The exact derivation of the term gobo is unclear, however, many in theatrical lighting believe it stands for GOes Before Optics

A gobo is an object placed inside, or in front of a light source to control the pattern of the emanating light and shadow.  

In photography, the term ‘gobo’ refers to a flag or other apparatus, placed in the path of the light to casts a shadow or texture.
In theatrical lighting, a gobo is a pattern placed inside the gate, at the focal point between the source, and the lens. The projected pattern can be in sharp focus to produce the desired image, or in soft focus to create a texture.
Gobos are available in many sizes, with A-size being the largest standard size used in common theatrical fixtures.
A-size gobos are the most popular to create a soft texture with the luminaire output.
B-size is preferred for projecting images which will be in sharp focus, as it is challenging to achieve a sharp focus from edge to center across the entire A-size pattern with most theatrical luminaires.
Incandescent luminaires can produce undesired halation, where light spills beyond the desired image area, but there are a few solutions.
Bench focus the fixture is the best option, but if that isn’t enough, you can use a do-nut in the color frame holder, but this will reduce output.
In an ETC Source 4, the longer filament of the 750 Watt lamp will also create more halation. You may find better success with the 575 Watt lamp.
Because of the flat emitter array, LED luminaires tend to produce clean, sharp gobo images. Running the barrel out of focus for a softer image creates a rainbow halation. Best to first achieve sharp focus and utilize a light diffusion to soften the projected image.
Please visit Apollo Design or Rosco to select gobo patterns. Please note, ordering from a single gobo producer will save shipping cost. Apollo requires a minimum of six metal gobos for drop shipment.
When ordering gobos from XS, please include the manufacturer’s number as well as pattern name, and the desired size. If unsure about size, please include the luminaire brand and model.