Thermal deformation plays an important role in large diameter optics system with ultra-thin mirror. For analysing the specific and quantitive thermal deformation of ultra-thin mirror
a simulating method of temperature distribution by Zernike polynomials is pressented. The temperature is spreaded to different thermal modes having their own meanings in thermal field. For example
the tilt thermal mode means that temperature is high in one end and low on the other end. Summing thermal deformation introduced by different thermal modes to get overall deformation based on two rules is as follows: firstly
temperature is scalar and could be summed after divided. Secondly
every thermal mode introduces small deformation which could be simply added. Considering the relative difficult of theoretical analysis in elasticity
finite element method is used. Mirror model is established for large and thin systems in space without gravity and thermal gradient along radius. The fixed point locates in the center of mirror. To demonstrate this method
some geometric parameters are used from NGST(next generation space telescope). And finally
calculation reveales that different thermal modes introduces different types of surface errors:thermal modes of piston
tilt
focus
astigmatism
coma
spherical
mainly create figure errors of focus
tilt
focus
tilt
tilt
spherical
respectively. Also
thermal deformations could be much different: focus
coma and spherical thermal mode create relative large deformation
which means mirror is sensitive to these kinds of thermal modes. With the same 2 K temperature difference across whole mirror surface
thermal mode introducing larger thermal deformation is coma(56 μm PV)
spherical & focus (18 μm PV) and focus (19 μm PV). In contrast
tilt thermal mode and coma thermal mode can create figure errors no more than 0.07 μm (PV) which almost have no effects on mirror function. Also
to find which kind of thermal mode is more possible
the environment should be put into consideration. Using Zernike polynomials to simulate mirror temperature distribution can give some meaningful findings and is proved to be useful for analysis of ultra-thin mirror.
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references
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