## 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 ?`

`Marvin replies`

`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.`

`Proof : `

`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 )`

`In use:`

`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.*