Review header creation process

Oct 13, 2025 by wbrum.org 81 views

Introduction

In the realm of optics and image processing, creating a header from a table with one point source at (0, 0) and arbitrary CDELT is a crucial step in calculating the footprint of an image. However, a peculiar behavior has been observed in the image_plane_utils module of the scopesim.optics package. This article delves into the issue, providing examples and insights into the problem.

The Issue

When creating a header from a table with one point source at (0, 0) and arbitrary CDELT, the calc_footprint function returns an unexpected result. Specifically, the footprint is not calculated correctly, resulting in an array of identical values instead of the expected array with varying values.

Example 1: Minimal Header

Consider a minimal header created from a table with one point source at (0, 0) and arbitrary CDELT:

from scopesim.optics import image_plane_utils as impu
hdr = impu.header_from_list_of_xy([0], [0], 1)

Running the calc_footprint function on this header results in:

array([[0.5, 0.5],
       [0.5, 0.5],
       [0.5, 0.5],
       [0.5, 0.5]])

However, the expected result should be:

array([[0., 0.],
       [0., 0.],
       [0., 0.],
       [0., 0.]])

This discrepancy suggests that the header creation process is not handling the edge case correctly.

Example 2: Larger Header

Things get worse when creating a header with a larger size:

hdr = impu.header_from_list_of_xy([-1, 1], [-1, 1], 10)

The resulting footprint is:

array([[5., 5.],
       [5., 5.],
       [5., 5.],
       [5., 5.]])

This result is even more unexpected, as the values are not even close to the expected range.

Example 3: Correct Result

To illustrate the correct behavior, consider a header created with a smaller size:

hdr = impu.header_from_list_of_xy([-1, 1], [-1, 1], 1)

The resulting footprint is:

array([[-1., -1.],
       [-1.,  1.],
       [ 1.,  1.],
       [ 1., -1.]])

This result is as expected, indicating that the issue lies in the header creation process.

Conclusion

The weird behavior observed in the image_plane_utils module is a result of incorrect header creation for trivial table headers. The issue is not limited to a single example, as it affects multiple cases with different sizes and CDELT values. To resolve this issue, a thorough review of the header creation process is necessary to ensure that edge cases are handled correctly.

Recommendations

To address this issue, the following recommendations are made:

Review header creation process: A thorough review of the header creation process is necessary to identify the root cause of the issue.
Edge case handling: Ensure that edge cases, such as trivial table headers, are handled correctly to prevent unexpected results.
Unit testing: Implement unit tests to verify the correctness of the header creation process and identify any potential issues.

Introduction

In our previous article, we explored the weird behavior observed in the image_plane_utils module of the scopesim.optics package. This behavior occurs when creating a header from a table with one point source at (0, 0) and arbitrary CDELT. In this article, we will delve into the Q&A section to provide more insights and clarify any doubts.

Q: What is the root cause of the issue?

A: The root cause of the issue lies in the header creation process. Specifically, the header_from_list_of_xy function is not handling the edge case of trivial table headers correctly.

Q: What is a trivial table header?

A: A trivial table header is a header created from a table with one point source at (0, 0) and arbitrary CDELT. This is an edge case that is not handled correctly in the current implementation.

Q: Why is the issue not limited to a single example?

A: The issue is not limited to a single example because it affects multiple cases with different sizes and CDELT values. This suggests that the root cause of the issue lies in the header creation process, rather than a specific example.

Q: How can I reproduce the issue?

A: To reproduce the issue, you can create a header from a table with one point source at (0, 0) and arbitrary CDELT using the header_from_list_of_xy function. For example:

from scopesim.optics import image_plane_utils as impu
hdr = impu.header_from_list_of_xy([0], [0], 1)

Running the calc_footprint function on this header will result in the unexpected output.

Q: What is the expected output for a trivial table header?

A: The expected output for a trivial table header is an array with values ranging from -1 to 1, depending on the size and CDELT values. For example:

array([[-1., -1.],
       [-1.,  1.],
       [ 1.,  1.],
       [ 1., -1.]])

Q: How can I fix the issue?

A: To fix the issue, you can modify the header_from_list_of_xy function to handle the edge case of trivial table headers correctly. This may involve adding additional checks or modifying the calculation of the header values.

Q: What are the implications of this issue?

A: The implications of this issue are that users may receive incorrect results when working with trivial table headers. This can lead to errors or inconsistencies in their calculations.

Conclusion

In this Q&A article, we have explored the weird behavior observed in the image_plane_utils module and provided answers to common questions. By understanding the root cause of the issue and the expected output for trivial table headers, users can take steps to fix the issue and ensure accurate results in their calculations.