Part of the WCS proposal in Paper I is a new set of keywords known as
WCSNAMEa. These keywords provide a name for
each of the 26 + 1 alternate WCS representations. The purpose for
this name is to permit a user of a FITS file to choose the coordinate
system(s) best suited to the needs of particular application(s).
'A', and similarly for all 26 + 1 alternate and primary versions of WCS. As a corollary I propose that explicitly given values of
WCSNAMEa be required to be at least two characters in length.
CTYPE1 = 'AMPLIFIER_X'
CTYPE2 = 'AMPLIFIER_Y'
A CCD may have more than one amplifier. In such a case there is one amplifier WCS for each amplifier on the CCD. Each WCS will typically have integral offsets from the others, and their relative orientations may be flipped or even transposed.
The pixel coordinate values are numbered starting with 1 for the initial pixel which is part of the normal imaging array, and the coordinate values increase by 1 for each successive pixel that is read from the detector.
The world coordinate system values on the detector correspond to un-binned pixels.
In keeping with longstanding FITS practice, the centers of the pixels have integral coordinate values. The edges of the pixels have half-integral coordinate values.
The geometry of the amplifiers on the silicon of many CCDs creates non-image, pre-scan pixels which must be read prior to the actual image pixels. These pixels should have non-positive coordinate values.
The amplifier coordinate system only makes sense if a single FITS HDU consists of data from a single amplifier. If the pixel data from more than one amplifier are combined into one image, then the amplifier WCSNAME should not be used.
This definition is adapted directly from
the work of Frank Valdes at NOAO
CTYPE1 = 'CCD_X'
CTYPE2 = 'CCD_Y'
In this case ``detector'' typically means a single CCD, but the actual definition is more precise. It is conceivable that a single CCD could have multiple amplifiers each of which reads out a rectangular array of pixels which is disjoint or non-contiguously oriented from the others. In that case the CCD would be classified as multiple CCDs for the purposes of WCSNAME.
This definition is intended to match the definition used by Frank Valdes at NOAO .
WCSNAMEa for DEIMOS
WCSNAMEa values which are or may be used in the FITS headers of DEIMOS images. They are not yet suitable for adoption. They are documented here for the sake of discussion by the FITS community and for the sake of DEIMOS users.
CTYPE1 = 'IMAGE_X'
CTYPE2 = 'IMAGE_Y'
CTYPE1 = 'DETECTOR_X'
CTYPE2 = 'DETECTOR_Y'
It is not clear whether a detector coordinate system which includes gaps would be a legal use. Neither is it clear whether such a scheme would be understood by various different software implementations of FITS readers.
In the practice of the NOAO mosaic detector the enumeration of the
pixel values in detector coordinates is continuous across CCD
boundaries as if they were contiguously abutted.
Within the range of legal detector coordinate values there are no
values that correspond to locations between the CCDs.
The usage for DEIMOS makes the same presumption.
CTYPE1 = 'PANE_X'
CTYPE2 = 'PANE_Y'
PANEn keywords of the DEIMOS mosaic CCD readout system. Its purpose is to define a single, intuitively natural, coordinate system which can refer unambiguously to any pixel in the DEIMOS mosaic. Pane coordinates presume that the overall array of pixels is rectangular. Pane coordinates permit CCDs to be rotated in 90 degree increments.
Pane coordinates will typically have no gaps between CCDs. Nevertheless, if the most natural description of a mosaic detector layout includes gaps, they are legal.
The pane coordinate system adopts the more common, but non-FITS,
notion that the centers of pixels are half-integral. Therefore
the initial corner has coordinate value 0.0, and its center is at
0.5. However, when referred to in the integer-valued
PANEn control keywords, the entire initial
pixel is referred to as pixel 0.
In practice for DEIMOS this means that its coordinate values
differ by one-half pixel (as real-valued coordinates) or one pixel
(as integer-valued coordinates) from that of the ``detector''
CTYPE1 = 'GAP_X'
CTYPE2 = 'GAP_Y'
CTYPE1 = 'SLITMASK_X'
CTYPE2 = 'SLITMASK_Y'