Table of contents

The measurement of precise absolute fluxes for stellar sources has been pursued with increased vigor since the discovery of dark energy and the realization that its detailed understanding requires accurate spectral energy distributions (SEDs) of redshifted Ia supernovae in the rest frame. The flux distributions of spectrophotometric standard stars were initially derived from the comparison of stars to laboratory sources of known flux but are now mostly based on calculated model atmospheres. For example, pure hydrogen white dwarf (WD) models provide the basis for the HST CALSPEC archive of flux standards. The basic equations for quantitative spectrophotometry and photometry are explained in detail. Several historical lab-based flux calibrations are reviewed; and the SEDs of stars in the major online astronomical databases are compared to the CALSPEC reference standard spectrophotometry. There is good evidence that relative fluxes from the visible to the near-IR wavelength of ∼2.5 μm are currently accurate to 1% for the primary reference standards, and new comparisons with lab flux standards show promise for improving that precision.

We present new BVI photometry containing 40,000 stars of the Galactic globular cluster M15 (NGC 7078), covering a 25^' × 25^' area centered on the cluster with a magnitude range from the tip of the red giant branch to 3 mag below the main sequence turnoff. Using α-enhanced Dartmouth Stellar Evolution Program models, we find an age of 13.0 ± 1.0 Gyr and distance modulus of (m - M)_V = 15.4 ± 0.1 through isochrone fitting. Unlike previous works, we find good agreement between the observed completeness-corrected stellar luminosity function and models.

ESA and NASA recently selected two 5 μm cutoff Teledyne H2RG sensor chip assemblies (SCA) for flight on the James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec). These HgCdTe SCAs incorporate Teledyne's "improved barrier layer" design that eliminates the degradation that affected earlier JWST H2RGs. The better indium barrier, together with other design changes that Teledyne phased in from other programs over the years, has improved the performance and reliability of JWST's SCAs. In this article, we describe the measured performance characteristics that most directly affect scientific observations including read noise, total noise, dark current, quantum efficiency (QE), and image persistence. As part of measuring QE, we inferred the quantum yield over the full NIRSpec pass band of λ = 0.6–5 μm and found that it exceeds unity for photon energies E_γ > (2.65 ± .2)E_g, where E_g is the HgCdTe bandgap energy. This corresponds to λ ≲ 2 μm for NIRSpec's 5 μm cutoff HgCdTe.

Photogenerated charge in thick, back-illuminated, fully-depleted CCDs is transported by electric fields from the silicon substrate to the collecting well at the front gate of the CCDs. However, electric fields transverse to the surface of the CCD—with diverse origins such as doping gradients, guard rings around the imaging area of the sensor, and physical stresses on the silicon lattice—displace this charge, effectively modifying the pixel area and producing noticeable signals in astrometric and photometric measurements. We use data from the science verification period of the Dark Energy Survey (DES) to characterize these effects in the Dark Energy Camera (DECam) CCDs. The transverse fields mainly manifest as concentric rings (''tree rings'') and bright stripes near the boundaries of the detectors (''edge distortions'') with relative amplitudes of about 1% and 10% in the flat-field images, respectively. Their nature as pixel size variations is confirmed by comparing their photometric and astrometric signatures. Using flat-field images from DECam, we derive templates in the five DES photometric bands (grizY) for the tree rings and the edge distortions as a function of their position in each DECam detector. These templates can be directly incorporated into the derivation of photometric and astrometric solutions, helping to meet the DES photometric and astrometric requirements.

We have designed, manufactured, and characterized an 8-bit 5 Giga samples per second (Gsps) ADC printed circuit board assembly (PCBA). An e2v EV8AQ160 ADC chip was used in the design and the board is plug compatible with the field programmable gate array (FPGA) board developed by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) community. Astronomical interference fringes were demonstrated across a single baseline pair of antennas using two ADC boards on the Yuan Tseh Lee Array for Microwave Background Anisotropy (AMiBA) telescope. Several radio interferometers are using this board for bandwidth expansion, such as Submillimeter Array; also, several experimental telescopes are building new spectrometers using the same board. The ADC boards were attached directly to the Reconfigurable Open Architecture Computing Hardware (ROACH-2) FPGA board for processing of the digital output signals. This ADC board provides the capability of digitizing radio frequency signals from DC to 2 GHz (3 dB bandwidth), and to an extended bandwidth of 2.5 GHz (5 dB) with derated performance. The following worst-case performance parameters were obtained over 2 GHz: spur free dynamic range (SFDR) of 44 dB, signal-to-noise and distortion (SINAD) of 35 dB, and effective number of bits (ENOB) of 5.5.

