A Quetzal is a Herschel track-based gun with a period lower than 62. Standard stable Herschel conduits do not permit guns with periods less than 62, so alternate components are required. The name is short for Quetzalcoatlus (a giant pterosaur), and expresses the fact that Quetzal guns are enormous and emit gliders. The term emu refers to the gliderless Herschel tracks of periods below 62, and describes how the loops are flightless (they emit no gliders).
The Fx77 Herschel conduit can potentially be made to emit gliders at periods as low as 51. However, no Quetzal of period less than 54 has yet been found.
The original Q54 gun was created by Dietrich Leithner in January 1998. It is a huge Herschel loop populated with oscillators, which destroy the first natural gliders of the Herschels before they have chance to collide into the other Herschels. A pipsquirter, which is welded to one of the other oscillators, is used to liberate a glider from the Herschel track.
The Q55 gun was first explicitly constructed by Stephen Silver, and dwarfed the other Quetzals due to period constraints. Specifically, the unlucky coincidence of the track period and the length of the 77-step conduit both being divisible by 11 resulted in the loop requiring 44 corners.
Noam Elkies suggested using a linear track, where gliders emitted from the track are reflected using p5 reflectors to re-synthesise the initial Herschel. Stephen Silver constructed such a gun shortly after assembling the original Quetzal-55 gun. This method was later applied to the p54 gun, but was much more difficult due to the timing constraints caused by the even period.
The Q56 gun is a relatively compact Herschel loop, again built by Dietrich Leithner. It uses p4 and p8 oscillators, even though p7 oscillators could have been used. Unlike the p54 gun, it has not been improved upon by using an open track.
The Q57 gun uses a large p3 oscillator to release a glider from an Fx77 Herschel conduit in the p57 Herschel loop.
- Main article: Period-59 glider gun
The Q59 gun, built by Adam P. Goucher in 2009, is a large assembly of p59 Herschel tracks, rephasers and p44-like engines. It is larger even than the original Quetzal-55 gun, due to the immense amount of rephasing required. Jason Summers noticed that one half of the gun could be removed and replaced with a single reflector, due to the serendipitous fact that the reflector has exactly the right spacing to complete the gun.
A period-60 glider-emitting Herschel loop was constructed by Dieter Leithner in January 1998. It uses a unix to liberate gliders from a corner of the loop. Due to its period, the gun is minuscule compared to the other Quetzals, with a bounding box of less than 10 000 cells.
- Main article: Period-61 glider gun
The first Q61 gun was built by Luka Okanishi. It uses a lightweight spaceship stream as a glider-to-Herschel converter, as well as a small stable Herschel-to-2 glider converter.
- Quetzal at the Life Lexicon