36 research outputs found
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
Using Cosmic Rays detected by HST as Geophysical Markers I: Detection and Characterization of Cosmic Rays
The Hubble Space Telescope (HST) has been operational for over 30 years and
throughout that time it has been bombarded by high energy charged particles
colloquially referred to as cosmic rays. In this paper, we present a
comprehensive study of more than 1.2 billion cosmic rays observed with HST
using a custom written python package, \texttt{HSTcosmicrays}, that is
available to the astronomical community. We analyzed dark calibration
files taken as part of routine calibration programs for five different CCD
imagers with operational coverage of Solar Cycle 23 and 24. We observe the
expected modulation of galactic cosmic rays by solar activity. For the three
imagers with the largest non-uniformity in thickness, we independently confirm
the overall structure produced by fringing analyses by analyzing cosmic ray
strikes across the detector field of view. We analyze STIS/CCD observations
taken as HST crosses over the South Atlantic Anomaly and find a peak cosmic ray
flux of . We find strong evidence for two spatially
confined regions over North America and Australia that exhibit increased cosmic
ray fluxes at the level.Comment: 48 pages, 30 figures, submitted to Ap
Cool white dwarfs as standards for infrared observations
In the era of modern digital sky surveys, uncertainties in the flux of stellar standards are commonly the dominant systematic error in photometric calibration and can often affect the results of higher level experiments. The Hubble Space Telescope (HST) spectrophotometry, which is based on computed model atmospheres for three hot (Teff>30000 K) pure hydrogen (DA) white dwarfs, is currently considered the most reliable and internally consistent flux calibration. However, many next-generation facilities (e.g. Harmoni on E-ELT, Euclid, and JWST) will focus on IR observations, a regime in which white dwarf calibration has not yet been robustly tested. Cool DA white dwarfs have energy distributions that peak close to the optical or near-infrared, do not have shortcomings from UV metal line blanketing, and have a reasonably large sky density (≃4 deg−2 at G < 20), making them, potentially, excellent calibrators. Here, we present a pilot study based on STIS + WFC3 observations of two bright DA white dwarfs to test whether targets cooler than current hot primary standards (Teff<20000 K) are consistent with the HST flux scale. We also test the robustness of white dwarf models in the IR regime from an X-shooter analysis of Paschen lines and by cross-matching our previously derived Gaia white dwarf catalogue with observations obtained with 2MASS, UKIDSS, VHS, and WISE