This paper describes the advantages and challenges of applying microelectromechanical accelerometer systems (MEMS accelerometers) in order to attain precise, accurate, and stateless positioning of telescope mounts. This provides a completely independent method from other forms of electronic, optical, mechanical or magnetic feedback or real-time astrometry. Our goal is to reach the subarcminute range which is considerably smaller than the field-of-view of conventional imaging telescope systems. Here we present how this subarcminute accuracy can be achieved with very cheap MEMS sensors and we also detail how our procedures can be extended in order to attain even finer measurements. In addition, our paper discusses how can a complete system design be implemented in order to be a part of a telescope control system.

Astronomy is undergoing a methodological revolution triggered by an unprecedented wealth of complex and accurate data. The new panchromatic, synoptic sky surveys require advanced tools for discovering patterns and trends hidden behind data which are both complex and of high dimensionality. We present DAMEWARE (DAta Mining & Exploration Web Application REsource): a general purpose, web-based, distributed data mining environment developed for the exploration of large data sets, and finely tuned for astronomical applications. By means of graphical user interfaces, it allows the user to perform classification, regression, or clustering tasks with machine learning methods. Salient features of DAMEWARE include its ability to work on large datasets with minimal human intervention, and to deal with a wide variety of real problems such as the classification of globular clusters in the galaxy NGC1399; the evaluation of photometric redshifts; and, finally, the identification of candidate Active Galactic Nuclei in multiband photometric surveys. In all these applications, DAMEWARE allowed us to achieve better results than those attained with more traditional methods. With the aim of providing potential users with all needed information, in this paper we briefly describe the technological background of DAMEWARE, give a short introduction to some relevant aspects of data mining, followed by a summary of some science cases and, finally, provide a detailed description of a template use case.

In this paper, we use the Cramér-Rao lower uncertainty bound to estimate the maximum precision that could be achieved on the joint simultaneous (or two-dimensional) estimation of photometry and astrometry of a point source measured by a linear CCD detector array. We develop exact expressions for the Fisher matrix elements required to compute the Cramér-Rao bound in the case of a source with a Gaussian light profile. From these expressions, we predict the behavior of the Cramér-Rao astrometric and photometric precision as a function of the signal and the noise of the observations, and compare them to actual observations—finding a good correspondence between them. From the Cramér-Rao bound, we obtain the well-known fact that the uncertainty in flux on a Poisson-driven detector, such as a CCD, goes approximately as the square root of the flux. However, more generally, higher-order correction factors that depend on the ratio B/F or F/B (where B is the background flux per pixel, and F is the total flux of the source), as well as on the properties of the detector (pixel size) and the source (width of the light profile), are required for a proper calculation of the minimum expected uncertainty bound in flux. Overall, the Cramér-Rao bound predicts that the uncertainty in magnitude goes as (S/N)^-1 under a broad range of circumstances. As for the astrometry, we show that its Cramér-Rao bound also goes as (S/N)^-1 but, additionally, we find that this bound is quite sensitive to the value of the background—suppressing the background can greatly enhance the astrometric accuracy. We present a systematic analysis of the elements of the Fisher matrix in the case when the detector adequately samples the source (oversampling regime), leading to closed-form analytical expressions for the Cramér-Rao bound. We show that, in this regime, the joint parametric determinations of photometry and astrometry for the source become decoupled from each other, and furthermore, it is possible to write down expressions (approximate to first order in the small quantities F/B or B/F) for the expected minimum uncertainty in flux and position. These expressions are shown to be quite resilient to the oversampling condition, and become thus very valuable benchmark tools to estimate the approximate behavior of the maximum photometric and astrometric precision attainable under prespecified observing conditions and detector properties.