Characterization of the NPOI Fringe Scanning Stroke
Abstract
We report on the results of an experiment to characterize the fringe scanning stroke on the Navy Prototype Optical Interferometer (NPOI) Fast Delay Line (FDL) strokes. The NPOI uses a heterodyne metrology laser system in its operations. It consists of a HeNe laser with a 2 MHz heterodyne component generated by an Acousto-Optic Modulator (AOM). One polarization is used as the 2 MHz clock, and the other is sent through the feed system twice and bounces off the piezo stroke modulators. We sampled both signals at 50 MHz, and obtained stroke and cart combined motion at the frequency of the stroke modulated 2 MHz heterodyne signal. By counting zero-crossings in the reference and feed system signals, a rough position (to a wavelength) can be obtained. This can be further refined to the few-nanometer level by measuring the relative phases of the reference and feed system signals. This results in approximately 4000 positions measurements per 2 ms stroke with a precision of approximately 1 nm. We recorded stroke positions for approximately 500 strokes (1 s), for all but one of the six FDLs, under a variety of conditions: different stroke amplitudes, different cart speeds, and different cart positions in the FDLs. We then analyzed these data from a total of 100 tests to understand the deviation of the actual stroke from the ideal stroke. We found that the mean stroke differs from the ideal stroke, and that consecutive strokes differ from each other. We computed the effect of the non-ideal stroke on the science data. A non-ideal stroke results in leakage of fringe power between fringe frequencies. This leakage is not significant during most normal operations of the NPOI. However, when the squared visibilities of baselines on the same spectrograph differ by large amounts (a factor of 10), care should be taken. Ideally, high- and low-visibility baselines should be placed on different spectrographs.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 2006
- Accession Number
- ADA477238
Entities
People
- Anders M. Jorgensen
- Dave Mozurkewich
- Donald J. Hutter
- G. C. Gilbreath
- Henrique Schmitt
- J. T. Armstrong
- James Murphy
- Marc Sapantaie
- Robert Hindsley
- Thomas A. Pauls
Organizations
- Los Alamos National Laboratory