Asteroids with primitive compositions, in the C taxonomic complex (see DeMeo et al. 2015, Asteroids IV) have flat spectral reflectances from B to H bands (roughly 0.4 to 1.7 microns). Additionally, their polarimetric properties are consistent across these wavelengths (see Masiero et al. 2022, PSJ). This means that they can be used as superior calibrators for polarimetric instrumentation, as opposed to stars that show chromatic changes in degree of polarization and sometimes even angle of polarization.
The primary difficulty for using asteroid calibrators is that the expected degree and angle of polarization will vary (predictably) with the observing geometry at the time of measurement. Asteroids will therefore need to checked at the expected time of observation to determine if they are viable calibrators.
The below tool will check a handful of objects that have confirmed non-chromatic polarization behavior. Enter the date of observation (any day in the 2022-2062 window), and it will report which asteroids are available (Solar elongation >60 deg) as polarized or unpolarized standards.