Upside down and backward images

The image in a telescope is normally upside down or reversed left to right. Figure 5.2 shows why. The objective lens flips the image over, but the eyepiece does not flip it again, so if there are no other mirrors or prisms, what you see is

Exit pupil Eyepiece | (Ocular)

Exit pupil Eyepiece | (Ocular)

Aperture

(size of telescope)

Eye Focal relief ler|gth of

Focal length of telescope

Eye Focal relief ler|gth of

Focal length of telescope eyepiece

Figure 5.1. Important dimensions of a telescope.

Aperture

(size of telescope)

Parallel rays from another star higher In sky

Parallel rays from another star higher In sky

Parallel rays from star In center of field

Figure 5.2. Why the image in a telescope is upside down.

Parallel rays from star In center of field

Figure 5.2. Why the image in a telescope is upside down.

inverted. (The eyepiece adds power to the lens of your eye but does not form a separate image of its own.) Extra lenses, prisms, or mirrors could put the image right side up, but they would add cost and bulk, as well as the risk of losing light or reducing image quality. Since "up" and "down" don't mean much in the sky anyhow, astronomers have worked with inverted images for centuries.

But as Figure 5.3 shows, that's not the whole story. Most modern telescopes do include another mirror or prism, a diagonal, so that observers don't have to crane their necks. The diagonal flips the image, but only in one dimension. Accordingly, what you actually see is right side up (because the mirror has flipped it vertically) but reversed left to right (because in the horizontal dimension it's still inverted).

Many of the maps in this book are reversed to match such a view. Except for the "R" maps of the American Association of Variable Star Observers, other astronomical publications do not do this. Most star maps are either right side up (north up, east to the left) or inverted (south up, east to the right) to match

No telescope No reflections

Erect image Jj

LIKE THIS

Telescope with 0 or 2 reflections

Inverted image

Was this article helpful?

0 0

Post a comment