"Fighting compartment observation methods
The commander must have uninterrupted 360 degree vision. To achieve this, a commander's cupola was introduced for the KV-1S and then T-34.
The KV-1S commander's cupola was equipped with four mirrored periscopes, providing 360 degree view range with 22-40 meters of dead space. This cupola was unsatisfactory for the following reasons:
- Mirror periscopes were destroyed by nonpenetrating shots.
- Mirror periscopes were inconvenient to use (the commander was looking 30 degrees up), and were low in quality, distorting the image.