How hard is this to do?

I had a go Saturday night, with a 4.7mm EP in my 10" Dob - should have got it but dont think i did.
I thought I may have had it, but Carl (remormalised) had a look and reckoned it wasnt.
Have many of you been able to split it? How close to Antares it it - would a 10mm been better?
Thanks