CRVALn

CRVALn is a keyword used in the FITS (Flexible Image Transport System) format, which is a standard for storing and transmitting astronomical data. The keyword is part of the World Coordinate System (WCS) specification and is used to define the reference value for the nth axis of a coordinate system. In the context of FITS, the WCS is a set of conventions for describing the relationship between pixel coordinates in an image and the corresponding world coordinates on the sky.

The CRVALn keyword is typically used in conjunction with other WCS keywords, such as CTYPEn, CDELTn, and CRPIXn, to fully define the coordinate system of an image. The value of CRVALn is the world coordinate value at the reference pixel, which is defined by the CRPIXn keyword. The CTYPEn keyword specifies the type of coordinate system (e.g., celestial, spectral), and the CDELTn keyword defines the increment per pixel along the nth axis.

For example, in a celestial coordinate system, CRVAL1 might represent the right ascension (RA) at the reference pixel, and CRVAL2 might represent the declination (Dec) at the reference pixel. The units of CRVALn depend on the type of coordinate system being used. In celestial coordinates, RA is typically given in degrees, while Dec is given in degrees.

The CRVALn keyword is an essential component of the WCS, as it provides the necessary information to transform pixel coordinates into world coordinates and vice versa. This is crucial for astronomical data analysis, as it allows astronomers to accurately interpret and compare data from different images and instruments.