HIREX

Insolation of Primary Mirror 

Overview:

This study, like others related to the mirrors, is split into two parts, that examining the on-axis optical system and that examing the off-axis optical system. The off-axis primary mirror illumination from the sun is constant and uniform; this is one of its prime advantages. The on-axis primary is subject to penumbra from the secondary mirror. This causes an uneven illumination pattern on the primary mirror; one which moves across the mirror as the observatory is repointed. The moving illumination pattern causes a time dependant heat input, making the resulting deflected mirror shape nearly impossible to predict.

On-Axis Illumination Pattern:

The solar flux hitting the HIREX primary mirror was calculated based on the following assumptions:

Pointed at the Solar Center:

The attached figure shows the calculated illumination level normalized for the average, unblocked solar flux.

Pointed off the Center of the Sun:

The pattern remains constant as the telescope is pointed around the sun, but its center moves. When the telescope is pointed at the edge of the sun the pattern center moves 0.16m from the center of the mirror.

Penumbra from Other Sources:

The effects of the spider were examined. Under the assumption the the spider is 0.006m wide, there are places on the mirror which could have their solar flux reduced up to 3%. To avoid this, a 1mm wide secondary support was examined, this reducing the effect to .2%, which was ignored.

Work in Progress:

At present the radiative properties of the primary mirror coating are being measured. This information, coupled to a thermal model of the mirror itself will be used to determine the effect solar flux has on the mirror figure, in both the on-axis and off-axis optical design.

Questions, comments pcheimets@cfa.harvard.edu