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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.

Results of the commissioning of the first Gemini Multi‐Object Spectrograph (GMOS) are described. GMOS and the Gemini–North telescope act as a complete system to exploit a large 8 m aperture with improved image quality. Key GMOS design features such as the on‐instrument wave‐front sensor (OIWFS) and active flexure compensation system maintain very high image quality and stability, allowing precision observations of many targets simultaneously while reducing the need for frequent recalibration and reacquisition of targets. In this paper, example observations in imaging, long‐slit, and multiobject spectroscopic modes are presented and verified by comparison with data from the literature. The expected high throughput of GMOS is confirmed from standard star observations; it peaks at about 60% when imaging in the \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $r^{\\prime }$ \\end{document} and \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $i^{\\prime }$ \\end{document} bands, and at 45%–50% in spectroscopic mode at 6300 Å. Deep GMOS photometry in the \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $g^{\\prime }$ \\end{document} , \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $r^{\\prime }$ \\end{document} , and \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $i^{\\prime }$ \\end{document} filters is compared to data from the literature, and the uniformity of this photometry across the GMOS field is verified. The multiobject spectroscopic mode is demonstrated by observations of the galaxy cluster A383. Centering of objects in the multislit mask was achieved to an rms accuracy of 80 mas across the 5 \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $\\farcm$\\end{document} 5 field, and an optimized setup procedure (now in regular use) improves this to better than 50 mas. Stability during these observations was high, as expected: the average shift between object and slit positions was 5.3 mas hr−1, and the wavelength scale drifted by only 0.1 Å hr−1(in a setup with spectral resolution of 6 Å). Finally, the current status of GMOS on Gemini–North is summarized, and future plans are outlined.

Stars appearing in CCD images obtained over 224 nights during the course of 69 observing runs have been calibrated to the Johnson/Kron‐CousinsBVRIphotometric system defined by the equatorial standards of Landolt (1992, AJ, 104, 340). More than 15,000 stars suitable for use as photometric standards have been identified, where “suitable” means that the star has been observed five or more times during photometric conditions and has a standard error of the mean magnitude less than 0.02 mag in at least two of the four bandpasses, and shows no significant evidence of intrinsic variability. Many of these stars are in the same fields as Landolt’s equatorial standards or Graham’s (1982, PASP, 94, 244) southern E‐region standards but are considerably fainter. This enhances the value of those fields for the calibration of photometry obtained with large telescopes. Other standards have been defined in fields containing popular objects of astrophysical interest, such as star clusters and famous galaxies, extending Landolt‐system calibrators to declinations far from the equator and to stars of subsolar chemical abundances. I intend to continue to improve and enlarge this set of photometric standard stars as more observing runs are reduced. The full current database of photometric indices is being made freely available via a site on the World Wide Web or via direct request to the author. Although the contents of the database will evolve in detail, at any given time it should represent the largest sample of preciseBVRIbroadband photometric standards available anywhere.

We present optical photometric and spectral data of the peculiar Type Ic supernova SN 2002ap. Photometric coverage includesUBVRIbands from 2002 January 30, the day after discovery, through 2002 December 12. There are five early‐time spectra and eight in the nebular phase. We determine that SN 2002ap is similar to SN 1997ef and the gamma‐ray burst–associated SN 1998bw with respect to spectral and photometric characteristics. The nebular spectra of SN 2002ap present the largest Mgi] λ4571 to [Oi] λλ6300, 6364 ratio of any supernova spectra yet published, suggesting that the progenitor of SN 2002ap was a highly stripped star. Comparing the nebular spectra of SN 1985F and SN 2002ap, we notice several similar features, casting the classification of SN 1985F as a normal Type Ib supernova in doubt. We also present nebular modeling of SN 2002ap and find that the object ejected ≳1.5M ⊙of material within the outer velocity shell of the nebula (∼5500 km s−1) and synthesized ∼0.09M ⊙of56Ni.

