... FITS¹

FITS, the ``Flexible Image Transport System'' is the de facto standard for the interchange of astronomical data.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... value.²

It may also be known as the ``invalid'', ``flagged'' or even ``magic'' value, but the meaning is the same.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... package.³

The Starlink application packages CCDPACK (SUN/139) and POLPACK (SUN/223) make extensive use of these facilities.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... structures.⁴

Warning: you should not write hierarchical FITS header items as this violates the FITS standard (although data from outside sources may sometimes contain them).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.