Precision astrometry with SuperCOSMOS

We describe the new, fast, high-precision microdensitometer SuperCOSMOS. Some aspects of hardware and software design that enable high-precision astrometry from photographic plates are explained. We show that the positioning repeatability of the measuring machine is less than 0.1 μ μ m standard error in either coordinate, and the absolute positional accuracy is about 0.15 μ m standard error. Furthermore, measurements of the same plate in different orientations show that the sampling errors are small (e.g. ∼ 0.2 μ m, rising to ∼ 1.0 μ m at the plate limit, for stellar images in a IIIaJ emulsion), thus allowing the extraction of relative positional information from Schmidt plates at accuracies less than 1 μ m. We demonstrate that SuperCOSMOS is capable of measuring the positions of bright stars (i.e. those more than ∼ 4 mag above the plate limit) to a precision ∼ 0.5 μ m with survey–grade photographic plates employing fine–grained emulsions.     

Digital stacking of photographic plates with SuperCOSMOS

Photographic Schmidt plates are among the most effective tools in wide-field astronomy. One of the principal difficulties of photographic plates when compared to modern detectors is lack of image depth. We present a technique for stacking plates digitized using the SuperCOSMOS microdensitometer, aimed at maximizing the signal-to-noise ratio in faint images. The efficacy of several image combination algorithms is tested by stacking plate frames in the presence of spurious images. We find that an 'average sigma clipping' type pixel rejection in conjunction with our weighting scheme is most effective in delivering a clean, high signal-to-noise ratio stack. The gain in limiting magnitude obtained from stacking is found to be consistent with that expected: Δ M ∼1.5 for a stack of 16 good-quality plates.     

The Edinburgh-Cape blue object survey

A brief summary is given of the basic plan and aims of the Edinburgh-Cape Blue Object Survey. We relate some of the difficulties encountered and progress made during the last few years, including the publication of Zone 1 and the discovery of a new class of variable star—the sdB rapid pulsators. The current status of the survey is described.     

Alignment of spiral galaxies in the local supercluster

Evidence is presented indicating the non-random alignment of spiral galaxies in the Local Supercluster. The form of this effect is such that the spin angular momentum vectors of intermediate-type spirals are coherently aligned in space. The results suggest the formation of galaxies in the Local Supercluster according to the fragmentation hypothesis.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Monte-Carlo simulations of galaxy systems

Computer models of galaxy fields are generated using the Monte-Carlo simulation technique described in Paper I (MacGillivrayet al., 1982b). In the models, various scenarios for the true 3-dimensional distribution of galaxies (e.g., clustering on continuous or hierarchical scales) are incorporated and the galaxies projected onto a synthetic photographic plate. All factors affecting the light from distant galaxies are taken into account in the simulations and also factors affecting the detection and measurement of faint galaxy images on photographic plates scanned with an automatic plate-measuring machine. The model fields are compared, both qualitatively and statistically, with a field obtained from measures with COSMOS on a deep photograph taken with the UK Schmidt Telescope. The results indicate that the observed distribution of faint galaxies on photographic plates can only be represented by a model involving second order clustering of galaxies. However, the exact nature of this second-order clustering cannot be unambiguously ascertained from the 2-D distributions, both a uniformly-populated supercluster model and the cellular model being able to represent statistically the observed projected distribution of galaxies.     

The SuperCOSMOS Sky Survey – II. Image detection, parametrization, classification and photometry

In this, the second in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the methods for image detection, parametrization, classification and photometry. We demonstrate the internal and external accuracy of our object parameters. Using examples from the first release of data, the South Galactic Cap survey, we show that our image detection completeness is close to 100 per cent to within ∼1.5 mag of the nominal plate limits. We show that for the B _J survey data, the image classification is externally > 99 per cent reliable to B _J∼19.5 . Internally, the image classification is reliable at a level of > 90 per cent to B _J∼21 , R ∼19 . The photometric accuracy of our data is typically ∼0.3 mag with respect to external data for m >15 . Internally, the relative photometric accuracy in restricted position and magnitude ranges can be as accurate as ∼5 per cent for well-exposed stellar images. Colours are externally accurate to σ _{B − R , R − I} ∼0.07 at B _J∼16.5 , rising to σ _{B − R , R − I} ∼0.16 at B _J∼20 .