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 The Telescope

Revised 06-05-23  (Previously a subarticle to Large Telescopes Pave Way for New Discoveries, May 6, 2021).

All telescopes are made with a primary objective lens or mirror.  In the case of a Refractor telescope (earliest type used by Galileo) it will have a glass convex lens, and a Reflector telescope (invented by Newton) will have a spherical or parabolic mirror.  Other types of telescopes utilize both of these features, known as Catadioptric, with some examples being Schmidt-Cassegrain and Muksutov-Cassegrain.  

Major large telescopes of the world are primarily based on the Newtonian Reflector.  Firstly, because it allows a direct light path to a camera - a type of camera that is composed of all sensors and no lenses.   In general, glass lenses will exhibit chromatic aberration, although this flaw is managed in amateur astronomy by way of corrective methods since smaller telescopes don’t always detect a difference.  Secondly, a large reflecting mirror is much easier to make than a large glass lens.  Only the surface of a parabolic mirror needs to be perfect compared to a glass lens which must be perfect throughout its entire thickness and volume.

It is necessary to take a picture through a telescope because certain objects in Space cannot be detected by the human eye, but can be detected by a camera.  A camera is actually more sensitive to all kinds of light - especially faint light - than our eyes.  This is done by a practice called long-exposure photography where the shutter is left open for long times such as thirty minutes.  The result is then translated by way of digital processing so the human eyes can see the result on a screen monitor, or if you choose, printed out in hard copy.  This is how photographs of very distant nebulae or far away galaxies are accomplished.

In order to practice long-exposure photography the telescope needs to follow, or track, the object in the night sky.  If you ever observed the night sky, you will notice everything moves right to left (when facing north) in just a matter of minutes. The star constellations are still identical but since the Earth is rotating, it causes the appearance that the sky is moving from east to west.  But there are other motions to account for such as the Earth’s orbit about the Sun, a planet’s own orbital movement about the Sun (when observing a planet), the motion of galaxies (a spiral galaxy that is twirling), and the translation of all celestial objects due to the Big Bang expansion.

The first two obvious motions, the Earth’s rotation and revolution about the Sun, are taken care of by what Astronomers call Sidereal Time.  It is a type of timekeeping based on reference to the fixed stars.  Stars are relatively fixed in the Milky Way Galaxy when viewed from Earth.  Since the Earth’s orbit is in the same sense (direction) as its rotation, the time for one day to pass in Sidereal Time will be faster when compared to Solar Time, which is the time according to our everyday clocks and is relative to the Sun.  

A point on Earth directly facing the Sun (high noon) occurs every 24 hours regardless of where Earth is in its orbit.  This is called Solar Time (everyday clocks).  However, a distant star “sees” a point on Earth (directly facing the star) which is moving in the same sense, or direction, for both Earth’s rotation and revolution.  Hence, the star sees a faster rate of time which is less than 24 hours, or 23 hours and 56 minutes.  This is Sidereal Time (about 4 minutes faster than Solar Time).

 

Sidereal Time is really only of interest to Astronomers since it is related to how to track objects in the sky with a telescope. The remaining motions of galaxies that rotate and translate are not so simple but can be accounted for in advanced radio astronomy.  

Autoguiding is a technique of taking rapid photos, or loop recording, of a supposedly fixed point in the sky (such as a distant star) to “learn” detect if there are any additional relative motions it may have. ²  Once it has learned (recorded) the motion of the distant object, it can simply play it back to track the object for long-exposure photography.  Autoguiding is used in amateur and professional astronomy along with periodic-error-correction (PEC) which is described as accounting for a motor’s gear backlash (“slop” in the tracking gears from possibly non-premium equipment).  Since autoguiding “picks up everything in the wash” as far as additional motions or movements, it does not really matter if the motions are caused by galaxies, orbital perturbations, or inaccurate motor gears - as along as the motion is somehow consistent small (which it should be whether from a galaxy, or motor gears moving constantly in one direction).

.. autoguiding “picks up everything in the wash”..

For observing the night sky without photographing, only sidereal tracking is needed which can be performed manually on the least expensive amateur telescopes. 

Another feature of earth-based telescopes is how they are mounted - or how they are situated on Earth - so it will point to any part of the sky.  One of these is called Alt-azimuth, similar to a surveyors tripod, which is simply up-and-down (altitude)  and  left-to-right (azimuth) axes to cover the night sky, or what Astronomers call the “celestial sphere”.  The other type of mount is Equatorial which also has two rotational axes but with the addition that the entire mount can tilt in the same direction as the Earth’s axis (sometimes called the Polar Axis).  Since an equatorial mount is tilted in the same direction parallel to the Earth’s axis, tracking objects is simplified by rotating the telescope only on the azimuth axis (or more correctly stated as Right Ascension since the telescope is now polar aligned).  Equatorial mounts are preferred in amateur astronomy since it simplifies tracking issues (again, most likely due to nonpremium equipment).¹  But Altazimuth mounts will be preferred in professional astronomy since it greatly simplifies construction of large telescopes.  Large altazimuth telescopes can run on extensive tracking systems using software and autoguiding systems in place of expensive and clumsy physical mounting structures.  Autoguiders can fit in the palm of one’s hand (more or less an inexpensive gadget) and tracking software only requires a good software engineer.

 

 

1.  Some may contend that the basic Equatorial Mount used in amateur astronomy is for educational purposes and that there are no “equipment deficiencies” as telescope manufacturers imply.  For instance, equatorial mounts may reveal the earth’s precession, which is otherwise disguised by altazimuth mounts but known to professional astronomers.  Likewise, it follows that there are really no deficiencies in motor tracking gears but telescope manufacturers wish not to elude to discussions over complex relative motions (such as those of galaxies).  Consequently, the educational aspects of amateur astronomy will need to be pursued in University courses or through available astrophysics articles.

2.  In addition to telescope mount errors, real time autoguiding detects the apparent motion of stars which exhibit fluctutations from atmospheric refraction.  This small apparent motion is due to instability of the atmosphere and related to the "Seeing" conditions.  Other atmospheric conditions are "Transparency " and the commonly known  "Cloud Coverage".  (0% Cloud Coverage is ideal).