Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 247)

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bulb: Bulb Exposures
With [ z 2: Long exp. noise reduction ], you can reduce the noise
generated during long exposures (p.191).
For bulb exposures, using a tripod and Remote Switch RS-80N3 (sold
separately) or Timer Remote Controller TC-80N3 (sold separately) is
recommended (p.258).
Using mirror lockup (p.256) with bulb exposures is also possible.