LBC Active Optics
Focus and wavefront corrections are performed using two routines: WRS (wavefront reconstruction software) and FPIA (focal plane image analysis). WRS is based on the “Donut” method (Tokovinin and Heathcote, 2006) and is used to remove large aberrations at the beginning of the night or whenever active optics are cleared. The less computationally intensive FPIA is used to refine aberration corrections throughout the night.
These routines use extra-focal pupil images from the science array produced by moving the primary mirrors down the optical axis by -0.8mm. To improve readout and analysis time, only a sub-array of chip 2 is read out (y = [1201:2608], full x range). In the illustration below, the white rectangle near the bottom of chip2 denotes the region that is used for focus and collimation. It was moved to a region that is slightly below the optical axis (marked by the yellow circle labeled ‘target’) after it was noticed that the chips are flatter in this region, and a focus offset is applied to produce best-focus around the optical axis.
An extra-focal image with zero aberration would appear as a perfect image of the telescope entrance pupil. Deviations from this ideal pupil image can be used to describe and correct for aberrations in the primary mirror and LBC corrector:
- Pupil size indicates departure from ideal focus
- Centration of the central obscuration indicates coma
- Ellipticity of the pupil indicates astigmatism
- Relative size of central obscuration indicates spherical aberration
Empirically determined scaling relations are used to apply Zernike corrections (Z4-Z8, Z11 and Z22) to remove the aberrations. How FPIA works is described in more detail in Prime Focus Active Optics with the Large Binocular Telescope.