Newsletter 12 (2000) 01-04
We describe the first release of data from the SuperCOSMOS Sky Survey programme, the South Galactic Cap survey. This consists of a 3 colour (BRI), one colour (R) at 2 epochs, digital sky survey based on high Galactic latitude (|b|>60o) Schmidt survey plates covering ~5000 square degrees - it is the first digitised sky survey to include both colours and proper motions. Positions are tied to the International Co-ordinate Reference Frame via the Tycho-ACT catalogue and are externally accurate to ~0.3 arcsec; proper motions (also zero-pointed on the extragalactic frame) are typically accurate to ~10 mas yr-1. Photometry in BRI is accurate to ~0.2m and is tied to external CCD zeropoints with field-to-field zeropoint errors minimised using field overlap regions. We describe a simple database interrogation example and show the results. Finally, we describe the future plans for expanding the survey to cover the full southern sky. For full details access the survey homepage on http://www-wfau.roe.ac.uk/sss/.
The database access software consists of a number of separate web forms allowing pixel and/or object catalogue subsets to be extracted. These files are then transfered to the user's home institution for browsing and manipulation. Users can get images with object catalogue data over the same area over areas up to 15 arcmin; in addition users can obtain object catalogue information alone over much larger regions of up to ~10o on a side. The data file formats used are primarily FITS for image data and FITS binary tables for object catalogue data; however object catalogues are also provided in the ESO tab-separated list format (useful for SkyCAT-type applications and remote server communication - see later) and ASCII format for readability. When accessing the database, the user specifies the survey colour (for example BJ or I) of primary interest; image data and parameters for that colour are then provided. Moreover, pairing between different colour/epoch plates on the same field centre has been done using a 3 arcsec proximity criterion. This enables image colour and proper motion information to be given (when available) for any query. If for any reason an object does not appear on a given plate, then null values are returned for that colour. Primary image parameters given include position, proper motion, colour shape and image classification. By comparison with high precision external datasets, we find that position measures are accurate to better than ~0.3 arcsec; proper motions are accurate at typically ~10 mas yr-1 for a median first/second epoch baseline of 15 yr. Colours are accurate to ~0.2m and image classification is >90% reliable down to BJ~20. The above figures for absolute astrometric and photometric accuracy are true, global external errors when measured against external data. Such errors are dominated by positionally dependent systematic errors for well-exposed images. The internal random errors on scales less than ~1o within any particular plate for well exposed images (ie. those more than ~3m above the respective plate limits) are much better than this - eg. stellar positions are relatively accurate to better than ~0.1 arcsec and magnitudes to ~0.05m. The area currently covered by the survey is in the range RA > 22h; RA < 3h; +2.5o > DEC > -67.5o. For full details and much more description see the web pages.
Here we describe a simple database interrogation example. Figure 1 illustrates the results of a query for a BJ image in a rich cluster of galaxies in SGP field 349. To obtain these data, load up http://www-wfau.roe.ac.uk/sss/pixel.html in your hypertext browser and input co-ordinates 0 3 21.1 -35 1 50 into the first box; click the equinox to B1950 and type in 10.0 to the field size. Then simply click on `send'. After a few seconds, a results page will appear indicating that the extracted files are ready to be transferred. Shift-click1 on the FITS button to transfer the FITS image file (which also contains binary table extensions listing parameters of the objects within the image). This FITS image can be input into your favourite image display utility; however we recommend the Starlink utility GAIA since this SkyCAT-based software allows you to `browse' your image data along with catalogue parameter data from the SGC survey (or indeed from other sources). Figure 2 illustrates this. To load up object catalogue data, if your GAIA/SkyCAT version can read FITS tables, then simply input the same FITS image as a `local catalogue' under the `data servers' menu. Alternatively, the pixel extraction software also extracts a catalogue in the ESO/SkyCAT tab-separated ASCII list format - simply transfer this file back in addition to the image, and again load it in as a `local catalogue'. Once catalogue data are loaded in GAIA/SkyCAT, detections are overplotted as ellipses in the image display and you can click on any image to instantly access all the parameterised data, as is shown by the highlighted object in Figures 1 and 2. Conversely, you could click on an image line in the object list to find out which object that it is in the image window. As shown in Figure 2, you have access to astrometric, photometric and image class parameters in a very convenient form. Our web pages also have an object catalogue server at http://www-wfau.roe.ac.uk/sss/obj.html. This allows you to extract object lists alone in plain ASCII, tab-separated ASCII or FITS binary table format over areas up to ~100 square degrees, with many options for selection. For example, using the same co-ordinates as before, the object catalogue web form allows you to extract a 60 arcmin radius circular area in the magnitude range 13 < BJ < 16 for use as fiducials for a 2dF run. When extracting large datasets, we recommend using the compact FITS binary table format and a utility like Starlink's CURSA to browse/manipulate your data.
Our immediate plans are as follows. We aim to finalise the web interrogation system to handle batch-mode queries, large scale (ie. > 100 square degrees) generalised queries and remote server queries from observing tools or web-based image/catalogue manipulation tools such as GAIA/SkyCAT/CURSA. This means that users will be able to use SGC data in a `transparent' way without explicitly accessing the interactive web forms. We will also enhance the online documentation and publish a detailed user guide plus technical description in a refereed journal. Over the medium term, we will be installing a 2 Terabyte disk system - this is enough space to hold data similar to the SGC but for the entire sky at the 20x H-compress factor currently employed. We estimate that we will have finished scanning the entire southern sky in BR by the end of this year and will have completed the full BR(x2)I southern sky by mid-2002; several more years would be needed to do a similar job in the northern hemisphere. Questions and comments concerning the SuperCOSMOS Sky Surveys should be addressed to the authors (email: N.Hambly and M.Read, both AT-roe-DOT-ac-DOT-uk). We would particularly like to encourage any potential users requiring large object catalogue datasets over the full SGC region to contact us directly, specifying their needs.
), 2000-04-07, update: 2006-12-11