Title: Establishing the Photometric Capability of ULTRAcam Through a Study of the Type II-Plateau Supernova, SN 2017eaw

Abstract: The ability to perform precision photometry with a CCD imaging camera mounted on a professional telescope is essential for most astronomical research. The Mt. Laguna Observatory (MLO) 40-inch reflecting telescope has recently undergone an upgrade to its imaging camera, transitioning from CCD447 (“CCD2005”) to E2V Ultracam (“ULTRAcam”). The earlier camera, which was only tested and found to have had a nonlinear response sensitivity (meaning that the number of detected photons did not scale linearly with source brightness) after years of use, provides a cautionary tale. Given this, we seek to: (1) Pro-actively assess the new camera’s photometric properties through all-sky surveys of well-studied photometric “standard stars” on multiple nights, in order to discover and correct any unexpected irregularities; and (2) provide an external test of the verified (and, corrected, as needed) system through the photometric study of a well-observed transient source (a supernova), for which derived values may be compared with those obtained at other observatories.

For (1), we conducted independent, all-sky absolute photometric calibrations of the instrument on two dates separated by nearly a year. The results from both nights were consistent with each other, and revealed no trends with stellar brightness, color, or observational airmass. From these data, we establish the “color terms” for the (BV RCIC) filters and camera system, and find them to be small (less than 0.1), indicating a good match with the standard system. Further, we develop and make available for general use a “fringe frame” that corrects for the significant fringing found in all I-band images obtained with ULTRAcam; a reduction pipeline is provided for use with any future photometry conducted with ULTRAcam. For (2) we photometrically followed the “Type II-P” supernova SN2017eaw, sampling its development from 20 — 133 days post explosion, and find close agreement (mean offsets < 0.01 magnitude in all filters for contemporaneous observations) between our resulting light curves and those of another, recently published, study by Buta & Keel (2019). These tests, corrections, and comparisons all build confidence in the use of ULTRAcam for standard photometry going forward.

Finally, we use our photometry in tandem with spectroscopy obtained through collaboration with the SuperNova SpectroPOLarimetry project (SNSPOL) to derive a Standardized Candle Method (SCM) distance to NGC 6946, the host galaxy of SN 2017eaw. We find a distance of 6.60 +/- 0.59 Mpc, in general agreement with prior distance estimates obtained to this galaxy through other independent techniques.