You can do it with a fixed mount using the natural sidereal motion to drift the sun across the slit as I experimented with here with my LHIRES
http://www.threehillsobservatory.co.uk/astro/spectra_29.htm
With Buil’s other instrument the SolEx, it is essentially a high resolution spectrograph which is mounted on a telescope and the mount is normally driven at x times sidereal, taking advantage of improvements in camera sensitivity and speed to reduce the scan time so the result is less reliant on stable conditions. The details of this device are yet to be revealed though
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