Mixing Bounds Expressions

In the last example both axis co-ordinates and pixel indices were mixed in the same subscript expression. In fact, any of the features described earlier may be combined when specifying an NDF section, the only restrictions are as follows.

1. When the shape of the resulting section is expressed in pixel indices, the lower bound must not exceed the upper bound in any dimension.

2. If the bounds for an axis are specified by centre and width values (rather than as lower and upper bounds), then a WCS value should not be used with a pixel index. That is, the centre and width values must both refer to the same co-ordinate system.

Thus, all the following might be used as valid specifications for NDF sections

ndf(3.7)

ndf(,5:)

ndf(-77:01h23m45s,,4)

ndf(66$\sim$9,4:17)

ndf($\sim$5,6$\sim$)

ndf($\sim$,:)

ndf(5500.0$\sim$150,)

ndf(3.0$\sim$1.5,-78.06D-3:13.0545,,,,)

Many other combinations are obviously possible. In cases where some bounds are given in pixel indices and some in WCS co-ordinates, two boxes will be formed; one representing the pixel-index bounds and one representing the WCS bounds. The actual NDF section used will be the overlap of the two boxes. The pixel box will inherit any pixel index limits supplied in the bounds expression, and will use default values for any missing limits. These default pixel-index bounds are just the bounds of the full NDF. Likewise, the WCS box will inherit any WCS limits supplied in the bounds expression, and will use default values for any missing limits. The default WCS limits are the bounds of a box that just includes the whole pixel box.