| article: Superresolution Frame Combining in Astronomy |
What is Superresolution?
Superresolution is the approach to retrieve a high resolution output image from low resolution input images.
This may not be confused by the method called "digital zoom" in consumer equipment which is a simple interpolation to create more resolution.
Interpolation - no matter if linear, bicubic or more complex - can never really create a high resolution image with more information than the input image.
But superresolution can.
Superresolution - to a certain degree - can be achieved from a single input frame by means of sophisticated mathematical algorithms.
But this article will focus on using a series of input frames like it is common for CCD astronomy.
How does Superresolution work?
The question is: if the superresolution output image contains more information than one low resolution input image, where does that information come from?
An astronomical photo will contain stars.
If the stars are sampled correctly there will be at least 2 pixels forming the star.
Two pixels per star is called critical sampling and you can influence that by matching your telescope or optics to your camera.
Click here for more detial.
For any of these stars on the low resolution image we can now calculate the centroid of the star to sub-pixel precision.
This sub-pixel information will be used for superresolution.
Let's assume we want to achive a 2x superresolution.
For just one frame this additional information would be fairly useless.
But a series of frames must be registered and stacked to form a combined (summed or averaged) final image.
During this process of stacking we can use the known sub-pixel precise information to either shift a frame by 0 pixel or 1 pixels depending on the calculated value being below or above 0.5.
This would create our 2x superresolution.
Keep in mind that if all frames would be 100% identical (for example by a douplication of files on the harddrive) the superresolution must fail.
In fact it is necessary that from one frame to the other the object is slightly shifted arround on the CCD chip.
This is the fact for almost any astronomical images due to tiny tracking errors and variations in seeing.
Some telescope guiding applications do allow for a "drizzeled" guiding introducing some guide star offsets of about 0.5 pixels on purpose.
This could be useful in wide angle photography where the seeing or tracking errors do not effect the image so much.
Day Light Example of Superresolution
For reference purpose a high resolution frame was taken using the Canon EOS 300D.
The low resolution frames were taken with a Starlight Xpress SXV-8MC camera and the astroScopius software.
To create the necessary "drizzle" I was carefully walking around the tripod to shake the floor ( patent applied for :) )
The superresolution was done by Registax.
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