Mirror F Ratio Confusing Me

[Tenacious Del]

I've been surfing the net. Just looking at basic DIY stuff. No plans yet to build anything. Sometimes I see mirrors for sale or being used and they have specs similar to

6 inch F5 mirror.

How can the F ratio be figured out until it is in the scope? Couldn't that mirror be any F ratio whilst it is out of a scope? I thought the F ratio was to do with the focal length and diameter of a telescope. How can a mirror that is not installed in a telescope yet have an F ratio? :ahh!:

[orion]

The focal ratio(f) is derived from the focal length(F) of the mirror divided by the diameter(D).......f=F/D

Every mirror whether in a telescope or not makes a focus.
For instance if you are holding a mirror and making the reflection from say, the moon on a wall. You will see that if you move back and forth with the mirror the image on the wall will change.
When you have a clear image of the moon on the wall that means that you have focus.

Now the distance from the image on the wall to the mirror is your focal length.
There are other ways of checking the length but I think this is the easiest.

Remember every mirror inside or outside the telescope has a focal length.

[timthelder]

How can the F ratio be figured out until it is in the scope?


http://en.wikipedia.org/wiki/Focal_length

Every mirror has a focal point.The focal point will vary depending on how the mirror has been ground.

You can have two 6 inch mirrors,the focal length of one being 900mm,the other being 1000mm.


When you have a clear image of the moon on the wall that means that you have focus.There are other ways of checking the length but I think this is the easiest.


I'll try and remember to do that...:biggrin:

[Radar]

I don't really know much about this topic but I imagine the lower the focal length of a mirror, the more convex the mirror actually is.

Ray

[Tenacious Del]

okay, cool, thanks for that guys. so a secondary has to be in just the right spot? telescope making just got harder :frown:

[orion]

If you are thinking about making a telescope this program will make it a lot easier. http://www.dalekeller.net/ATM/newtonians/newtsoft/newtsoft.htm

[Tenacious Del]

thanks Ed, that is cool software.

I'm just toying with the idea. I have to get my head around all the hard stuff first. but I like the idea of building my own telescope.

Del

[CanisMajorTom]

Interesting thread. I had no idea about this either. :confused: