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 ?
(+) 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 ?
Notes on eyepiece projection for enthusiastic insomniacs
Ford Prefect asks: (With apologies to Douglas Adams)
What is the magnification and effective focal ratio of a telescope when I project the image of Jupiter into my camera with a 6mm eyepiece, and can I then work out the expected size of Jupiter on my film ?
The magnification is ( v - f ) / f
Where v = image distance (in practice the distance of the eyepiece from the back of the camera).
f = focal length of the eyepiece.
The image size on the film is the primary image size multiplied by magnification.
The focal ratio of the system is the ratio of the instrument multiplied by magnification.
1/f = 1/v + 1/u (the lens formula )
1/u = 1/f - 1/v
1/u = ( v - f ) / fv
but Magnification = v/u
so M = v ( v -f ) / fv
or (v - f ) / f "QED"
( or v/f - 1 )
I have a 6mm eyepiece and its about 50mm away from the 'film' (these days, that's the digital image sensor).
The telescope is a 300mm f/6 reflector
Magnification = 7.3
Focal ratio = 44
Effective focal length is 13 m.
Did it work in practice ?
Yes ! The image size of Jupiter at the prime focus of the 300 F/6 is given by ( Theta . F / 206265 )
where Theta = apparent size of Jupiter i.e. 60" arc F = 1800mm, so Jupiter is 0.5 mm in diameter.
After eyepiece projection, the image should be magnified up to 7.3 x 0.5 mm
The measured diameter on the processed film was 3.8mm !!
This note last modified: 5th Feb 2015 22:05.
(+) 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 ?