Description
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 was found to have had a nonlinear response sensitivity after several years of use. 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 irregularities; and (2) provide an external test 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 all-sky absolute photometric calibrations of the instrument on two dates separated by a year. The results from both nights were consistent, 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 for use a “fringe frame” that corrects for the fringing found in all I-band images obtained with ULTRAcam; a reduction pipeline is provided for future photometry conducted with ULTRAcam. For (2) we photometrically followed the “Type II-P” supernova SN 2017eaw, sampling its development from 20 – 133 days post explosion, and find close agreement (mean offsets < 0.01 magnitude) between our resulting light curves and those of a recently published study by Buta & Keel (2019). These tests, corrections, and comparisons all build confidence in the use of ULTRAcam for standard photometry. 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.
