XMM-Newton Science Analysis System


imrebin (tools-1.68.1) [xmmsas_20230412_1735-21.0.0]

Pixel dimensions

The ratio of output-to-input pixel dimensions are specified by the parameters binxsize and binysize; in addition, the starting row or column can be defined using parameters offsetx and offsety. At present, only integer values are allowed for these four parameters.

The working of these is perhaps best shown by some examples. I'll restrict myself to the x axis for the sake of simplicity; the working in the y dimension is of course identical in character.

All pixel indices are taken to start at 1 not 0.

• Example 1: input is 44 pixels wide, binxsize=1, offsetx=0 (the default). In this case, as one might expect, the output image contains the same number of columns as the input.

• Example 2: input is 51 pixels wide, binxsize=2, offsetx=0 (the default). In this case the output image is 26 pixels wide. The first column of the output image contains columns 1 and 2 of the input image, the second contains 3 and 4, and so forth. The last (26th) column of the output contains just column 51 of the input.

• Example 3: input is 28 pixels wide, binxsize=5 (the present default), offsetx=3. 28 divided by 5, rounded to the next highest integer, equals 6. The offsetx value shifts the input image to the right so that the first output column contains not columns 1 to 5 of the input but only columns 1 and 2; columns 3 to 7 of the input go into column 2 of the output, columns 8 to 12 go into 3, and so forth. However now we see that 6 output columns are not sufficient, because column 6 of the output contains only columns 23 to 27 on the input, leaving the last input column unallocated. Hence the output in this case is broadened to 7 columns, the last containing just column 28 of the input.

As you can see, the dimensions of the output are adjusted in every case to be just large enough that no input columns (or rows) are omitted.

Offset values greater than or equal to the number of input pixel dimensions in the output pixels are not useful since in all cases the output with offsetx=$N$ for example is the same as for offsetx=$N$ modulo binxsize.

As seen in the second and third examples above, in cases in which there is not a perfect alignment between the old and new image boundaries, some `dummy' input pixels can become included in output pixels at the edges of the image: in example 3 for example there are 3 dummy input columns in the first output column and 4 in the last output column. These dummy pixels are not included when the output values are calculated (see section 2.2).