The design requirements for the primary mirror sub-assembly of a space camera are: its dynamic stiffness is above 90 Hz; the synthesis face figure error (P-V value) for the primary mirror is less than 632.28 nm

and the maximum deflection is less than 30 μm

when the primary mirror sub-assembly is subject to 1 g gravity and a uniform temperature rise of 4℃ in X

Y

Z directions. Analysis and calculations show that if the primary mirror sub-assembly is a rigid support

the synthesis face figure error will go far beyond the design requirement; but if it is a flexible support

the error will generally be within the limits specified. The primary mirror sub-assembly must therefore be a flexible support design

and the determination of height and thickness of mirror supporting ribs is the key to a good flexible support design. If flexibility is too high

dynamic stiffness and the maximum deflections of the primary mirror sub-assembly can not satisfy the design requirement; if flexibility is too low

synthesis face figure errors will not meet the design requirements. It is concluded from calculations that when the flexible support rib is 46 mm high

and 1.7 mm thick

the response value of the primary mirror sub-assembly satisfies the design requirements.