SparsePak IFU / Performance

Overview

A full description of the performance can be found in

SparsePak: A Formatted Fiber Field-Unit for The WIYN Telescope Bench Spectrograph. I. Design, Construction, and Calibration , Bershady, M. A., Andersen, Harker, J., Ramsey, L. W., Verheijen, M. A. W. 2004, PASP, 116, 565 (preprint: [local pdf] [astro-ph/0403456])

and in the forthcoming paper:

SparsePak: A Formatted Fiber Field-Unit for The WIYN Telescope Bench Spectrograph. II. On-Sky Performance, Bershady, M. A., Andersen, D. R., Verheijen, M. A. W., Westfall, K. B., Crawford, S. M., Swaters, R. A. 2003, submitted to ApJSupp

The above results were based on the following measurements made during instrument commissioning:

• Lab test reults on FRD and throughput from 450 nm to 800 nm;
• On-telescope comparisons to Densepak and Red Hydra cables at 515 nm, 660 nm, and 860 nm;
• On-telescope performance at 515 nm, 660 nm, and 860 nm: resolution, vignetting, and sky levels using the (echelle); and
• On-telescope scattered light tests at 5150 and 8650 A (O11 & O6 of the echelle).

A summary of the primary results are presented here.

Fiber transmission (throughput) of 13 of the 82 SparsePak fibers and the SparsePak Reference Cable, measured using the test-bench in our lab, as described in the text and illustrated in Figure 13. Symbols denote different fibers, as identified in the key, and are offset in wavelength within groups for presentation purposes. Light symbols (gray instead of black) are fibers for which the throughput variance across bands was larger than 15%. These fibers either had measurement-log notes indicating unsatisfactory setup of the pin-hole image on the fiber face, or beam-mode measurement variances indicating the lamp stability was poor. As such, these measurements should be viewed as suspect, consistent with the unusually high or low measured transmittance. The large, grey circles are the median of the 8 good fiber measurements. The solid curve is the expected transmittance of 25.4m of Polymicro ultra-low OH- fused-silica fiber, based on Polymicro's figures, combined with two silica-air interfaces (3.43% per interface). The dotted curves represent the normalized broad-band filter transmission, convolved with the CCD response function. From left to right: B, V, R, I. For V,R,I, note the excellent agreement between the measured and expected values, the measurable difference between different fibers. The B-band measurements appear high relative to expectations. The dashed curve curve takes into account the effective band-pass given the expected fiber transmittance. Not taken into account is the spectrum of the light source. The likely effective wavelength of the B-band measurements is 4900A. (Ref: Fig. 5, Bershady et al. 2004, PASP, 116, 565.)

[LEFT] Relative encircled energy (EE) at an output f-ratio of f/6.3 as a function of wavelength for 4 of the SparsePak fibers (labeled) fed with a f/6.3 input beam. These represent four different measures of FRD wavelength dependence for the 500 micron core Polymicro ultra-low OH^- fibers. Error-bars represent estimated observational uncertainties. Wavelength offsets of points are for presentation purposes. (Ref: Fig. 8, Bershady et al. 2004, PASP, 116, 565.)

[RIGHT] Fractional output encircled energy (EE) from fibers as a function of WIYN Bench Spectrograph collimator focal ratio (bottom axis) or collimator focal length (top axis). This assumes a 152mm collimated beam diameter and that all fibers are fed with a f/6.3 beam. The mean SparsePak beam profile (thick, solid line) the range for the 13 measured SparsePak fibers (grey shaded area), and reference cable (thin solid curve) are based on laboratory measurements (cf. Figure 15) using the f/6.3 input beam shown as a thin solid curve with solid circles. Comparable curves, as measured {\it on the telescope} for Densepak (300 micron fibers), and the two Hydra cables (with ``blue,'' 310 micron and ``red,'' 200 micron fibers), are shown for comparison (private communication, P. Smith & C. Conselice; see text for further details). These measurements use the WIYN f/6.3 beam (accounting for the central obstruction -- see text), shown as the thin solid curve with open triangles. The very bottom scale (relative spectral degradation) indicates how the spectral resolution of the Bench would alter (worst case) due to changes in system demagnification as a function of changes in the collimator focal length at fixed camera focal length. The current Bench has a f/6.7 collimator for a 152mm collimated beam. This figure illustrates the effects of FRD on light losses for the Bench Spectrograph, and how optimization trades might be made between throughput and spectral resolution for redesign of the Bench Spectrograph collimator. (Ref: Fig. 10, Bershady et al. 2004, PASP, 116, 565.)

Detailed table is forthcoming with publication of second SparsePak commissioning paper. For now see table under setups.