The origin of the "wcs-rd2xy" module

[m105...@gm.astro.ncu.edu.tw]

Nice to meet all of you,

I have a question about where the origin (0, 0) of the coordinate given by wcs-rd2xy, the ordinary lower left or the upper left like array in python?

[Dustin Lang]

Per the FITS standard, the first pixel is coordinate (1,1).  FITS does not say whether pixel (1,1) should be displayed as the *bottom*-left or *top*-left.  If you read a FITS image into a python array, array element [0,0] is pixel (1,1).

cheers,
--dustin

[m105...@gm.astro.ncu.edu.tw]

Thanks for your reply!

So, if the result of wcs-rd2xy is (500, 1000), it implies that its position on python output data array is just right at array[1000][500] !?

Dustin Lang於 2018年3月29日星期四 UTC+8上午1時10分08秒寫道：

[Dustin Lang]

Nope, wcs-rd2xy gives 1-indexed coordinates, so (500,1000) would be the python array position a[999,499].

cheers,
--dustin