## And second...

Share practical advice and tips on making your own kit

### And second...

Hello,

I am finally getting around to making a truss type newtonian, having had all or most of the materials for quite some time.

Having also had the primary mirror for some time I have completely forgotten whether it is f6 or f8 (definitely one or the other).

I have an idea of setting it up in order to bring an image to focus and measuring to get the focal length... it is a 200mm mirror.

I am not sure that this will allow for the secondary to focuser distance though or whether this will have any effect.

Do any of the optics experts out there know of a simple way to check the focal length?

Thanks

Steve
skye at night

Posts: 523
Joined: Mon Feb 13, 2012 6:42 pm

### RE: And second...

Rough way: Do this indoors; just focus a distant object (e.g. clouds, Moon) through a window onto the bit of wall adjacent to the window. The focal length is the distance from the mirror to the wall. Should be easily good enough to tell you whether it is f/6 or f/8.

Another rough way, but doesn't require distant object: Mount the mirror vertically (making sure it can't fall and damage itself or anything else) Hold a light (penlight, LED) to the side of your head near your eye so that you can see its reflection. Move the light. If its reflection moves in the same direction, move away from the mirror. If the reflection moves in the opposite direction, move closer to the mirror. There comes a point where the light appears to fill the mirror. Your eye/the light is now at the centre of curvature. The focal length is half this distance. Again, for speed and ease, if you just test it at a distance equivalent to f/7 (i.e. 112" from the mirror), if the reflection moves the same way as the light, it is f/8; if it moves the opposite way it is f/6.
sftonkin

Posts: 2826
Joined: Mon Feb 13, 2012 6:22 pm

### RE: And second...

make sure you dont focus the sun on to the wall.
andrew shields

Posts: 408
Joined: Mon Feb 13, 2012 6:42 pm

### RE: And second...

Thanks for that..
your first method was roughly what i had planned...

plus.. I only need to check if it is f6 or f8.. so if it comes near to focus at a distance of either 1200 or 1600 I should know.

Thanks

Steve

skye at night

Posts: 523
Joined: Mon Feb 13, 2012 6:42 pm

### RE: And second...

[quote]ORIGINAL: andrew shields

make sure you dont focus the sun on to the wall.
[/quote]

Spoilsport!

[color=#993300]<thoroughly_irresponsible>[/color]
[color=#0099CC]Focusing the Sun on [i]anything[/i] with an 8" is great fun [:D] -- not as much fun, mind, as using one of those metre-square Fresnels that Edmund Scientific used to sell (anyone know where these can be had nowadays?) and which could blast a hole through an aluminium drinks can or write your initials in a tarmac road....[/color]
[color=#993300]</thoroughly_irresponsible>[/color]
sftonkin

Posts: 2826
Joined: Mon Feb 13, 2012 6:22 pm