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C. D. Mackay

Publications and source records attributed to C. D. Mackay.

11 recordsLinked to original sources
arXiv2008

Getting Lucky with Adaptive Optics: Fast AO Image Selection in the Visible with a Large Telescope

We describe the results from a new instrument which combines Lucky Imaging and Adaptive Optics to give the first routine direct diffraction-limited imaging in the visible on a 5m telescope. With fast image selection behind the Palomar AO system we obtained Strehl ratios of 5-20% at 700 nm in a typical range of seeing conditions, with a median Strehl of approximately 12% when 10% of the input frames are selected. At wavelengths around 700 nm the system gave diffraction-limited 35 milliarcsecond FWHMs. At 950 nm the output Strehl ratio was as high as 36% and at 500 nm the FWHM resolution was as small as 42 milliarcseconds, with a low Strehl ratio but resolution improved by factor of ~20 compared to the prevailing seeing. To obtain wider fields we also used multiple Lucky-Imaging guide stars in a configuration similar to a ground layer adaptive optics system. With eight guide stars but very undersampled data we obtained 300 milliarcsecond resolution across a 30X30 arcsec field of view in i' band.

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arXiv2007

The LuckyCam Survey for Very Low Mass Binaries II: 13 new M4.5-M6.0 Binaries

We present results from a high-angular-resolution survey of 78 very low mass (VLM) binary systems with 6.0 <= V-K colour <= 7.5 and proper motion >= 0.15 arcsec/yr. 21 VLM binaries were detected, 13 of them new discoveries. The new binary systems range in separation between 0.18 arcsec and 1.3 arcsec. The distance-corrected binary fraction is 13.5% (+6.5%/-4%), in agreement with previous results. 9 of the new binary systems have orbital radii > 10 AU, including a new wide VLM binary with 27 AU projected orbital separation. One of the new systems forms two components of a 2300 AU separation triple system. We find that the orbital radius distribution of the binaries with V-K < 6.5 in this survey appears to be different from that of redder (lower-mass) objects, suggesting a possible rapid change in the orbital radius distribution at around the M5 spectral type. The target sample was also selected to investigate X-ray activity among VLM binaries. There is no detectable correlation between excess X-Ray emission and the frequency and binary properties of the VLM systems.

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arXiv2006

Discovery of Five Very Low Mass Close Binaries, Resolved in the Visible with Lucky Imaging

We survey a sample of 32 M5-M8 stars with distance < 40pc for companions with separations between 0.1 arcsec and 1.5 arcsec and with Delta m_i <5. We find five new VLM binaries with separations between 0.15 arcsec and 1.1 arcsec, including a candidate brown dwarf companion. The raw binary fraction is 16% +8/-4% and the distance bias corrected fraction is 7% +7/-3%, for companions within the surveyed range. No systems with contrast ratio Delta m_i>1 were found, even though our survey is sensitive to up to Delta m=5 (well into the brown dwarf regime). The distribution of orbital radii is in broad agreement with previous results, with most systems at 1-5AU, but one detected binary is very wide at 46.8+/-5.0AU. We also serendipitously imaged for the first time a companion to Ross 530, a metal-poor single-lined spectroscopic binary. We used the new Lucky Imaging system LuckyCam on the 2.5m Nordic Optical Telescope to complete the 32 very low mass star i' and z' survey in only 5 hours of telescope time.

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arXiv2003

Photon counting strategies with low light level CCDs

Low light level charge coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy. We achieve this by optimising a transfer function which translates the digitised output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on photon counting accuracy and effective signal-to-noise. We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about twenty photons per pixel and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio.

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arXiv2002

The Las Campanas IR Survey. II. Photometric redshifts, comparison with models and clustering evolution

The LCIR Survey, using the Cambridge IR Survey Instrument (CIRSI), reaches H \~ 20-21 over ~1 deg^2. We present results for 744 arcmin^2 in which public UBVRI data exist. We compare optical-IR colours with predictions of a semi-analytic hierarchical model (SAM) and find reasonable agreement. We also determine photometric redshifts, finding a median z of z_m ~ 0.55. We compare N(z) of different spectral types with models, showing that the observations are inconsistent with simple PLE models while the SAM provides a reasonable fit to the total N(z) but underestimates the number of z ~ 1 red spectral types. We also present N(z) for samples of red objects (EROs). We find that EROs with R - H > 4 and H < 20.5 have z_m ~ 1; redder EROs have higher z_m. For 19 < H < 20, EROs with R - H > 4 comprise ~18% of the observed galaxy population while in the SAM they contribute only ~4%. We also determine the angular correlation function w(theta) for magnitude, colour, spectral type and photo-z selected samples and use the estimated N(z) to derive the spatial clustering xi(r). Parametrizing xi(r) by xi(r_c,z)=(r_c/r_*(z))^(-1.8) (r_c comoving), we find that r_*(z) increases by ~1.5-2 times from z = 0 to z ~ 1.2. We interpret this as a selection effect - the galaxies selected at z ~ 1.2 are intrinsically very luminous. When galaxies are selected by absolute magnitude we find no evidence for evolution in r_* over this z range. Extrapolated to z = 0, we find r_*(0) ~ 6.5 h^-1Mpc for red galaxies and r_*(0) ~ 2-4 h^-1Mpc for blue galaxies. We also find that while the angular clustering amplitude of EROs with R - H > 4 or I - H > 3 is up to four times that of the whole galaxy population, the spatial clustering length r_*(z=1) is ~7.5-10.5 h^-1Mpc which is only a factor of ~1.7 times r_* for non-EROs lying in a similar z and luminosity range.

