When putting datatable inside an ajax callback, how do you access settings?

dllabs

Due to having dynamic column names I am using datatables inside an ajax callback, rather than using datatables' own ajax feature. (This was advised somewhere on this board, though I can't seem to find it now.)
I cannot figure out how to set settings, such as ""serverSide": true", inside the ajax callback, however.

This works (no settings):

    $.ajax( {
        "url": "get_table.php",
        "type": "POST",
        "data": {
            "s1": series_1,
            "s2": series_2,
            "s3": series_3,
            "t1": topic_1,
            "q1": question_1
        },
        "success": function ( json ) {
            $(document).ready(function() {
                $('#data_table').dataTable(
                     json//<<<<<============================= this works!
                )
            })  
        },
        "dataType": "json"
    } );

but my attempts to use settings produce a surprising error:

    $.ajax( {
        "url": "get_table.php",
        "type": "POST",
        "data": {
            "s1": series_1,
            "s2": series_2,
            "s3": series_3,
            "t1": topic_1,
            "q1": question_1
        },
        "success": function ( json ) {
            $(document).ready(function() {
                $('#data_table').dataTable(
                    {
                        "ajax":json //<<<<<<<========================== this breaks!
                    }
                )
            })  
        },
        "dataType": "json"
    } );

This throws the error Uncaught TypeError: Cannot read property 'aDataSort' of undefined even though, in the working example, datatables didn't look for aDataSort, even though (as far as I can tell) "aDataSort" is deprecated in favour of "orderData", and even though the json data in question actually contains an aDataSort field anyway.

I've tried a few varieties of "ajax":, such as "sAjaxSource": (deprecated?), and got the same error.
How do I do it?

thanks

John