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Image Descriptors

APM/SuperCOSMOS analysis produces 32 4-byte parameters per detected object. As an example the INT Wide Field Camera survey pipeline produces the following set (which has evolved from the original APM set):

No. 		 Name 		 Description 

1 Seq. no. Running number for ease of reference, in strict order of image detections
2 Isophotal flux Standard definition of summed flux within detection isophote, apart from
detection filter is used to define pixel connectivity and hence which
pixels to include. This helps to reduce edge effects for all isophotally
derived parameters.
3 Total flux An attempt at generating automatically a total flux using a curve-of-growth
technique. The closest description as to how it works is given in Hall &
Mackay (1984). This is better than the isophotal flux in terms of systematic
error, but still not perfect and definitely worse for random error.
4 Core flux Best used if a single number is required to represent the flux for ALL
objects. Basically aperture integration with radius rcore (in the FITS
header) but modified to simultaneously fit `cores' in case of overlapping
5 X co-ord Intensity-weighted centre-of-gravity in X
6 Y co-ord Intensity-weighted centre-of-gravity in Y
7 Gaussian sigma These are derived from the intensity-weighted second moments. The
8 Ellipticity equivalence between them and a generalised elliptical Gaussian distribution
9 Position angle is used to derive Gaussian sigma = $(\sigma_a^2+\sigma_b^2)^{1/2}$
Ellipticity = $1.0-\sigma_a/\sigma_b $
Position angle = angle of ellipse major axis wrt x axis
10 Peak height in counts relative to local value of sky
11 Areal profile 1 Number of pixels above a series of threshold levels relative to local sky.
12 Areal profile 2 Levels are set at T, 2T, 4T, 8T ...128T where T is the threshold. These
13 Areal profile 3 can be thought of as a sort of poor man's radial profile. Note that for
14 Areal profile 4 deblended, ie. overlapping images, only the first areal profile is computed
15 Areal profile 5 and the rest are set to -1 flagging the difficulty of computing accurate
16 Areal profile 6 profiles.
17 Areal profile 7
18 Areal profile 8
19 Core 1 flux A series of different radii core/aperture measures similar to parameter 4.
20 Core 2 flux Together with parameter 10 these give a simple curve-of-growth analysis from
21 Core 3 flux peak pixel, $1/2\times$ rcore, $\surd2\times$ rcore, $2\times$ rcore, $2\surd2\times$ rcore
22 Core 4 flux
23 RA RA and Dec explicitly put in columns for overlay programs that cannot, in
24 Dec general, understand astrometric solution coefficients. Derived exactly from
WCS in header and XY in parameters 5 & 6
25 Classification Flag indicating what image is most probably: -1 stellar, +1 non-stellar, 0 noise
26 Statistic An equivalent N(0,1) measure of how stellar-like an image is, used in
deriving parameter 25 in a `necessary but not sufficient' sense
27 Core 5 flux $4\times$ rcore, extra to ensure $\sim99$% of PSF flux
28 Sky level Interpolated sky level from background tracker
29 Child/parent Flag for parent or part of deblended deconstruct
30 Blank
31 Blank
32 Blank

This will be enhanced to include error estimates for a TBD subset of the parameters.

As an alternative the SDSS archive has a much larger set of parameters per object (see for example - many are error estimates on parameters analogous to the above. Others include different flux measures - eg. Petrosian magnitudes, and list-driven flux measures expressed in asinh magnitudes whereby flux estimates at the same position in all other wavebands are obtained given a significant detection in any one passband. It is proposed that the parameters derived froma WFCAM image in a single passband will be similar to the above INT-WFC set. Input from the WFCAM user community (especially UKIDSS) would be welcomed to help finalise the details of the parameter set, but it is clear that any significant enhancement lies outside the scope of the WFCAM Science Archive as achievable with existing resources and would require additional resources, eg. from AstroGrid or specific science projects.

Over the last few years, provision for user access to large databases has grown from simple position and proximity-based querying capability to highly flexible systems employing `structured query language' (SQL) syntax. Examples of the former are the UK photographic archives at Cambridge (APM) and Edinburgh (SuperCOSMOS):
while an example of the latter is the 2MASS archive:
The most recent large-scale archive, that of the SDSS, employs an extended SQL (SXQL) syntax in a flexible database system (SX) which sits on top of the commercial object-oriented database management system Objectivity; details are available at and see also Thakar, Kunszt & Szalay (2001) and references therein. The object-oriented approach brings many advantages, not the least of which is enabling whole database searches for specific queries to be executed quickly provided the database is indexed (or `tagged') in an appropriate way. A simple example of an SXQL procedure is:
// Select all stars in a given RA range

FROM sxStar
RA() BETWEEN 15 AND 16 && g < 20

where the the select-from-where construct allows the user to choose the parameters to be output, the `class' of object to be searched, and the search criteria to be applied to all images in the searched class.

A mirror site for the SDSS EDR is being established at Edinburgh. Given the manpower effort expended on the state-of-the-art SDSS system and its obvious flexibility, it is proposed that it be employed as the basis for the WFCAM Science Archive. Experiments are underway at Edinburgh with SX/Objectivity; these will include implementation on a Beowulf cluster for enhanced access speed. The general feeling is that SX is likely to become the community-wide standard since much development has already been done on it.

next up previous
Next: References Up: wfcpdr4 Previous: Data Quality Monitoring
Nigel Hambly 2002-10-02