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Step-by-Step Guide to extracting IAM catalogue data from the SHS

This 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.

  • Step 1 - assuming you already know the equatorial co-ordinates in RA (hours minutes seconds) and Dec (degrees, minutes, seconds) of your region of interest (for either equinox B1950 or J2000) or you know the Galactic l,b co-ordinates (in decimal degrees) you can proceed directly to the form located at ./haobj.html. Packages such as coco allow you to convert between various equinoxes and coordinate systems (galactic, ecliptic, equatorial etc).

  • Step 2 - fill in the on-line form using the boxes and buttons provided. There are three sections to the form dealing with:

    The various options for each section are described here

    Once the form is filled in click on "send". An example of a filled-in catalogue extraction form as a screen capture is given here.

  • Step 3 - if the requested part of the plane is up on disk you should see a results page something like this.

    If the scanned data has not yet been put on-line or you have requested a region outwith the survey area then the form should return an "error" message. Similarly if the form cannot handle the data you have entered (eg if the size of the extracted IAM catalogue data is excessively large) then you will be informed. If you think you have entered sensible parameters and the chart of scanned data (located here) indicate the survey field has been put on-line then please email with the search parameters used and the messages returned.

  • Step 4 - To save the selected catalogue format data file to your local disk space click (in Netscape use shift + mouse button 1, in Internet Explorer use the right mouse button + `Save target as') on the `FITS.gz' button. Some browsers are configured so that the FITS file is automatically uncompressed on download. If the file is saved in the compressed format then use the Unix command gunzip to uncompress it. (PC/Windows users can use packages such as Winzip to uncompress files)

Form options

Search Parameters

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)

Coordinate system:
Select `J2000' (default) or `B1950' equinox from the menu if using RA/Dec or `Galactic' if using l,b coordinates

X-Y region size:
Enter values for the size of the rectangular region to be extracted in X and Y in arcminutes; If Y=0 then a circular extraction is performed with radius=X. A seamless catalogue across several survey field boundaries can be created.

Galactic Plane survey waveband:
Select either the H-alpha or matching SR survey from which you wish to extract the IAM data. The on-line availability of the Galactic Plane H-alpha survey fields can be found under Galactic Plane coverage.

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.

Output Parameters

Equinox of output coordinates:
Regardelss of the equinox of the position used to define the centre of the extracted IAM catalogue region the output RA/DEC co-ordinates can be returned in either B1950 or J2000. Note that l,b co-ordinates are always also returned for SHS IAM data.

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.

Expert Parameters

The choices in this section enable more tailored selection of some of the IAM parameters and are designed for more advanced users. The default values offered have been found to return catalogues which generally contain a minimum number of spurious sources based on empirical results with previous SuperCOSMOS surveys. More experimentation may prove beneficial when dealing with the SHS data due to the increased level of 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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