Dark Energy Camera (DECam) is a new prime focus, wide-field imager for the V. M. Blanco 4-m telescope at Cerro Tololo Inter-American Observatory (CTIO) and is the science instrument for the Dark Energy Survey Collaboration (DES). The innstrument includes a large, five-lens optical corrector mounted on a hexapod mechanism for fine alignment, filters, and a 520 Megapixel focal plane; all integrated in a new cage structure identical to the original telescope prime focus assembly. Installation of this 8,600 kg instrument requires the removal of the primary mirror from the telescope, the removal of the old prime focus cage, the new integration of new cage, and fine adjustment of large, over-contrained mechanism followed by telescope reassembly. A large facility shutdown was scheduled for this upgrade and several tools, fixtures, monitoring systems and procedures were developed in order to identify and then recover the optical alignment of the telescope, to control the distribution of stresses during the tuning, and to maintain the balance of the telescope with significant added mass. The final goal has been to keep the high performance of the telescope for both the existing f/8 Ritchey-Chretien mounted instruments and the new DECam now in commissioning, Blanco allowed a flip of prime focus structure so that the f/8 secondary mirror, mounted on the back of the cage, points towards the primary mirror for Ritchey-Chretien observations. DECam will maintain this capability by attaching the existing f/8 mirror cell to the front of the new cage when not observing with prime focus. The challenges presented in handling large elements, real-time monitoring, alignment, verification and feedback are described.
