6.4 Warp ¶
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 1989 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 warping 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.
Observational reasons for warping an image are listed below:
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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 them.
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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.
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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 easily.
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 grid.
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Optical distortion: 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.
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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.