Special Note Concerning Some Flux Spectra Obtained during Campaign 73

Special care should be used when comparing some spectra obtained from UTC 2015 June 18 until the end of the observing campaign on UTC 2015 June 22 with spectra from other epochs. Although the sky was typically clear during this period, smoke from wild fires in Arizona and in northern Mexico drifted over Mt. Bigelow. As a result, spectra of several blazars may be found to be redder than on other nights during Campaign 73. The effects of the smoke varies from object to object and is impossible to calibrate out of the flux data given the existing calibration observations. It is recommended that, if possible, spectra obtained during this period be checked against spectra taken earlier in the campaign to determine if you wish to included these data in your analysis. Generally, observations made at higher airmasses will be most affected by the smoke.

Polarization measurements and spectra are unaffected by the smoke. In addition, the differential photometry obtained during these nights is valid. Affected spectra are still useful for the identification of spectral features and for measuring their equivalent widths.