checking the f number of a mirror

[peeceeTal2M]

Does anyone know of a way to check the f number / focal length of a mirror? I know it can be done at Orion Optics but my son in law would like to start on building the telescope tube before mirror goes in for recoating and he needs to know its focal length to calculate the tube length.

Any help appreciated.

Phill

[MikeP]

Google seems to offer some solutions. There are some methods here http://home.comcast.net/~cassarole/Instructions/Measuring_Focal_Length.htm

Mike

[peeceeTal2M]

Thanks for the link mike, lots of useful information.

[marxy]

I was gonna suggest that you set up the mirror against the wall and shine a torch on it.

Get a piece of paper and note where the light is brought to focus. This gives you the focal length of the mirror.

Hope the build goes well

[Kaptain Klevtsov]

Checked mine by propping it up facing a window. By putting a piece of paper between the mirror and window (off to one side a little, or it gets in the way ), you can get an image on the piece of paper. The focal length of the mirror = the distance from mirror to paper.

Kaptain Klevtsov

[Ajohn]

There are several ways. One is stand the mirror on edge and hold a torch and eye level with each other and on axis. Move the torch side to side. Prior to the centre of curvature it will move one way and past it the direction of movement will switch. The focal length is half of that. This is really a test for a sphere not a parabola but should still work, look for the image change over in the central part of the mirror. The nice thing is it's twice the focal length which 1/2's the error. If it's difficult to get the correct point mask the mirror to about 1/2 of it's normal size. If it's an F5 mirror you may have to do that. When the eye and torch are at the correct point the mirror will also be filled with light.

Kaptains method will work but it's important that the object is a long way away. The best thing to use is the sun,moon or a very distant street light etc.

You can also knock up a very simple faucoult tester. Mirror on edge. A small torch bulb masked with say a 2 or 3mms hole in the mask and a ruler to use as a knife. The height of the torch hole wants to be roughly at the same height as the center of the mirror. Look past the bulb with it close to your eye and locate the reflection of the 3mms hole. The mirror will fill with light when your in the correct place. Then use the rule to slowly cut off the beam of light entering your eye. The shadow will move in a different direction just as the torch mentioned above. You can tie the radius of curvature down to a couple of mms easily that way. If you play about carefully you will see the centre of the mirror darken evenly all over. That will be even closer. The radius of curvature is the average of the distance from the centre of the mirror and the hole and the "knife". Could also use a knife instead of a ruler. Anything like that will do. Best mounted on a wooden block if you want to see foucault shadows.

John

If the mirror is dirty but the coating looks ok you should give it a long warm shower. Maybe up to a day. Other ways of cleaning them usually wrecks the coating - much depends on if it's an over coated mirror. Most are and they will clean up this way.