Proposed conventional values for FITS WCSNAMEa keywords

The FITS WCS papers are ready for public inspection. They are visible at the websites of Eric Greisen and Mark Calabretta.

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).

Proposed Default Values and Limits on Values of WCSNAMEa

I propose that the default value of WCSNAMEA should be '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.

Proposed Conventional Values of WCSNAMEa

For optical astronomy there are certain world coordinate systems whose meaning can and should be universally recognized. For these world coordinate systems it makes sense to declare conventional values of the WCSNAMEa keywords.
WCSNAMEa = 'amplifier'
CUNITn = 'pixel'
(DEIMOS version = A)
A world coordinate system whose integer coordinate values correspond directly to an array of pixels on detector with a raster geometry (e.g., a CCD).

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 .

CUNITn = 'pixel'
(DEIMOS version = C)
The CCD WCS is chosen as one of the amplifier WCS that apply to a particular detector. Aside from the choice of one canonical amplifier, this WCS is in all other respects identical to the amplifier WCS. One CCD WCS can be applied to any regularly spaced rectangular grid of pixels.

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 .

Documentary Values of WCSNAMEa for DEIMOS

The above values are ones that I would propose to be adopted by the FITS community with little change. The values below are some additional 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.
WCSNAMEa = 'image'
CUNITn = 'pixel'
(DEIMOS version = I)
The image WCS is a redundant reiteration of the default FITS pixel coordinate system. The initial pixel along each axis has coordinate value 1, and the coordinate value increments by one for each pixel. The centers of the pixels have integral coordinate values, and the edges have half-integral values. This is equivalent to IRAF logical coordinates .

WCSNAMEa = 'detector'
CUNITn = 'pixel'
(DEIMOS version = D)
This coordinate system is intended to follow the definition created by Frank Valdes for the NOAO mosaic detector . It is effectively an extension of the notion of CCD coordinates. It presumes that a mosaic detector is comprised of a rectangular array of identical CCDs. It permits these CCDs to be flipped, but not rotated, with respect to each other. (Therefore a pinwheeled configuration of CCDs, or CCDs with differing pixel sizes, cannot be described.)

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.

WCSNAMEa = 'pane'
CUNITn = 'pixel'
(DEIMOS version = P)
This coordinate system is used by the 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'' coordinate system.

WCSNAMEa = 'gap' (aka 'ccd2gap')
CUNITn = 'pixel'
(DEIMOS version = G)
This coordinate system is an interim test scheme which is like pane coordinates except that it incorporates the gaps between each of the CCDs. It is intended to be used as a means of instructing an image display how to layout a mosaic image such that it approximately matches the actual mosaic geometry.

WCSNAMEa = 'slitmask'
CUNITn = 'mm'
(DEIMOS version = M)
This coordinate system has not yet been implemented for DEIMOS, but could be implemented at any time. Its coordinate values give the location on the slitmask as measured in the coordinate system which is used for milling the masks.

Steve Allen <>
$Date: 2003/07/28 18:33:34 $