Foci and Mirrors

This week’s discussion topic will attempt to answer the question:

Suppose your Newtonian reflector has a mirror with a diameter of 20 cm and a focal length of 2 m. What magnification do you get with eyepieces whose focal lengths are: a. 9 mm, b. 20 mm, and c. 55 mm?

From my textbook:

The magnification of a reflecting telescope is equal to the focal length of the primary mirror divided by the focal length of the eyepiece lens:

Magnification = Focal Length of Primary / Focal Length of Eyepiece

In the question stated above, the three different eyepieces will result in the following magnifications:

2000 mm / 9 mm = 222X
2000 mm / 20 mm = 100X
2000 mm / 55 mm = 36X

Moss XX14g (Oct 2016)
Moss XX14g (Oct 2016)

From the Orion XXg Instruction Manual (p. 20-21):

Magnification, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece. Therefore,
by using eyepieces of different focal lengths, the resultant
magnification can be varied.

For my Orion XX14g (shown above), for example, which has a focal length of 1650mm, using the eyepieces (shown below), I would have the following magnifications:

1650 mm / 9 mm = 183X
1650 mm / 26 mm = 64X
1650 mm / 38 mm = 43X
Moss Eyepieces (Sep 2017)
Moss Eyepieces (Sep 2017)

Depending on what I plan to observe for an evening will determine what eyepieces I primarily employ.  If I’m observing star clusters, galaxies (like M31) or nebulae, I would probably stick with the wide field of view and less magnification eyepieces.  But when I want to observe a transit of a moon across the Jupiter or the gaps in the rings of Saturn, I would definitely want as much magnification as I could manage without sacrificing brightness of the image being viewed.  The more I magnify and zoom in on an object, like Mars for example, the less light I have to work with and the darker the image appears to me through the eyepiece.

The ASKC’s Powell Observatory boasts a large Newtonian telescope called the Ruisinger Telescope, which has an f/5 mirror and focal length of 3810 mm. The Society calls the Ruisinger a 30 inch (762 mm) mirror, but in reality, it is 29 inch (736 mm) because a 1/2-inch around the periphery is masked off. Because of this, the real f/ratio is precisely f/5.18.  Using the same formula, with the same eyepieces from the original question, the following magnifications are possible using the Ruisinger Telescope at Powell Observatory:

3810 mm / 9 mm = 423X
3810 mm / 20 mm = 191X
3810 mm / 55 mm = 69X

The focal ratio of a telescope refers to the speed of its optics and is a limitation related to the brightness and sharpness of an object.