Redshift Maker

The purpose of the Redshift Maker is to derive photometric redshifts from optical imaging data, in this case the INT Wide Field Survey (WFS). The user selects a target RA and Dec coordinate, the WFS database is queried for images containing that position, images are retrieved and a tool SExtractor is run in each of them to create catalogues of objects. The resulting catalogues are federated and then input to the photometric redshift application (HyperZ). The user gets back a catalogue with source positions, photometry and redshifts. See here for a diagram.

How to run the Redshift Maker from the Astrogrid Workbench

The steps below summarise the actions that the user has to perform in order to run the Redshift Maker workflow using the Workbench.  Start the Workbench, click on the Data Analysis tab and click on Science Workflows.

The user then needs to enter user name, password and community. After that the user is presented with a choice of templates from which the Redshift Maker Workflow can be selected.

Parameters

The next widget allows the user to specify the parameters needed to run the workflow.

























First the user has to input the required coordinates (RA and Dec) in decimal degrees (J2000). These coordinates will be used to query the WFS database for images which have covered that position. There are two additional options. The first one allows the user to save the images which will be used in MySpace. Otherwise the images will be retrieved from the data archive and the extraction algorithm performed on-the-fly and the user will not have access to them. This is useful if the user wants to inspect the images with e.g. Aladin and overplot the objects extracted -- this is on by default. Since the Wide Field Camera is a four chip camera, it is also possible to select if one wants to run the workflow for all the chips or only for the chip including the selected position -- this is off by default.
Note that for the workflow to run at the selected position there must be WFS images in all of the following five optical bands: U, g, r, i and Z. See the WFS home page for info on observed fields. This figure shows the coverage of the WFS as derived from the VO Registry entry. This can be inspected from the portal Resources page -- galactic latitude and longitude are shown in magenta and cyan respectively.

Finally click on OK to run the workflow.

Saving your workflow

The workflow is a prepackaged product but crucially can be modified by the user beyond the basic parameter configuration given in the previous section. For example, the user may want to modify the extraction parameters or the photometric redshift galaxy templates. This will be explained below but to do that the user needs to save a copy of the workflow, preferably in their MySpace, as follows. Select a suitable location from MySpace, click on New File (top left icon in the panel) and save the workflow.

Viewing the Job Status

A new window appears specifying that the job has been submitted. After clicking on Ok, a Jobs Monitor page is presented from which one can follow the progress of submitted jobs.

Clicking on the job and selecting View in Portal from the panel or from the drop down menu will take the user to a web page where one can follow in detail which steps have been completed.

Output Files

The output files are saved into MySpace and are the following:

1. Images (if user selected to save them) in votable/intwfs/Run*.fits
2. SExtractor files in votable/sex_?.vot
3. Band merged catalogues in votable/xmatch_*.vot
4. Photometric redshift catalogue (containing all the information from previous files) in votable/zphot*vot

Note that thes files always have the same names regardless of when executed or which coordinates have been used. In other words, they will be overwritten in successive workflow runs.

The files can be read using utilities like Topcat or Aladin (e.g. this flash movie.)

Customising the workflow

If the user has saved the workflow it can be customised as required. In order to do that go to the portal web page, click on Workflows, Open and browse to the saved workflow file. On loading, the user sees the following (without the colours and explanation):

This figure shows the different blocks of the workflow and what each of them does. The first  block is just parameter settings.  In fact the first four Set correspond to the the parameters introduced by the user in the Parameter Editor above. In order to change them just click on them, change their values and click on Update. The next Scripts deal with the connection to the WFS database and image retrieval. The user does not need to modify them. In case the user wants to alter the SExtractor parameters, go to the SExtractor Step and click on it, as shown in the figure below:

The user can input his own configuration and parameter files -- these can be files sitting on MySpace or in a web server. The same applies for the configuration of the photometric redshift tool.

Improvements

A list of improvements and enhancements that will be incorporated into future releases of this service are at RedshiftMakerNext

Changelog

References

• Wide Field Survey at http://www.ast.cam.ac.uk/~wfcsur
• Sextractor info at http://terapix.iap.fr/rubrique.php?id_rubrique=91
• HyperZ info at http://webast.ast.obs-mip.fr/hyperz/ (and as described in "Photometric Redshifts based on standard SED fitting procedures" M. Bolzonella, J.-M. Miralles, R. Pelló, 2000, A&A 363, 476-492)

Author: Eduardo Gonzalez-Solares