A bit more checking shows that it Depends on the manufacturer. Meade is 95mm, Celestron is 110mm and the Antares is 95mm
there is a bit more information at:
http://www.celestron.com/c3/support3...articleid=2243
Remember that the chip for the QHY10 is recessed by 20mm from the outside connection so you will need 90mm of spacing to the shoulder of the reducer. also be aware that it will not illuminate the full chip of the QHY10. You will get some viginetting of the image. I had this problem when I was using my .63 reducer with my C9.25. You can reduce the effect by taking good flats.
Good Luck
Paul
