In an alt-alz ( Dobsonian ) mount since the mirror, no matter how it points skywards, is always symmetrical about a vertical plane and hence a true floatation cell will not give astigmatic mirror.

• 地平经纬式（多布森装置）无论如何指向天空其反射镜均总是与垂直面成对称因而壹个真正的浮动镜室不会产生象散误差。