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I am wondering how to cause a parameter file to look for its value in the header of the image. Specifically I am trying to find out how to make datapars FWHMPSF read its value from the header of the file that is being photted. Unlike exposure, airmass, or filter where I can enter the header field where its value can be found I can't enter anything but a real value for the FWHMPSF.Any ideas would be appreciated.

Short of modifying the code for the task, you can't do this. Some, but not all tasks, have adopted a convention that a '!' in the param says to use the param value as a header keyword to be read for the value, in other cases the param is meant to explicitly refer to a keyword.If you need this behavior, the best way is to write a small wrapper script that uses HSELECT to get the value and then sets the datapars value explicitly.Cheers,
-Mike