[task #16280] Warp also returns the error of the output pixels

[Mohammad Akhlaghi]

URL:
  <https://savannah.gnu.org/task/?16280>

                 Summary: Warp also returns the error of the output pixels
                 Project: GNU Astronomy Utilities
               Submitter: makhlaghi
               Submitted: Mon 24 Oct 2022 10:49:04 AM BST
         Should Start On: Mon 24 Oct 2022 12:00:00 AM BST
   Should be Finished on: Mon 24 Oct 2022 12:00:00 AM BST
                Category: Warp
                Priority: 5 - Normal
              Item Group: Enhancement
                  Status: None
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: pedram
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00


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Follow-up Comments:


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Date: Mon 24 Oct 2022 10:49:04 AM BST By: Mohammad Akhlaghi <makhlaghi>
Currently Warp will only takes an input image and produces an output pixel
grid. However, astronomical data are often accompanied with their standard
deviation image (containing the error in each pixel that has been
kept/propagated from the very start of the reduction). 

Within a pipeline, this "error" image also needs to be warped with the input
(and then stacked) so we can propagate the errors of each observation to the
final stacked image.

In the current implementation (as of version 0.19), a user will have to
manually create a variance image from their error image (because errors get
added after being taken to the square root of 2), then run Warp on it (similar
to how they warp their input).

However, the WCS calculation of the pixel edges will be the same between the
input and error images, so will the polygon clipping and fractional area
measurement. So it is a waste of time, RAM and CPU to run Warp twice in such
scenarios. 

It is more efficient for Warp to also have an option called '--instd' and
'--stdhdu' (similar to MakeCatalog) to let the user provide an STD image. When
the user provides this image (it is optional), Warp will read that image, take
it to the power of 2 (to become variance), and then do the warping also on the
variance image in the exact same way it works on the input's pixel values. It
will finally take the square root of the final variance in each output pixel
and return the error (standard deviation) in each output pixel as a separate
HDU in the output file.

Of course, the power-of-2 and the square-root operations can take extra CPU,
so like MakeCatalog, we can have a '--variance' option that would lead it to
assume that the error image is already the variance, and it would also just
output the variance of the output (not standard deviation).

This should be pretty easy to implement, and we can test it with the 'SKY_STD'
extension of NoiseChisel's outputs on the image that are to be warped (like
the example in the book
<https://www.gnu.org/software/gnuastro/manual/html_node/Moire-pattern-and-its-correction.html>).







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