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Al,
There's a couple of options:
Now that you have a calibrated spectra you don't need the zero order..just use the Ha and Hb lines...
Alternatively add a 4 degree prism ( Edmunds) and turn it into a Grism; this will then bring the first order spectra back close to the optical axis.
I made one up for the P-H gratings I have and it worked as advertised.
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