Apodization. Now there's a word I hadn't heard before. Stumbled on to it whilst searching for examples of the diffraction spikes produced by four vane curved spiders similar to the one on my newt truss that had just finished a rebuild. Found this lot of useful spike pattern images
http://www.astronet.co.kr/xe/making/177075
Then stumbled on apodization in this cloudy nights thread (towards the bottom of the page).
http://www.cloudynights.com/topic/49...-spikes/page-3
I'll let wikipedia explain it.
http://en.wikipedia.org/wiki/Apodization
Here's the science
http://www.ejournal.unam.mx/rmf/no511/RMF51116.pdf
Seem to be mainly used to resolve very close double stars and if your desperate to minimise your spikes.
You learn something every day!!
Cheers
Stephen