Special Note Concerning Flux Spectra Obtained during Campaign 110.9

Extreme caution must be used if flux spectra from Campaign 110.9 are used for further analysis and comparison with other sets of data. Spectropolarimetry and spectrophotometry were obtained with a non-standard instrumental configuration of SPOL. The very short Campaign 110.9 was used as a testbed for a 600l/mm diffraction grating with a bluer blaze than the grating normally used. Also, a new camera lens was used that is sensitive down to the atmospheric cutoff at about 3200 Å. As a result, the available polarization and flux spectra span the range of 3300-7550 Å instead of the usual spectral coverage of 4000-7550 Å. To gain sensitivity at the blue end of the spectrum, it was necessary to observe without the L38 order blocking filter. This introduces a 2nd-order overlap at wavelengths longer than 6300-6400 Å. It is found that this contamination does not significantly affect the polarization spectra. However, the flux spectra of objects are affected depending on how blue they are. Corrections for the order overlap tend to be small for most of the blazars observed, leading to insignificant adjustments to the reported broad-band photometric measurements. Corrections for the contaminating 2nd-order light at the red end of the spectra, however, are not perfect and could cause inaccuracies in the flux spectra presented in the data archive, especially with regards to the continuum slope for >6300 Å.

In addition to the order-overlap issue, flux calibration of the spectra turned out to be difficult at the blue end of the spectrum (3300-4100 Å). This is caused largely by a fairly complex grating efficiency function in the blue and the fact that the blue-sensitive lens has a much more restricted range of focus as a function of wavelength than the normal camera lens used for the monitoring program. The best focus was set at 5000-6000 Å, resulting in quite poor focus at both ends of the spectra. This issue will be noted most easily for objects with prominent emission lines falling between 3300 Å and 4500 Å. It is not recommended that measurements of these lines be used in any subsequent analysis of these spectra.