Also if you mount the SA behind the lens you are limted to about f5 otherwise the resolution deteriorates unacceptably because of the severity of the aberations due to the steep light cone through the grating. For lenses with apertures up to 30mm (eg 35mm f1.2, 50mm f1.7 etc) mouting the lens close to the front element gives excellent sharp spectra without significant vignetting.
Here is an example of comet spectrum I took using a modified webcam and a 29mm fl lens with a 100 l/mm grating in front. (an early test of the SA concept)
http://www.threehillsobservatory.co..../spectra_4.htm
near the bottom of the page
Cheers
Robin