Testing of the LSST's photometric calibration strategy at the CTIO 0.9 meter telescope

The calibration hardware system of the Large Synoptic Survey Telescope (LSST) is designed to measure two quantities: a telescope's instrumental response and atmospheric transmission, both as a function of wavelength. First of all, a "collimated beam projector" is designed to measure the instrumental response function by projecting monochromatic light through a mask and a collimating optic onto the telescope. During the measurement, the light level is monitored with a NIST-traceable photodiode. This method does not suffer from stray light effects or the reflections (known as ghosting) present when using a flat-field screen illumination, which has a systematic source of uncertainty from uncontrolled reflections. It allows for an independent measurement of the throughput of the telescope's optical train as well as each filter's transmission as a function of position on the primary mirror. Second, CALSPEC stars can be used as calibrated light sources to illuminate the atmosphere and measure its transmission. To measure the atmosphere's transfer function, we use the telescope's imager with a Ronchi grating in place of a filter to configure it as a low resolution slitless spectrograph. In this paper, we describe this calibration strategy, focusing on results from a prototype system at the Cerro Tololo Inter-American Observatory (CTIO) 0.9 meter telescope. We compare the instrumental throughput measurements to nominal values measured using a laboratory spectrophotometer, and we describe measurements of the atmosphere made via CALSPEC standard stars during the same run

Measurement of telescope transmission using a Collimated Beam Projector

With the increasingly large number of type Ia supernova being detected by current-generation survey telescopes, and even more expected with the upcoming Rubin Observatory Legacy Survey of Space and Time, the precision of cosmological measurements will become limited by systematic uncertainties in flux calibration rather than statistical noise. One major source of systematic error in determining SNe Ia color evolution (needed for distance estimation) is uncertainty in telescope transmission, both within and between surveys. We introduce here the Collimated Beam Projector (CBP), which is meant to measure a telescope transmission with collimated light. The collimated beam more closely mimics a stellar wavefront as compared to flat-field based instruments, allowing for more precise handling of systematic errors such as those from ghosting and filter angle-of-incidence dependence. As a proof of concept, we present CBP measurements of the StarDICE prototype telescope, achieving a standard (1 sigma) uncertainty of 3 % on average over the full wavelength range measured with a single beam illumination

Calibration Hardware and Methodology for Large Photometric Surveys

Photometric surveys such as the Dark Energy Survey (DES), the Legacy Survey of Space and Time (LSST), and Pan-STARRS are and will continue to be increasingly large sources of data for the astronomical community. Type Ia supernova (SNe Ia) cosmology in particular stands to make large gains in statistical power for measurements of dark energy, but this increase in statistical power must be matched by a corresponding decrease in systematic uncertainties associated with SNe Ia measurements. Flux calibration stands out as a dominant systematic uncertainty in current-generation SNe Ia cosmology. Determination of atmospheric chromatic variability and variations in instrument throughput contribute heavily to uncertainty in flux calibration. We present two calibration systems built to increase the precision of flux measurements in astronomical surveys, with the ultimate goal of reaching 1 mmag precision. The Collimated Beam Projector (CBP) projects a field of monochromatic ``stars'' of known relative brightness onto the focal plane of a telescope. By performing aperture photometry on the ``stellar'' images and comparing to an internal CBP monitoring photodiode, estimates of the telescope's throughput can be made. We have tested this system on the StarDICE telescope at the Laboratoire de Physique Nucl\'{e}aire et des Hautes \'{E}nergies (LPNHE), and achieved throughput uncertainties at the $\sim2$\% level for 400 nm $0.44$) are less likely to flare than the intermediate population (at the 99.97 \% level). It is posited that the same angular momentum loss mechanism (if it exists) that produces the bimodal population of M dwarf rotators may be responsible for powering flares in intermediate rotators, as they quickly evolve from rapidly to slowly rotating