We present progress on the stellar surface imaging project recently funded by the U. S. National Science Foun- dation. With the unique layout of the Navy Precision Optical Interferometer (NPOI) in combination with data acquisition and fringe-tracking upgrades we expect to be able to substantially exceed the imaging fidelity and resolution of any other interferometer in operation. The project combines several existing advances and infras- tructure at NPOI with modest enhancements. For optimal imaging there are several requirements that should be fulfilled. The observatory should be capable of measuring visibilities on a wide range of baseline lengths and orientations, providing complete Fourier (UV) coverage in a short period of time. It should measure visibility amplitudes with good SNR on all baselines as critical imaging information is often contained in low-amplitude visibilities. It should measure the visibility phase on all baselines. The technologies which can achieve this are the NPOI Y-shaped array with (nearly) equal spacing between telescopes and an ability for rapid configuration. Placing 6-telescopes in a row makes it possible to measure visibilities into the 4th lobe of the visibility function. By arranging the 12 available telescopes carefully we can switch, every few days, between 6 different 6-station chains which provide symmetric coverage in the Fourier plane without moving any telescopes, only by moving beam relay mirrors. The 6-station chains are important to achieve the highest imaging resolution, and switching rapidly between station chains provides uniform coverage. Coherent integration techniques can be used to obtain good SNR on very small visibilities. Coherently integrated visibilities can be used for imaging with standard radio imaging packages such as AIPS. The commissioning of one additional station, the use of new data acqui- sition hardware and fringe tracking algorithms are the enhancements which are making this project a reality. The New Classic data acquisition system, based on a powerful Stratix FPGA and fast Direct Memory Access module, upgrades the existing Classic beam combiner to allow for continuous data recording across all baselines available with 6 telescopes. It also provides the computing power and software environment necessary for im- plementing the 6-station, 5-baseline fringe-tracking algorithms. In separate papers we discuss the New Classic data acquisition system and the fringe-tracking algorithms in greater detail. In this paper we will focus on an overview of the project. We will describe the observation planning, logistics of the observations, and discuss the current status of the project including preliminary results and simulations of expected future results.
The Navy Precision Optical Interferometer (NPOI) has a station layout which makes it uniquely suited for imaging. Stellar surface imaging requires a variety of baseline lengths and in particular long baselines with resolution much smaller than the diameter of the target star. Because the fringe signal-to-noise ratio (SNR) is generally low on such long baselines, fringe-tracking cannot be carried out on those baselines directly. Instead, baseline bootstrapping must be employed in which the long baseline is composed of a number of connected shorter baselines. When fringes are tracked on all the shorter baselines fringes are also present on the long baseline. For compact sources, such as stellar disks, the shorter baselines generally have higher SNR and making them short enough that the source is unresolved by them is ideal. Thus, the resolution, or number of pixels across a stellar disk, is roughly equal to the ratio of the length of the long baseline to the length of the short baselines. The more bootstrapped baselines, the better the images produced. If there is also a wide wavelength coverage, wavelength bootstrapping can also be used under some circumstances to increase the resolution further. The NPOI is unique in that it allows 6-station, 5-baseline bootstrapping, the most of any currently operating interferometer. Furthermore, the NPOI Classic beam combiner has wavelength coverage from 450 nm to 850 nm. However, until now, this capability has not been fully exploited. The stellar surface imaging project which was recently funded by the National Science Foundation is exploiting this capability. The New Classic data acquisition system, reported separately, is the hardware which delivers the data to the fringe-tracking algorithm. In this paper we report on the development of the fringe-tracking capability with the New Classic data acquisition system. We discuss the design of the fringe tracking algorithm and present performance results from simulations and on sky observation.
The Navy Precision Optical Interferometer (NPOI) has been recording astronomical observations for nearly two decades, at this point with hundreds of thousands of individual observations recorded to date for a total data volume of many terabytes. To make maximum use of the NPOI data it is necessary to organize them in an easily searchable manner and be able to extract essential diagnostic information from the data to allow users to quickly gauge data quality and suitability for a specific science investigation. This sets the motivation for creating a comprehensive database of observation metadata as well as, at least, reduced data products. The NPOI database is implemented in MySQL using standard database tools and interfaces. The use of standard database tools allows us to focus on top-level database and interface implementation and take advantage of standard features such as backup, remote access, mirroring, and complex queries which would otherwise be time-consuming to implement. A website was created in order to give scientists a user friendly interface for searching the database. It allows the user to select various metadata to search for and also allows them to decide how and what results are displayed. This streamlines the searches, making it easier and quicker for scientists to find the information they are looking for. The website has multiple browser and device support. In this paper we present the design of the NPOI database and website, and give examples of its use.
The New Classic data acquisition system is an important portion of a new project of stellar surface imaging with the NPOI, funded by the National Science Foundation, and enables the data acquisition necessary for the project. The NPOI can simultaneously deliver beams from 6 telescopes to the beam combining facility, and in the Classic beam combiner these are combined 4 at a time on 3 separate spectrographs with all 15 possible baselines observed. The Classic data acquisition system is limited to 16 of 32 wavelength channels on two spectrographs and limited to 30 s integrations followed by a pause to ush data. Classic also has some limitations in its fringe-tracking capability. These factors, and the fact that Classic incorporates 1990s technology which cannot be easily replaced are motivation for upgrading the data acquisition system. The New Classic data acquisition system is based around modern electronics, including a high-end Stratix FPGA, a 200 MB/s Direct Memory Access card, and a fast modern Linux computer. These allow for continuous recording of all 96 channels across three spectrographs, increasing the total amount of data recorded by a an estimated order of magnitude. The additional computing power on the data acquisition system also allows for the implementation of more sophisticated fringe-tracking algorithms which are needed for the Stellar Surface Imaging project. In this paper we describe the New Classic system design and implementation, describe the background and motivation for the system as well as show some initial results from using it.