Astigmatism and focal reducer issue

[En1gma]

Hi all

I was having a punt on ccd imaging last night with my ss dsi II and I came accross the problem of astigmatism using a 0.5 focal reducer

firstly, I'll list my eq

ed100 f/9 skywatcher branded
gso 1.25" 0.5 focal reducer

the position of the focal reducer is on the nosecone of the dsi

now my questions
what is the best way to achieve a flat field? (read: cheapest)
Will I need a corrector?
If I up this to a 2" fr. Will it still have astigmatism?
What do others do to eliminate this issue?

Thanks in advance

rob