We have created a digital spectral library, using low resolution optical spectra, of photometric and spectral standard stars. The data were acquired using the Cassegrain spectrograph installed on the 1.9 m Radcliffe telescope at the South African Astronomical Observatory. The library consists of optical wavelength (≃3500-7500 Å) spectra for main sequence and giant stars encompassing those most commonly observed in the Galaxy, namely the late-B, A, F, G, K, and early- to mid-M stars. We intend that our standard star spectra will be especially useful for spectral classification of stars in the field and Galactic clusters alike and will have high pedagogic value when included into representative \"Introductory Astronomy\" or \"Stellar Astronomy\" curricula for undergraduate astronomy major and minor programs. We exploit the spectral library in order to derive spectral types for 76 optically and X-ray selected members of the young open cluster NGC 6475. Comparison of spectral type, optical and infrared photometric data to theoretical colors derived from spectral type show that the reddening of the cluster is EBV = 0.068 ± 0.012 (1σ = 0.058), a vector consistent with earlier surveys. Our analysis also highlights the utility of such spectra in rejecting cluster nonmembers, thereby allowing the creation of a clean sample of bona fide cluster members for follow-up science observations.

We have designed, constructed, and tested an optical polarimeter for use with the Virginia Military Institute (VMI) 0.5 m, f/13.5 Cassegrain telescope. Our instrument is based on the common dual-beam design that utilizes a rotatable half-wave plate and Wollaston prism to image starlight onto a CCD detector after it has passed through a broadband filter. The usable field of view is ≲10′′ and the operational range of the instrument is 400-700 nm. Measurements of unpolarized stars demonstrate that the instrumental polarization is ≲0.05%. Observations of seven standard stars were in agreement with their accepted values by an order of Δp(%) ≲ 0.23 for the degree of polarization and Δθ(°) ≲ 0.94 for the position angle.

We describe the flattening of scientific CCD imaging data using a dome flat-field system. The system uses light emitting diodes (LEDs) to illuminate a carefully constructed dome flat-field screen. LEDs have several advantages over more traditional illumination sources: they are available in a wide range of output wavelengths, are inexpensive, have a very long source lifetime, and are straightforward to control digitally. The circular dome screen is made of a material with Lambertian scattering properties that efficiently reflects light of a wide range of wavelengths and incident angles. In this paper, we compare flat fields obtained using this system with two types of traditionally-constructed flat fields: twilight sky flats and nighttime sky flats. Using photometric standard stars as illumination sources, we test the quality of each flat field by applying it to a set of standard star observations. We find that the dome flat-field system produces flat fields that are superior to twilight or nighttime sky flats, particularly for photometric calibration. We note that a ratio of the twilight sky flat to the nighttime sky flat is flat to within the expected uncertainty; but since both of these flat fields are inferior to the dome flat, this common test is not an appropriate metric for testing a flat field. Rather, the only feasible and correct method for determining the appropriateness of a flat field is to use standard stars to measure the reproducibility of known magnitudes across the detector.

We discuss the photometric performance and calibration of the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The stability and accuracy of WFPC2 photometric measurements is discussed, with particular attention given to charge-transfer efficiency (CTE) effects, contamination effects in the ultraviolet (UV), and flat-field accuracy and normalization. Observational data are presented from both WFPC2 observations and ground observations using a system similar to that flown. WFPC2 photometric systems are defined both for the ground and flight systems. Transformations between these systems and the Landolt UBVRI system are presented. These transformations are sensitive to details in the spectra being transformed, and these sensitivities are quantified and discussed. On-orbit observations are used to revise the prelaunch estimates of response curves to best match synthetic photometry results with observations, and the accuracy of the resulting synthetic photometry is discussed. Synthetic photometry is used to determine zero points and transformations for all of the flight filters, and also to derive interstellar extinction values for the WFPC2 system. Using stellar interior and atmosphere models, isochrones in the WFPC2 system are calculated and compared with several observations.

We present CCD observations of 102 Landolt standard stars obtained with the Ritchey‐Chrétien spectrograph on the Cerro Tololo Inter‐American Observatory 1.5 m telescope. Using stellar atmosphere models, we have extended the flux points to our six spectrophotometric secondary standards, in both the blue and the red, allowing us to produce flux‐calibrated spectra that span a wavelength range from 3050 Å to 1.1 μm. Mean differences betweenUBVRIspectrophotometry computed using Bessell’s standard passbands and Landolt’s published photometry were determined to be 1% or less. Observers in both hemispheres will find these spectra useful for flux‐calibrating spectra, and through the use of accurately constructed instrumental passbands, will be able to compute accurate corrections to bring instrumental magnitudes to any desired standard photometric system (S‐corrections). In addition, by combining empirical and modeled spectra of the Sun, Sirius, and Vega, we calculate and compare synthetic photometry to observed photometry taken from the literature for these three stars.