The effect of pass-bands on broad-band optical photometry of SN1987A

The response functions of the VRI pass-bands in use at CTIO and at SAAO can be reconstructed from the color transformations to the standard system. The CTIO I-band is found to have a much bluer long-wavelength cut-off than does the one used at SAAO. In consequence, the Ca IIλ8600 emission lines, which are very strong in the spectrum of SN1987A, are included in the I measurement at SAAO but are excluded from the CTIO photometry. This is the probable reason for the disagreement of up to 0.3 mag between the values for IC obtained at the two observatories.