The DESI Fiber View Camera System

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is a 5000 fiber multi-object spectrometer now being installed at the prime focus of the 4 m Mayall telescope at Kitt Peak. Using DESI to measure ∼35 million galaxy redshifts and using the Baryon Acoustic Oscillation (BAO) technique to measure distances, the results will probe the nature of the recently discovered mysterious component of our universe called dark energy. Computer controlled robotic positioners move the 120 μm diameter fibers to positions of galaxies whose location on the sky have been obtained in a previous target selection imaging survey. To achieve good throughput the fibers should be centered on the target position to within 3 μm. The robotic positioners however are only capable of a 50 μm precision on their first move. To achieve the desired precision, the Fiber View Camera (FVC) system has been implemented. The FVC, located near the hole in the primary mirror of the Mayall telescope, has been designed to take an exposure of the focal plane, located at the prime focus some 12 m above the FVC, after the robotic positioners have completed their first move. The FVC is intended to measure the fiber locations with a precision of 3 μm and issue a set of fiber coordinate corrections for the second move correcting the fiber positions by the robotic positioners. Tests show that after two iterations better than 99% of the fibers will be in their intended location to within the desired precision. This paper describes the design of the FVC system, the R&D program preceding the final design, and the tests that have been carried out to demonstrate that the FVC can achieve the required precision.

Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument

“…More details will be published in Silber et al (2022), in preparation. Previous descriptions include Schubnell et al (2016), Leitner et al (2018), Baltay et al (2019), andFagrelius et al (2020). Figure 7 shows the fully assembled FPA, a side view of the focal surface including the tips of most of the 5020 fiber positioners, and an image of the illuminated focal surface obtained with the FVC.…”

“…The key requirements on the FVC system are to return centroid measurements with < 3 µm accuracy with a < 5 second cycle time. Baltay et al (2019) describe the FVC system in detail.…”

“…The FVC system observes the location of the fiber tips and the fiducials through its own lens and the DESI corrector system, and therefore we require accurate knowledge of the distortions of both optical systems to compute the transformation from coordinates on the FVC to the focal plane coordinate system. Through the end of 2019 the FVC had a 600 mm f/4 Canon telephoto lens described in Baltay et al (2019), although like most telephoto lenses it had significant, non-axisymmetric distortions. We ultimately decided to switch to a simpler, achromatic lens in 2020 to minimize distortion.…”

“…These SBIG cameras were chosen because their CCD array and pixel size (3072 × 2048, 9 µm pixels) produced a 28 square arcminute FOV that was comparable to the 25 square arcminute FOV of the GFAs, and with somewhat higher sampling (0.13 /pixel vs. 0.2 /pixel) and better noise properties, although somewhat lower efficiency. The fiducials were identical to those produced by Yale (Baltay et al 2019) for the FPA. The cameras and fiducials were supported by a short, steel cylinder called the CI focal plate assembly.…”

The Continued Relevance of 4m Class Telescopes to Planetary Science in the 2020s

“…These technologies center around remote or automated observing, automated fiber placement, automated optimization of real-time scheduling, and automated data reduction and calibration. Many of these technologies are already in development at some of the current 4m facilities, such as the fiberpositioning technology ongoing for the DESI program (Baltay et al 2019) at the Kitt Peak 4-m, the many telescopes with remote observing and queue observing modes, and the robotic telescopes around the world. However, these are far from optimal or ubiquitous, and support for developments in these areas will be critical to making the most of our 4m telescope resources.…”