Moon (-12) and the planets) Sun                -27 Moon (full)     -13 Venus            -4.4 Mars             -2.9 Jupiter          -2.5 Mercury          -2 Sirius              -1 Saturn / Vega    0 Antares            1 Polaris             2 Uranus             4.7 (naked eye limit) 6 Neptune            7.8 (7x50 bino limit) 10 Pluto                 14
Norton's 2000 Star Atlas and Reference Handbook: Sirius (alpha Canis Major)  -1.5 Canopus (alpha Carina)      -0.7 Arcturus (alpha Bootes)      -0.04 Vega (alpha Lyra)             +0.03 Capella (alpha Auriga)       0.08 Rigel   (beta Orionis)          0.12 Betelgeuse (alpha Orionis)   0.5 Aldebaran (alpha Tau)          0.85 Spica (alpha Virgo)            1.0 Caster (alpha Gem)            1.6 Visible stars in telescope or binoculars (Sidgwick, Amateur Astronomer's Handbook) Objective/Mirror        Limiting diameter (mm / ")       Magnitude        50                  10.5       100                 12.0       150                 12.9       200                 13.5       250                 14.0       300                 14.4     Note. Long exposure digital photography using 'amateur' equipment is limited to about Mag. 20-21 (after which noise swamps any result). In comparison, professional Earth based telescopes are limited to approx Mag. 27, whilst the Hubble Space Telescope (HST) limits to approx Mag. 31.
This note last modified: 5th Feb 2015 22:05.


(+) 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 ?

(+) 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)

(+) 1036 How do I calculate FOV for Eyepiece projection ?

(+) 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 ?