P

Parabolic Mirror - A mirror whose cross-section is parabolic rather than spherical. Spherical mirrors are easiest to make but suffer from aberrations in certain optical configurations. Parabolic mirrors are used in Newtonian telescopes and Classical Cassegrain designs. A parabolic mirror is one of the easiest aspheric mirrors to make, making it still relatively inexpensive compared to a hyperbolic mirror.

Parallax - Refers to the angular offset between two points. This term is used to describe any error in the alignment of a pair of binoculars. It also refers to the effect of a nearby object shifting relative to a more distant object. If a finderscope is aligned on a nearby object, parallax may cause the finderscope and main telescope to be misaligned on a distant object. Parallax is also used to measure celestial distances. Astronomers measure how much a nearby star appears to move relative to more distant stars as Earth orbits the sun. Some simple trigonometry reveals the actual distance to the star.

Parallelogram - This device, which looks basically like the quadrilateral shape for which it is named, is used to support binoculars for astronomical viewing. A parallelogram makes observing at the zenith easy by keeping the binoculars above the observer and away from the tripod. It has the unique property of being height-adjustable for different observers while being able to stay pointed at a given location in the sky.

Parfocal - Optical components, such as eyepieces, that do not require refocusing when switching from one to the other are called parfocal. It is also possible to have an eyepiece that is parfocal with a camera, allowing finding and focusing through the eyepiece, without refocusing when the camera is attached.

Phase Coatings - These are optical coatings used on roof prism binoculars. The prisms used in such binoculars cause a phase shift where the transmitted light is shifted by half a wavelength due to reflection off the edge of the prisms. Phase coatings are applied to the prisms to correct for this phase shift and increase the overall brightness and contrast.

Photometry - Photometric measurements determine the brightness, or magnitude, of an object. These measurements are used to obtain information about variable stars, asteroids, supernovae, and more. See also, astrometry.

Pier - This is a stand for a telescope mount, similar to a tripod but differing in that it consists of a single post rather than three legs. Portable piers still have three legs that usually come off the bottom of the post, rather than coming out from right underneath the bottom of the telescope. Permanent piers are used in observatories and consist of just a post anchored into the floor of the observatory.

Piggyback - Anything that is mounted on top of a telescope is said to be piggybacked on that scope. Cameras are often piggybacked for wide-angle photography where the telescope is used to track and/or guide but is not used for the imaging itself. Also, smaller telescopes (usually compact refractors) are often piggybacked on larger scopes for the purposes of guiding or wide-angle imaging and observing.

Planetary Nebula - A planetary nebula is the remnant of a dying star. When a sun-like star runs out of fuel, it will cast off its outer atmosphere in an expanding cloud of gas. Planetary nebula are usually symmetrical, with some forming spherical shells while others form double-lobed, or bipolar, clouds. A star much more massive than the sun will actually explode--a supernova--rather than gently create a planetary nebula. Examples of planetary nebulae include the Ring Nebula and Dumbbell Nebula.

Planisphere - A planisphere is a flat, circular chart that can be set to any date and time. Once set, the charts shows the entire sky laid out exactly as it appears for that particular date and time. Planispheres are basic charts, usually showing only the brightest constellations and sometimes a few brighter deep-sky objects. A planisphere is the best way to start learning the night sky.

Polar Alignment - Equatorially mounted telescopes must be properly aligned with Earth's rotation axis in order to track properly (and to point accurately if they are computerized). Correctly orienting the mount with the celestial pole is termed polar alignment. There are various levels of accuracy required depending on the intended use of the telescope. For viewing, a mediocre polar alignment will suffice, while for long-exposure astrophotography, a very precise polar alignment is required.

Polar Axis - This is the axis of an equatorial mount which is aligned to the celestial pole in order to facilitate tracking for Earth's rotation. This is also called the right ascension axis.

Polar Scope - This is a small telescope that is aligned with the polar axis on an equatorial mount to make the process of accurately polar aligning the mount easier. It usually displays the offset from Polaris to the true north celestial pole. In the southern hemisphere this process is trickier since there is no bright polar star like Polaris, but the scope works in a similar way using fainter stars.

Polarizing Filter - This is a set of adjustable filters that is used to attenuate the light from bright objects such as the moon and planets. It works similarly to a neutral density filter but is adjustable, making it more versatile. One of the most popular filters for observing.

Porro Prism - These are the type of prisms used in traditional binoculars where each optical path is offset. This is a less compact design than a roof prism, and does not allow as close a focus range, but it is a simpler design and results in brighter images due to fewer required reflections (although modern coating technology makes the difference very small). Porro prism binoculars are typically less expensive than an equivalent roof prism.

Primary Mirror - This is the main mirror in a reflecting or catadioptric telescope design. It usually determines the light-gathering power of the telescope.