GNU Astronomy Utilities
Image warping is the process of mapping the pixels of one image onto a new
pixel grid. This process is sometimes known as transformation, however
following the discussion of Heckbert 198975 we will
not be using that term because it can be confused with only pixel value or
flux transformations. Here we specifically mean the pixel grid
transformation which is better conveyed with ‘warp’.
Image wrapping is a very important step in astronomy, both in observational
data analysis and in simulating modeled images. In modeling, warping an
image is necessary when we want to apply grid transformations to the
initial models, for example in simulating gravitational lensing (Radial
warpings are not yet included in Warp). Observational reasons for warping
an image are listed below:
Noise: Most scientifically interesting targets are inherently
faint (have a very low Signal to noise ratio). Therefore one short
exposure is not enough to detect such objects that are drowned deeply
in the noise. We need multiple exposures so we can add them together
and increase the objects’ signal to noise ratio. Keeping the telescope
fixed on one field of the sky is practically impossible. Therefore
very deep observations have to put into the same grid before adding
Resolution: If we have multiple images of one patch of the
sky (hopefully at multiple orientations) we can warp them to the same
grid. The multiple orientations will allow us to ‘guess’ the values of
pixels on an output pixel grid that has smaller pixel sizes and thus
increase the resolution of the output. This process of merging
multiple observations is known as Mosaicing.
Cosmic rays: Cosmic rays can randomly fall on any part of an
image. If they collide vertically with the camera, they are going to
create a very sharp and bright spot that in most cases can be separated
easily76. However, depending on the depth of the
camera pixels, and the angle that a cosmic rays collides with it, it
can cover a line-like larger area on the CCD which makes the detection
using their sharp edges very hard and error prone. One of the best
methods to remove cosmic rays is to compare multiple images of the
same field. To do that, we need all the images to be on the same pixel
Optical distortion: (Not yet included in Warp) In wide field
images, the optical distortion that occurs on the outer parts of the focal
plane will make accurate comparison of the objects at various locations
impossible. It is therefore necessary to warp the image and correct for
those distortions prior to the analysis.
Detector not on focal plane: In some cases (like the Hubble
Space Telescope ACS and WFC3 cameras), the CCD might be tilted
compared to the focal plane, therefore the recorded CCD pixels have to
be projected onto the focal plane before further analysis.
Read in other formats.
GNU Astronomy Utilities 0.4 manual, September 2017.