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arXiv2002

Diffraction-limited CCD imaging with faint reference stars

By selecting short exposure images taken using a CCD with negligible readout noise we obtained essentially diffraction-limited 810 nm images of faint objects using nearby reference stars brighter than I=16 at a 2.56 m telescope. The FWHM of the isoplanatic patch for the technique is found to be 50 arcseconds, providing ~20% sky coverage around suitable reference stars.

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arXiv2001

The Las Campanas Infrared Survey. III. The H-band Imaging Survey and the Near-Infrared and Optical Photometric Catalogs

(Abridged) The Las Campanas Infrared Survey, based on broad-band optical and near-infrared photometry, is designed to robustly identify a statistically significant and representative sample of evolved galaxies at redshifts z>1. We have completed an H-band imaging survey over 1.1 square degrees of sky in six separate fields. The average 5 sigma detection limit in a four arcsecond diameter aperture is H ~ 20. Here we describe the design of the survey, the observation strategies, data reduction techniques, and object identification procedures. We present sample near-infrared and optical photometric catalogs for objects identified in two survey fields. We perform object detection in all bandpasses and identify ~ 54,000 galaxies over 1,408 square arcminutes of sky in the two fields. Of these galaxies, ~ 14,000 are detected in the H-band and ~ 2,000 have the colors of evolved galaxies, I - H >3, at z > 1. We find that (1) the differential number counts N(m) for the H-band detected objects has a slope of 0.44 at H < 19 and 0.27 for H > 19. In addition, we find that (2) the differential number counts for the H detected red objects has a slope of 0.85 at H < 20 and 0.32 at H > 20, with a mean surface density ~ 3,000 degree^{-2} mag^{-1} at H=20. Finally, we find that (3) galaxies with red optical to near-IR colors (I-H > 3) constitute ~ 20% of the H detected galaxies at H ~ 21, but only 2% at H = 19. We show that red galaxies are strongly clustered, which results in a strong field to field variation in their surface density. Comparisons of observations and predictions based on various formation scenarios indicate that these red galaxies are consistent with mildly evolving early-type galaxies at z ~ 1, although with a significant amount of on-going star formation as indicated by the large scatter in their V-I colors.

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arXiv2001

Sub-electron read noise at MHz pixel rates

A radically new CCD development by Marconi Applied Technologies has enabled substantial internal gain within the CCD before the signal reaches the output amplifier. With reasonably high gain, sub-electron readout noise levels are achieved even at MHz pixel rates. This paper reports a detailed assessment of these devices, including novel methods of measuring their properties when operated at peak mean signal levels well below one electron per pixel. The devices are shown to be photon shot noise limited at essentially all light levels below saturation. Even at the lowest signal levels the charge transfer efficiency is good. The conclusion is that these new devices have radically changed the balance in the perpetual trade-off between readout noise and the speed of readout. They will force a re-evaluation of camera technologies and imaging strategies to enable the maximum benefit to be gained from these high-speed, essentially noiseless readout devices. This new LLLCCD technology, in conjunction with thinning (backside illumination) should provide detectors which will be very close indeed to being theoretically perfect.

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arXiv1995

Optical structure and polarization of the western hot spot of Pictor A

HST images of the western hot spot of the nearby FRII radio galaxy Pictor A resolve the hot spot into an amorphous region of highly polarized wisps. The high polarization (>~ 50%) indicates that the optical emission is synchrotron radiation and that the magnetic field structure has been well resolved. The magnetic field is oriented nearly perpendicular to the jet axis and apparently very uniform. We interpret these observations in terms of a compressed magnetic field at the working surface where the jet impinges on the intergalactic medium some 100 kpc from the nucleus of the host galaxy. Keywords: Polarization --- radiation mechanisms: nonthermal --- galaxies: active --- galaxies: individual: Pictor A --- galaxies: jets.

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arXiv1995

HST polarization map of the ultraviolet emission from the outer jet in M87 and a comparison with the 2cm radio emission

We present the first high resolution polarization map of the ultraviolet emission from the outer jet in M87. The data were obtained by the Faint Object Camera (FOC) on the Hubble Space Telescope. The polarization map has a resolution of 0.2 arcsec and was derived using data from three linearly polarized images combined with the best available calibration data. The ultraviolet emission is highly polarized (~40\%) with the magnetic vector aligned roughly with the jet axis, except in the region of the brightest knot (Knot A) where the position angle changes abruptly and the magnetic vector becomes perpendicular to the jet axis. A similar behaviour is seen in the 2cm VLA radio polarization map. By aligning the FOC and VLA data, we present ultraviolet--2cm spectral index, depolarization and rotation measure maps. We identify a region of high depolarization adjacent to Knot A. This is the first direct observational evidence that indicates the presence of a cloud or filament of dense thermal material which is mixed with the synchrotron emitting plasma of the jet. The interaction of the jet with this cloud is likely to be responsible for the sudden increase in the brightness of the jet at Knot A due to an induced shock. We suggest that the dark line seen in the 2cm radio data between Knot A and Knot C could be the shadow or magnetotail of the depolarizing cloud in the jet.

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