## 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)

### (+) 1035 How to calculate FOV for prime focus ?

### (-) 1036** How do I calculate FOV for Eyepiece projection ?**

**Notes on eyepiece projection for enthusiastic insomniacs**

*Eyepiece projection is when the camera sensor (without it's lens) is positioned above the eyepiece. The image is 'projected' by the eyepiece onto the sensor. Typically, eyepiece to sensor distance is fixed by the camera adaptor, however this distance results in additional magnification*

`Worked example. What is the magnification and effective focal ratio of my 300mm f/6 telescope when I project the image of Jupiter into my camera with a 6mm eyepiece (so I can work out the expected size of Jupiter on my film / DSLR sensor) ?`

*Out goal is to get an image of Jupiter 'filling the frame' i.e. covering as many pixels as possible, to give the best resolution*

`The magnification formula for a simple lens (eyepiece) is M = ( v - f ) / f `

`v = is the 'image distance' i.e. the distance of the eyepiece from the position of the camera film/sensor.`

`f = focal length of the eyepiece (eg 6mm)`

`The image size on the film is then 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 )`

`Back to our example, the distance from the 6mm eyepiece to the 'film' plane (image sensor) was estimated as 50mm.`

`(NB. The telescope is a 300mm f/6 reflector).`

`So Eyepiece Magnification is thus = (50 - 6) / 6 = 7.3`

`This gives us a combined focal ratio f/6 X 7.3 = 44, and an effective focal length of 44 x .3 = 13 m.`

`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 would be magnified to 7.3 x 0.5 mm = 3.65mm`

`The actual measured diameter on the processed film was 3.8mm !!`

`(the difference is likely due to the estimate of eyepiece to film distance of 50mm)`

*This note last modified: 11th Jun 2019 10:58.*