Suavi,
You don't need a flattener with spectroscopy!
The dispersion angle from the grating is approx. 3.8 degrees/ 100 l/mm, so for the 200 l/mm grating the focal plane of the spectral image ( actually an arc) is spanning an arc that extends just over 7 degrees off axis.
(See
http://users.erols.com/njastro/faas/articles/west01.htm)
The blue portion of the spectra will be "closer" to the zero order image focus and the red will require you to re-focus further inwards.
Hope this helps.