Kitchen floor experiment. Looking for the fp

Kitchen floor experiment. Looking for the fp

I placed my 4.5 f/8 910mm fl on the kitchen floor 8 feet beneath a light bulb. With no lens I pulled my hand away until I saw a focused image on my palm. I held a 32 mm eyepiece and examined the image. It was a clear image of the lightbulb filament.

The primary to the diagonal distance is about 750mm and the distance from from the diagonal to the filament image is also about 750mm.

What was I seeing?


Stu

Okay; I think I've got it. Your image was 1,500 mm (750mm + 750mm) from the primary mirror. The reason it was so far was that the object you were focusing on was extremely close to the mirror. For stars, or other distant objects, the focal plane would be at 910 mm.

Hugh Bartlett

>>>What was I seeing?

Stu
----

An image of the filament. 8)

The equation which governs focal lengths is:

1/FL = 1/F1 +1/F2 or

FL = (F1xF2)/(F1+F2)

Plugging in your numbers (8 feet and 1500mm) and cheating by using MathCad to handle the units, the result is

FL= 928mm, pretty decent measurement...

Jon