Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

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IRAF is distributed by the National Astronomy Observatory, which is operated by the Associations of Universities for Research in Astronomy, Inc., under the cooperative agreement with National Science Foundation.

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... $U_{\rm 336}$

The light from the undetected stellar population is dominated by light from upper main sequence stars in a narrow colour range; the deviation from the mean colour for the bulk of these stars over the field is expected to be up to a few tenths of a magnitude.

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