what is the closest reasonable focus distance for an old 700mm focal length telescope, 60mm objective?

Question

I just got it at an auction.

I want to set up the finderscope before I go further with this.

Answer 1

The focus distance depends on how much travel you have in your focuser. When a telescope is focused at infinity, the image plane is at the focal length (f) away from the lens (here that is 700 mm). As you focus on nearer objects, your eyepiece must move farther away from the main objective lens. The distance away follows the lens equation. If the distance to the object is d, the distance from the objective lens to the eyepiece is e and the focal length is f, then:

1/d + 1/e = 1/f

Look at an example for your case. Say you have 4 inches (100 mm) of travel (t) in your focuser from where it focuses on a distant object until it is out as far as it can go. For the distant object:

1/(infinity) + 1/e = 1/f

Since 1/(infinity) = 0, e = f

Now see how close you can focus. the eyepiece distance now is the focal length plus the focuser travel:

1/d + 1/(f + t) = 1/f

Solving for d:

d = f(f + t)/t

For f of 700 mm and t of 100 mm, d = 5.6 m (18 feet)

Measure the travel in your focuser, use the above equation, and you can figure out your closest focus.

Answer 2

Theoretically, you can keep moving the eyepiece back until spherical aberrations prevent a good focus - maybe a meter or less. In practice you will be limited by the light cone becoming fatter as it passes through openings and being partially blocked. This is called vignetting, and results in darkening, first at the image periphery and then everywhere.

Answer 3

It depends on how far the eyepiece holder will crank out. The thin-lens formula says that

1/f1 + 1/f2 = 1/f

where
f = 700 mm
f1 = distance to near object
f2 = f + "eyepiece movement"

Suppose you can crank your eyepiece holder out by 30 mm.
Then f2 = 730 mm and f1 = 17 meters.

Answer 4

Good idea to align the finderscope first. Make sure you align it using the most distant object possible, if you're going to align it with something terrestrial.

If you purchase one or more extension tubes, you can get your telescope to focus even closer. Make sure you know if the barrel diameter is 1.25" (standard) or .96" (often used with cheap telescopes).