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 ?
(+) 0111 What is a Barlow ?
(+) 0112 What is a focal reducer ?
(-) 0114 What are Nebular filters ?
Observing with Nebular filters
Nebulae contain gasses that emit light when a nearby star shines on it or through it. Examples of such nebulae can be found in Orion (“Great” M42), Lyra (“Ring” M57), Vulpecular ( “Dumbbell” M27) and super nova remnants (“Veil” in Cygnus). Comet tails also emit light by a similar process. Hale-Bop had sodium tail.
Gasses emit light in only sharply defined regions of the spectrum, unlike stars which have what is described as a white light or “a continuous spectrum”. Filters that only pass the emission lines allow us to see the nebula in all its glory while all other light is removed. A filter that transmits only a few wavelengths is a “band pass” filter. Light pollution rejection filter block some areas of the spectrum while transmitting over a larger band width = these are broad band transmission filters.
Visible spectral lines from atoms in nebulae. (ref-1, 3)
Here is a list of the important nebulae lines in the visible region 700 nm(red) to 400(blue) nm
(nm=nano meter 1nm = 10-9m , 1A = 10nm)
Hydrogen Alpha Red 656.3 nm
Hydrogen Beta Green 486.1 nm
Hydrogen Gamma Blue 434.1 nm
Oxygen III Green 495.9 /500.7
Nitrogen II Red 654.8 / 658.4
Visible spectra lines in comets (Ref-2)
Sodium I Yellow doublet 589.0 / 589.6
C2 Swan Band (Ethane) Red 620 nm
ditto Blue 430 nm
Deep Sky Filter, or Light pollution rejection.
This is a broad band filter that blocks mercury and sodium vapour. It can be used for increasing contrast on galaxies and is suitable for CCD and photography.
Ultra High Contrast Filter.
These have narrow band pass filters that lets through Oxygen III and Hydrogen Beta. Good general purpose nebular filter.
Provides the highest contrast for planetary and brighter emission nebula. Best with larger apertures
Provides the highest contrast for faint Hydrogen clouds like the Horse Head and California. Best with larger apertures
1) Astronomical Spectroscopy, A.D. Thackeray (1961)
2) Observing Guide to Comets, British Astronomical Association (1996)
This note last modified: 5th Feb 2015 22:05.
(+) 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 ?