[fitsbits] Re: top or buttom

[Rob Seaman]

Juergen Liesmann <liesmann at t-online.de> inquires:

> Is the first pixel of a fits-image [...] at the top left corner
> or at the buttom left corner?

Neither (or maybe both).  In general this question doesn't mean anything
until you actually do something with the data.  Are we talking about
displaying the image on a computer screen?

FITS is a family of data transfer formats.  Most such questions of
interpretation are purposely outside the bounds of the standard.
Choices of how to display an image (FITS files may also contain
non-imaging data) rest with the observatories and individual
astronomers who are the users of FITS.

This is further complicated due to the support that FITS supplies for
various "world coordinate systems".  The pixels in a FITS image array
may map in an arbitrary fashion onto (for instance) the celestial sphere.
(In general the cameras on the telescopes may have any orientation with
respect to the sky.)

And finally the question becomes - the top left or bottom right corners
of exactly what?  Most astronomical image display tools allows flipping,
rotating or transposing the image axes in addition to zooming and panning
and a wide variety of other transformations.

However, in the case of IRAF usage (IRAF is a widely used astronomical
software package), the default behavior in the most typical usage would
be to interpret a FITS pixel array in normal (in scientific usage anyway)
right-handed Cartesian fashion, i.e. - NAXIS1 corresponds to the X axis,
NAXIS2 to the Y axis, X increases to the right if the image is displayed,
Y increases upward, and the origin is at the bottom left (of a computer
screen, for instance).

On the other hand, I've seen image displays in the past with the origin
placed in each of the four corners, with "X" vertical and "Y" horizontal,
and with the pixels painted onto the screen starting from each of the edges.

The "real-time displays" for modern instruments often fill the image
display window piecewise at the same time from various edges.  Perhaps
some other old FITS hand might even recount some boustrophedonic exploits...

Rob Seaman

-- 
seaman at noao.edu, http://iraf.noao.edu/~seaman
NOAO, 950 N Cherry Ave, Tucson AZ 85719, 520-318-8248
PGP: 98 8D 8B 49 74 9A 41 88  3A 43 87 54 51 BF 30 4B