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Tips and Grouping of Region Functions

The data subspace in some cases will produce more compact subspace specifications than in others. For range filters, it is always better to use the “in” operator than to specify ranges using $>$, $<$, etc. Though the two examples below are mathematically equivalent, the second example results in a more compact subspace specification than the first.

$\displaystyle RAWX < 50\ \&\&\ RAWX >25 \qquad \textrm{allowed, but discouraged} $

$\displaystyle RAWX\ in\ (25:50) \qquad \textrm{allowed} $

There is also a special subtlety with regards to the grouping of region function filters. When the parser sees the expression below,

$\displaystyle (A,B)\ IN\ ellipse(100, 200, 30, 40, 0)\ \&\&\ (A, B)\ in\ polygon(100,200,1,1) \qquad \textrm{allowed} $

it tries to combine these into one filter with two regions. If the cooridate specifications do not match, then it returns an error.

$\displaystyle (A,B)\ IN\ ellipse(100, 200, 30, 40, 0)\ \&\&\ (A, BX)\ in\ polygon(100,200,1,1) \qquad \textrm{NOT allowed} $

If, however, the region functions are isolated in parentheses, then the parser will put them in seperate filters. Thus the following expression is allowed.

$\displaystyle ( (A,B)\ IN\ ellipse(100, 200, 30, 40, 0) )\ \&\&\ ( (A, BX)\ in\ polygon(100,200,1,1) ) \qquad \textrm{allowed} $

Note that putting in the parentheses unnecessarily will result in a much less compact data subspace. The following expression is equivalent to the first one in this section, but will result in the creation of two subspace filters, instead of one.

$\displaystyle ( (A,B)\ IN\ ellipse(100, 200, 30, 40, 0) )\ \&\&\ ( (A, B)\ in\ polygon(100,200,1,1) ) \qquad \textrm{allowed, but discouraged} $

XMM-Newton SOC -- 2021-11-30