A common problem in astrophotography is column defects. A column defect occurs when a column of pixels is on even though that was not present in the actual image.

Here’s a sample of NGC 1977 – the Running Man nebula featuring two column defects:

It is a little difficult to see the column defects in this image because I resized the image so that it fits on this site’s pages, so here’s a version with the defects highlighted:

What you’re seeing is the column defects in a fully processed image. The column defects need to be corrected before integration (before stacking and aligning your images), so you correct these defects on the FITS files you captured from the telescope.

This article walks you through correcting these column defects using PixInsight’s CosmeticCorrection process.

The process is broken down into the following steps:

  1. Identify the locations of the column defects
  2. Use CosmeticCorrection to correct the defects
  3. Process your image to see the final result

Result

This is the result of fixing the column defects using PixInsight:

From this, you can see that the column defects are no longer present in the final image.

We’ll start by identifying where in the image the column defects reside.

Identify The Locations of Column Defects

Open your image that has the column defects on it, and start by enabling Readout Mode. Select Image – Mode – Readout. This mode allows you to see the location of your pointer on the screen along with other information.

Zoom into the image to review the column defect. Does it start somewhere in the image? In the case of this image, the defect on the right starts a little from the top of the image.

Does the defect span the whole column or does it end somewhere within the image? In the case of this sample image, the column defect on the left starts at the top of the image and stops about halfway through the image.

While zoomed-in, locate the beginning or end of your column defect. In this example, I located the end of the column defect on the left and placed my mouse pointer at the end of the column defect.

Click and hold your mouse button down and note the X and Y coordinates (this is why we enabled Readout Mode) – in this case, X=487 and Y=568 (ignore the numbers after the decimal). Take note of your numbers and record them somewhere safe (I used the Notepad application).

The column defect on the right starts about a quarter down the image and ends about three-quarters down the image, so I’ll have to locate the beginning and end of the defect and note the coordinates of both.

Here is what PixInsight displayed when I held my mouse button down at the beginning of the defect on the right:

And here is what PixInsight displayed when I held my mouse button down at the end of the defect on the right:

From this, I saw that the column defect started at 520, 204 and ended at 520, 711, so the defect runs from pixel 204 to 711 on the Y axis.

Using CosmeticCorrection to Fix The Column Defects

This section walks you through fixing the column defects.

1. From the PixInsight menu, select Process – Image Calibration – CosmeticCorrection. Alternately, you can find CosmeticCorrection under the Process – All Processes menu.

2. Click the Add Files button and add your original FITS files that you downloaded from your telescope service provider.

3. Note the Output section

PixInsight will correct the column defects and store the modified files in the location you provide. Left at its default, the Output directory will be the same as your input files directory and files will be suffixed with ‘_cc’. You can leave the other settings at their defaults.

4. Select Use Defect List and expand the settings under it

5. Enter the location of the column defect’s X coordinate in the box on the left and click the ‘Col’ radio button to indicate that you are specifying a column.

In this case, I entered 487.

5. Select the ‘Limit’ checkbox to indicate that you don’t want PixInsight to fix the whole column but instead want to provide your own starting and ending coordinates.

In this case, I entered 0 on the left because the defect starts at the top of my image, and then I entered 568 on the right because that’s where the defect ended when I looked at my image and clicked and held my mouse button down to view the coordinates.

6. Click the ‘Add defect’ button to save the defect.

7. Repeat step five and six for any other defects. In the case of my image, I entered 520 and used a Limit of 204 and 711 because those are the values PixInsight displayed when I looked at my image and clicked and held my mouse button down to view the coordinates.

8. Click the ‘Apply Global’ button to apply the correction

Your corrected files are saved as files having an extension of ‘.xisf’ and having ‘_cc’ appended to their original file names.

Process the modified xisf files as you would normally process them, and note that the column defect is gone now, as shown in the result earlier in this article.

Conclusion

In this article, you learned how to correct column defects using PixInsight. You learned how to identify where defects begin and end and learned how to use the CosmeticCorrection process.