Step-by-Step Guide to extracting IAM catalogue data from the SHSThis page describes the typical steps a user might go through when extracting SuperCOSMOS IAM data using the Catalogue extraction form Get a Catalogue under the SHS web interface to the AAO/UKST H-alpha survey of the Southern Galactic Plane. It is aimed at the first-time user and goes on to describe how the data can be manipulated and interrogated using a database package such as STARLINK's CURSA.
The form allows the user to extract a seamless region across several survey field boundaries. If the requested region of the survey is on-line an IAM catalogue of detected images is returned.
The maximum region possible would include a central field and the overlaps North, South, East and West. Due to the high stellar number density in many Galactic Plane fields and the larger number of IAM parameters available for the SHS IAM data, the total size of such an extracted IAM catalogue could be several hundred Mbytes. So although there is no current limit on the size for a catalogue one should consider the practical limitation on access/extraction speed when downloading IAM data.
The form should ignore all input characters apart from numbers and + or - and replaces them with blanks. i.e if you wanted to enter the RA and Dec as 08:09:32.6 -36:04:55 the form will work but it is really expecting 08 09 32.6 -36 04 55 so to avoid mis-interpretation try and stick to this format. For the l,b co-ordinate option you should always enter values in decimal degrees (e.g. 253.2914 -1.6125)
X-Y region size:
Galactic Plane survey waveband:
Detected object magnitude range: Extract objects fainter than MMM mags and brighter than NNN mags. The default range is 0 (bright limit) to 25.0 (faint limit) which would ensure that all objects detected in the selected region down to the survey limit would be returned.
Epoch of output coordinates: A value of 0.0 will output equatorial positions at the epoch of the H-alpha survey exposure, i.e. the date on which the exposure was taken at the telescope. Hence the returned positions will have been precessed from the selected equinox to the selected epoch.
Format of output catalogue/table: Catalogues can be returned and saved as ascii, tab seperated ascii or fits files according to the selection made. These catalogues can then be input to database manipulation packages such as tables (under iraf) or cursa. This will enable the data to be searched according to selected parameter ranges, plotted, paired with other catalogues based on positional tolerances etc.
spurious images though at this stage we see no need to change the parameters offered unless there is a particular reason for doing so.
Purge repeat/duplicate objects in overlap regions or keep all data: There is a considerable region of overlap in scanned data between adjacent survey fields. Consequently when a catalogue extraction region is specified which crosses an overlap region IAM data will exist from the associated SCOS scans in each field. This option allows the user to retain duplicate objects in overlap regions. This may be useful, for example, for identifying variable objects in these regions depending on the epoch differences between the adjacent exposures.
Output most useful subset of IAM data or output all 32 master IAM parameters: The default selection is to return the most useful set if IAM data. In fact this returns the same set of 31 IAM parameters which accompany SHS images extracted with Get an Image as fits table extensions but with the addition of an epoch value after the RA/DEC coordinates and a survey field identifier at the end and instead of a line identifier at the beginning. This gives a total of 32 rather than the 31 returned IAM parameters associated with the pixel data. Note that the number of IAM parameters selected in the most useful subset category actually only contains 12 independent parameters, the rest constituting equivalent values for the matching SR data and placeholders for the SR1/I data that will also be eventually incorporated.
Selection of IAM parameters from the master waveband will primarily return values associated with the selected waveband, i.e. it will not include all data paired in from the other waveband(s). These IAM parameters include the various moments of the pixel distrubution and image areas associated with each areal profile level etc. The lists of IAM parameters provided from each selection are given here.
Blended image option Blended images will be a particular feature of IAM data in the Galactic Plane survey due to the very crowded nature of star fields in much of the survey region. This option allows specific control for how these blended images are dealt with. There are 4 options available: Discard parent images of deblends Discard child images of deblends Include parent and child images of deblends Keep only unblended images (ie. discard parents & children)
Image quality threshold Images having quality flag greater than the threshold value entered will be discarded. e.g. if 99999999 is entered this will keep all images regardless of the value of the quality flag. The lower this integer value the higher the image quality. Use a threshold of 127 (highest quality) for the "best" catalogue. Other values for this parameter are (from Hambly et al 2001):
Notes: Unlike IAM data in the SSS there is no colour correction option to select. This option has been disabled due to the natire of the data in the Galactic Plane where the effects of variable extinction cloud the issue. Matches between object sin overlap regions in different IAM data sets are performed with a pairing radius of 3arcseconds.
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