Focal Length

[Kirk]

I have a bit of a mystery.

I have an Odyssey 8 scope listed as f/4.5. For some reason I have in my head that it is a focal length of 900mm.

Here's the thing, the math doesn't work. An 8 inch mirror is 208 mm so 900/208 =f/4.327 not f/4.5.

So, either Coulter's 8 inch mirrors were actually 200mm not 8 inches
(which works out to f/4.5) or the focal length is not 900 mm but actually 936mm (which also works out to f/4.5)

Which is more likely? A 200mm "8 inch" primary or a 936mm focal length?

Any thoughts?

~Kirk

[SEmert]

Which is more likely? A 200mm "8 inch" primary or a 936mm focal length?

Any thoughts?

~Kirk

Two:
Rounding errors/oversimplification! 4.5 is simpler to print than 4.327.

Use a tape measure and clear up the mystery??

[Kirk]

A tape measurer? Where's the fun in that!
Logical answers are sooooo over rated.



~Kirk

[ashutoshl]

Here's the thing, the math doesn't work. An 8 inch mirror is 208 mm so 900/208 =f/4.327 not f/4.5.

8" = 25.4 * 8 = 203.2mm
Thus, 900/203 = 4.43 ~ 4.5 (easier to remember)

I guess a 200mm (8") mirror is more likely for two reasons ---
1. Many of these instruments are manufactured in China, where metric system is more prevalent. There 1" is converted to 25mm.
2. Even though the mirror diameter is 203mm (8") it maybe chamfered at the edges, so that the actual concave surface would be ~ 200mm.