The figure below shows the telescope limits for the Keck telescopes. In the regions where an elevation limit of ~35° is shown, the telescope hits the Nasmyth deck. Note that this is a problem in the South and West for Keck II, while for Keck I it is more of a problem in the North and East.

Specific limits for the two telescopes:

Telescope Azimuth range Elevations accessible
Keck I 5.3° to 146.2° 33.3° to 88.9°
Elsewhere 18° to 88.9° (unvignetted), 0° to 18° (vignetted)
Keck II 185.3° to 332.8° 36.8° to 89.5°
Elsewhere 18° to 89.5° (unvignetted), 0° to 18° (vignetted)

The lower limit at other azimuths is shown as zero degrees, but in reality there is usually a software limit set at some positive elevation. In certain circumstances this limit may be decreased, to allow access to objects too far south to observe otherwise.