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Z. Ninkov, R. W. Slawson, D. A. McInnis, K. Hoheusle (C. I. S., R. I. T.)
We have been experimenting with using Tunable Liquid-Crystal Filters for the purposes of obtaining low-resolution (R~ 50--100) spectro\-photometric scans of stars in crowded fields. The LCTF's are Lyot polarization-interference filters consisting of a stack of retarding elements each of which contains a liquid-crystal waveplate whose retardance is electronically adjustable. Our procedure is to obtain a series of images of a star cluster as the filter is stepped through its range and then use PSF fitting to extract relative fluxes for stars in the field. Calibration involves correcting the observed fluxes for one or more stars in the field that have known spectral types to the intrinsic fluxes of stars of the same spectral type in a spectrophotometric atlas. Those corrections are then applied to the other stars in the field.
Here we discuss our experiences with applying this technique to a stack of images of NGC 4755, the "Jewel Box" cluster. As well, we present results of experiments to test how accurately spectral types can be determined from these low-resolution spectra considering the limitations imposed by spectral resolution, observational noise, and differential reddenning.