One of the most important things scientists need to know when using a spectrograph is precisely how light from different objects and different wavelengths travels through the instrument, and how the instrument changes and moves over the course of a survey. A new device developed at Penn State University adapts techniques from the fiber-optic telecommunications industry to demonstrate a very accurate and precise calibration of the SDSS-III APOGEE spectrograph.
The image shows the three APOGEE camera arrays illuminated with the output from the Fabry-Perot calibration device. The 200nm wavelength range of APOGEE is divided up into three different cameras. The light from each fiber is spread out across a row in this image. When APOGEE is observing the night sky, each row would be the light from a star. If you zoom in on the image you can see the large number of individual peaks that SDSS-III scientists can use to map out all of the details of the APOGEE instrument. (While the APOGEE instrument detects infrared light, we have represented the light in visual wavelengths so that you can see it.)
APOGEE currently uses thorium-argon and uranium-neon lamps for wavelength calibration. These lamps heat up small quantities of their respect elements; the elements in turn emit this energy at specific wavelengths characteristic of those elements. However, the number of lines available is fixed to the properties of the actual elements. Thorium-Argon lamps are commonly used in optical spectroscopy, but don’t have nearly enough bright lines in the near-infrared part of the spectrum to provide a good calibration. Uranium has plenty of lines in the near infrared, but they tend to blend together unless one uses a very high resolution spectrograph. Luckily, the telecommunications industry has developed efficient fibers and light sources in the near-infrared that can be adapted to some of these needs. The near-infrared sensitivity of the APOGEE instrument overlaps some of the standard telecom bands, making it possible to adapt and modify existing technology for use as calibration devices.
A team of scientists and engineers from several SDSS-III institutions recently tested out such a prototype test an entirely new way of calibrating APOGEE. The team brought together experts from several SDSS-III institutions, comprising Suvrath Mahadevan, Sam Halverson (Penn State), Fred Hearty, John Wilson (UVa), Jon Holtzman (NMSU), and Dimitry Bizyaev (APO). Using a Fabry-Perot interferometric cavity, the team was able to generate a precisely-controlled light source that had a stable fixed pattern of peaks. During a three-day run in February this calibration device was tested on the actual APOGEE instrument with great success. The new device, build entirely with commercially available components, consists of a single-mode fiber Fabry-Perot cavity illuminated with a supercontinuum source, and kept very stable with a thermoelectric temperature controller. The output of the cavity results in 400 emission peaks in the APOGEE wavelength regime. This light is fed to APOGEE’s 300 fibers and then passes through the volume-phase holographic grating that disperses the light to separate out different wavelengths onto different places on the camera. In the end, 400 emission peaks times 300 fibers results in 120,000 clearly-defined peaks in the APOGEE focal plane!. These peaks help astronomers to map out many of the important properties of the instrument: the line-spread function, scattered light, persistence effects, point-spread function, as well as providing a grid of stable markers across the focal plane to enable detailed tracking of any overall instrument drift.
The new calibration device is relatively insensitive to vibration and atmospheric pressure changes, and the single mode fiber mitigates issues of collimation and alignment that often affect larger Fabry Perot devices. Ongoing analysis of the dataset acquired over the three-day test is expected to help calibrate many aspects of the APOGEE instrument, including the mosaic Volume Phase Holographic grating at the heart of the APOGEE spectrograph.