Calculating Angle of Saturn from Earth Using a Refracting Telescope

[Fizzicist]

Homework Statement

Saturn is viewed through the Lick Observatory refracting telescope (objective focal length 18 m). If the diameter of the image produced by the objective is 1.7 mm, what angle does Saturn subtend when viewed from earth?

Homework Equations

$$\theta$$ = -y'/f1, where $$\theta$$ is the angle that Saturn subtends when viewed by the unaided eye, y' is the height of the image formed by the objective and f1 is the focal length of the objective.

The Attempt at a Solution

I used the image diameter as the image height (1.7 mm). It did not work. I don't see why that is wrong. Can someone please tell me?

 

[mgb_phys]

You are looking for the plate scale of the telescope - see this explanation
http://panisse.lbl.gov/~sed/telescope/obsguide/platescale.html

ps. be careful with the half-angle vs the full angle of the object in the equations.

 

[Fizzicist]

What exactly am I supposed to do with this? Use it for the image height? I am confused about what a plate scale is, and how I could use it to find the image height.

 

[Fizzicist]

I guess my question would be, what is the point of finding the plate scale? How does it relate to the image height?

 

[mgb_phys]

The plate scale is the image height/angle.
You know the image height and you want the angle.