When a refractor or catadioptric telescope is used for terrestrial observing (reflectors cannot be used terrestrially), the 90° star diagonal generally supplied with the scope gives an erect image, but one that is reversed left for right. Some people find this reversal disconcerting when attempting to follow the rapid movements of wildlife, when trying to read printing (the names or home ports of distant boats), etc. Also, for many people, the right-angle viewing position with a star diagonal (looking down to see straight ahead) is often inconvenient or uncomfortable. Removing the star diagonal and inserting an eyepiece directly into the scope gives a somewhat more comfortable straight-through viewing position, but an inverted image.
Replacing the star diagonal with a straight-through porro prism or image erecting lens, or with a 45° or 60° viewing angle image erecting diagonal, provides upright terrestrial images and allows a more comfortable viewing position. 45° viewing is generally more comfortable than straight-through viewing for watching treetop activity or for extended observing from a blind or back porch. You won’t get a crick in your neck from crouching over to look through the scope as you often will with a straight-through model on a too-short tripod. It is also more convenient for observing couples of varying heights who must share a single scope, as there’s little need to constantly raise and lower the tripod to a comfortable height for each observer. In addition, the left/right image orientation through the eyepiece is correct with a porro prism or image-erecting diagonal. This makes reading distant printing or following wildlife easy and natural. An image erector does not fit into the telescope’s existing star diagonal, which must be removed from the telescope for the image erector to work properly.
An eyepiece magnification of about 80x is the most usable with image erectors. Higher powers result in an amplification of the heat waves, mirage, and moisture in our atmosphere that can make the image unusable much of the time. In bright light, lower powers also result in the secondary mirror of a catadioptric scope being visible as a gray blur in the middle of the eyepiece field as your pupil contracts in the daylight to a size too small to see around the secondary mirror. Refractors do not have a secondary obstruction and can be used with lower power eyepieces.
Porro prisms use two offset pyramid-shaped prisms to correct the image and are consequently fairly large (about 2.5” in diameter). The 45° and 60° viewing image-erecting prisms generally uses a single multi-faceted Schmidt prism to provide a correct image. Consequently, they are about the size and shape of a normal 1.25” star diagonal.
An erecting prism’s internally folded light path increases the telescope focal length, resulting in a slight increase in magnification over that of an eyepiece and star diagonal. There is also a small light loss, which is usually of little consequence during daytime observing. Image erecting systems cannot be used with reflectors, as this type of scope does not have enough focuser in-travel to allow the longer light path of the image erector to focus.
Some people are tempted to use a 45-degree viewing image erecting system for astronomical observing, feeling that a correctly-oriented image that matches the orientation of a star chart would help them locate objects more easily. This use is not recommended, however, as the vertex in the prism puts a small spike of light on every bright point in the image. This is of no consequence in the daylight, as the spike is dim and the image is usually lit brightly enough to wash out the spike. At night, however, when the background is dim and dark, the spike of light is quite visible and reduces the resolution of binary stars, star cluster, and subtle lunar and planetary features. In addition, observing the sky with a porro prism or 45-degree prism leads to uncomfortable and contorted observing postures, as the eyepiece is pointed towards the ground when observing near the zenith.
Some image erecting systems, particularly the 45-degree viewing versions, can show a very faint straight line down the center of the prism if examined with the unaided eye in daylight. This is a vertex of the prism and is not a flaw in the optical system. It is not visible in an eyepiece field, day or night, if the prism is used as intended – in a telescope with an eyepiece.