Understanding LIMMAG and BACKMAG

Oct 13, 2025 by talynt.org 8 views

Introduction

In the context of astronomical data processing, the coaddition of visits is a crucial step in creating high-quality images. However, when using the pipeline.last.coadd.coaddVisits function, users may encounter unexpected results, such as non-realistic values for the LIMMAG. In this article, we will delve into the possible causes of this issue and provide guidance on how to troubleshoot and resolve it.

Understanding LIMMAG and BACKMAG

Before we dive into the problem, let's briefly explain what LIMMAG and BACKMAG represent. LIMMAG is the magnitude of the coadded image, while BACKMAG is the magnitude of the background noise. In a typical coaddition process, the LIMMAG value should be close to the average magnitude of the individual images, while the BACKMAG value should be a real number representing the background noise.

Non-Realistic LIMMAG Values

When coadding all images of CropID=10, the LIMMAG value is reported as 22.8, which is significantly higher than the typical LIMMAG value of 18.5. Additionally, the BACKMAG value is a complex number, indicating an error in the coaddition process. Similarly, when coadding all images of CropID=15, the LIMMAG value is reported as 22.9, and the BACKMAG value is 29.5, which is also higher than expected.

Possible Causes of Non-Realistic LIMMAG Values

There are several possible causes of non-realistic LIMMAG values when using the pipeline.last.coadd.coaddVisits function:

Incorrect image selection: If the images selected for coaddition are not representative of the typical images in the dataset, the resulting LIMMAG value may be skewed.
Inconsistent image quality: If the images selected for coaddition have varying levels of quality, the resulting LIMMAG value may be affected.
Coaddition algorithm issues: The coaddition algorithm used by the pipeline.last.coadd.coaddVisits function may be flawed, leading to non-realistic LIMMAG values.
Data corruption: The input data may be corrupted, leading to incorrect results.

Troubleshooting and Resolution

To troubleshoot and resolve the issue of non-realistic LIMMAG values, follow these steps:

Verify image selection: Ensure that the images selected for coaddition are representative of the typical images in the dataset.
Check image quality: Verify that the images selected for coaddition have consistent quality.
Inspect coaddition algorithm: Review the coaddition algorithm used by the pipeline.last.coadd.coaddVisits function to ensure it is correct.
Data validation: Validate the input data to ensure it is not corrupted.

Example Code

To demonstrate the issue, we will use the following example code:

import pandas as pd

# Load the products file
products_df = pd.read_csv('Products_CropID10.txt')

# Coadd the visits
coadd_df = pipeline.last.coadd.coaddVisits(products_df)

# Print the LIMMAG and BACKMAG values
print(coadd_df['LIMMAG'])
print(coadd_df['BACKMAG'])

Conclusion

In conclusion, non-realistic LIMMAG values when using the pipeline.last.coadd.coaddVisits function can be caused by a variety of factors, including incorrect image selection, inconsistent image quality, coaddition algorithm issues, and data corruption. By following the troubleshooting and resolution steps outlined in this article, users can identify and resolve the issue, ensuring accurate and reliable results.

Additional Resources

For further information on the pipeline.last.coadd.coaddVisits function and its usage, refer to the following resources:

Attachments

The following attachments are provided for reference:

Products_CropID10.txt
Products_CropID15.txt
Pipeline.last.coadd.coaddVisits Possibly Returns Non-Realistic LIMMAG: Q&A ====================================================================

Introduction

In our previous article, we discussed the issue of non-realistic LIMMAG values when using the pipeline.last.coadd.coaddVisits function. In this Q&A article, we will address some of the most frequently asked questions related to this issue.

Q: What is the typical LIMMAG value for a coadded image?

A: The typical LIMMAG value for a coadded image is close to the average magnitude of the individual images. In the case of the example provided, the typical LIMMAG value is 18.5.

Q: Why do I get non-realistic LIMMAG values when coadding all images of CropID=10?

A: There are several possible causes of non-realistic LIMMAG values when coadding all images of CropID=10, including:

Incorrect image selection
Inconsistent image quality
Coaddition algorithm issues
Data corruption

Q: How can I troubleshoot the issue of non-realistic LIMMAG values?

A: To troubleshoot the issue of non-realistic LIMMAG values, follow these steps:

Verify image selection
Check image quality
Inspect coaddition algorithm
Data validation

Q: What is the difference between LIMMAG and BACKMAG?

A: LIMMAG is the magnitude of the coadded image, while BACKMAG is the magnitude of the background noise. In a typical coaddition process, the LIMMAG value should be close to the average magnitude of the individual images, while the BACKMAG value should be a real number representing the background noise.

Q: Can I use the `pipeline.last.coadd.coaddVisits` function with other types of images?

A: Yes, the pipeline.last.coadd.coaddVisits function can be used with other types of images, but the results may vary depending on the specific image characteristics.

Q: How can I ensure that the input data is not corrupted?

A: To ensure that the input data is not corrupted, follow these steps:

Validate the input data
Check for inconsistencies in the data
Use data quality checks to identify potential issues

Q: What are some common pitfalls to avoid when using the `pipeline.last.coadd.coaddVisits` function?

A: Some common pitfalls to avoid when using the pipeline.last.coadd.coaddVisits function include:

Incorrect image selection
Inconsistent image quality
Coaddition algorithm issues
Data corruption

Q: Can I use the `pipeline.last.coadd.coaddVisits` function with other coaddition algorithms?

A: Yes, the pipeline.last.coadd.coaddVisits function can be used with other coaddition algorithms, but the results may vary depending on the specific algorithm used.

Q: How can I optimize the coaddition process for better results?

A: To optimize the coaddition process for better results, follow these steps:

Use a robust coaddition algorithm
Select images with consistent quality
Validate the input data
Use data quality checks to identify potential issues

Conclusion

In conclusion, the pipeline.last.coadd.coaddVisits function can be a powerful tool for coadding images, but it requires careful attention to detail to avoid non-realistic LIMMAG values. By following the troubleshooting and resolution steps outlined in this article, users can identify and resolve the issue, ensuring accurate and reliable results.

Additional Resources

For further information on the pipeline.last.coadd.coaddVisits function and its usage, refer to the following resources: