CRVAL2

CRVAL2 is a parameter used primarily in the context of astronomical data and spectroscopic or imaging observations stored in FITS (Flexible Image Transport System) files. It represents the coordinate value at the reference pixel along the second axis of an image or data array. In essence, CRVAL2 specifies the physical or celestial coordinate corresponding to the pixel position labeled as CRPIX2, which acts as the origin point in the coordinate system.

The purpose of CRVAL2 is to define the exact coordinate value (such as right ascension, declination, or wavelength) that aligns with the reference pixel. This parameter, together with other keywords like CRPIX2 (reference pixel position) and CDELT2 (coordinate increment per pixel), helps establish a precise mapping between pixel coordinates and real-world or celestial coordinates.

CRVAL2 is part of the WCS (World Coordinate System) standard, which facilitates the interpretation of astronomical images by associating pixel locations with formal coordinate systems. Its definition depends on the specific observational context and the type of data, with common examples including celestial coordinate systems (for declination or right ascension) or wavelength in spectral data.

Proper specification of CRVAL2 ensures accurate spatial or spectral referencing, enabling astronomers to analyze and compare data across different observations, instruments, or wavelengths. It is essential for tasks such as astrometric calibration, data overlay, and coordinate transformation in astronomical research.

Overall, CRVAL2 is a critical parameter in the WCS framework, providing a standardized way to annotate and interpret the coordinate information contained within FITS files, thus supporting precise astronomical data analysis and sharing.