[fitsbits] How is NAXISn supposed to work?

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On Mon, 27 Apr 2009, RPEHLM wrote:

> "Arrays of more than one dimension shall consist of a sequence such 
> that the index along axis 1 varies most rapidly, that along axis 2 
> next most rapidly,

if you are familiar with some programming languages, this is the 
"Fortran storage order" (which is different from the "C storage order")

> Does axis 1 equate to NAXIS1 and axis 2 to NAXIS2 etc.?

Yes ... in the sense that NAXIS1 is the number of pixels on axis 1,
NAXIS2 is the number of pixels on axis 2 etc.

> Should the value of NAXIS1 be greater than NAXIS2, which in turn 
> should be greater than NAXIS3 etc.?

NO, there is no implication to order axes by size !

> For example, take a 100 by 200 array, would the following be correct:
>
> NAXIS = 2
> NAXIS1 = 100
> NAXIS2 = 200

For me, which I'm used to the Fortran storage order, this is correct. 
100 pixels along the x axis and 200 pixels along the y axis. If you are 
used to another storage order it could be the other way round.

You should select your programming language, then select the library you 
want to use among those available on the net (or decide you want to 
write your own routines, which for simple cases may be worth), then 
install some common FITS image viewer (e.g. ds9 or fv).

Then you write your first FITS file with NAXIS1 sufficiently different 
from NAXIS2, and containing simple data, e.g. a vertical, horizontal or 
diagonal ramp, and try to visualize it with the viewer.

If you do some messy error it will look a mess (don't panic, it 
happens). If you instead just swap X and Y axes, the image will look 
flipped (most viewers have a way to flip back the data anyhow). Adjust 
your code until it comes out as you expect :-)


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