Amateur Astronomy

General Category => Equipment => Topic started by: Thomas on February 20, 2014, 08:38:10 PM

Title: Reflectors (Beginner's Telescopes Subtopic #1)
Post by: Thomas on February 20, 2014, 08:38:10 PM

While there are other optical designs in use by amateurs, by far most amateur instruments in this category are Newtonian reflectors.  This design was created by none other than Sir Issac Newton nearly 350 years ago.  The design is composed of a large primary mirror (in the shape of a paraboloid) and a smaller elliptical flat mirror.  The key attributes of the Newtonian telescope are:

• Lowest cost for a given aperture
  The Newtonian optical design is very simple.  There are just two optical surfaces.  The cheapest models in the < 10" aperture range will utilize either a cardboard or simple metal tube.  Many telescopes are sold with a mount that does not track objects being viewed.  This all results in the cheapest cost for a given aperture.
• Easiest telescope to make
  Because of the simple design, most amateur telescope makers will start with a Newtonian.
• Typically lowest focal length for a given aperture
  Focal ratios are almost always less than f/6 with ratios below f/5 being common.  Other optical designs are typically f/6 or greater with f/5 being uncommon.  Normally, this would mean that for a given aperture the Newtonian would offer you the greatest field of view, but for reasons that are probably best left to an intermediate topic, beginner Newtonians will typically have a smaller maximum field of view somewhat less than a refractor with a similar aperture.
• Secondary Mirror
  
  • Obstructed optical path
    Newtonians have a small mirror that redirects the light path out the side of the telescope for viewing.  This has a couple of effects optically.
    
    • The first, and probably least significant, is that some of the light entering the optical tube does not make it into the final image.  So compared to a design with an unobstructed optical path of the same aperture, the Newtonian's image will be somewhat dimmer.
    • The more significant effect, is that anytime something is in the optical path it degrades the image.  Conventional wisdom is that if the secondary mirror is 20% or less (by diameter) than the primary mirror the effect is negligible.  There are surely some that would argue with that position (just as there are probably some that would say 25% is OK).  Just keep in mind that as far as optical quality is concerned the smaller the secondary obstruction the better.
  • Collimation
    In order for the telescope to form images the secondary and primary mirrors must be aligned.  This process is called collimation.  Some newtonians will need to be collimated every time they are used and others will not.  Many people find the process intimidating, but it can be learned without too much difficulty.  It does require special tools that will most likely not come with your telescope.
• Open optical tube
  All optical surfaces are exposed to the environment.  Designs with long tubes open only on the end can be more susceptible to distortion due to air currents caused by temperature differences between the telescope and the environment (tube currents).
• Medium size for a given aperture
  The typically small focal ratio and bent optical path means that for a given aperture, Newtonians tend to have fairly short tubes.  They aren't the most compact design, but they're not bad.
• Coma aberration
  Coma is an aberration that causes a loss of sharpness that increases as the distance from the center of the field of view increases.  Stars at the edge may appear like little comets.
  
  • Telescopes will low focal ratios are affected by the aberration to a larger degree than telescopes with large focal ratios.
  • Coma correcting lenses available to reduce the aberration.

Learn about refractors (http://amateur-astronomy.net/forums/index.php/topic,10.0.html)
Learn about catadioptrics (http://amateur-astronomy.net/forums/index.php/topic,9.0.html)