Source of mirror reflectivity data
The spectral reflectance curve depends on the coatings applied.  For 'scopes used by people there is no point in maintaining reflectivity below 450 or above 700 nm.  You idea of &quot;visible&quot; range is, I fear somewhat optimistic.  You may be better off with an uncoated Al mirror (spectral curves easily found on the net) and giving up some signal in the photopic range.  -- or with a nice Ag overcoat, perhaps.
edit
Per request, some data on uncoated Al.
From   laserbeamproducts,

REFLECTIVITY OF PURE UNCOATED ALUMINIUM AT NORMAL INCIDENCE

Wavelength            % Reflectivity 
248nm                      92.6 
400nm                      92.0 
532nm                      91.6 
633nm                      90.7 
800nm                      86.8 
900nm                      89.0 
1um                          94.0 
3um                          98.0
10.6um                     98.7 
20um                        99.0 
100um                      99.4

An image published (by the author) at Researchgate