Paul,
Get one of these:
http://www.scopestuff.com/ss_fr1a.htm
You'll need a little adapter for it to fit on the end of the STi. Luckily the STi lens kit had that little adapter. It will help in brightening up guide stars and is useful for widening the FOV of the guide camera plus brighten the guide stars.
I don't know if you can fit it to an STXL. I read on Astromart someone selling an STXL11 and he has a .7X reducer on his guide chip. So is that standard or an accessory?
Greg.