Astronomy Astrophotography Photography

Fixing Transverse (Lateral) Aberration in AstroPixelProcessor (APP)

This is a quick guide and my experience on fixing transverse/lateral aberrations in light frames using AstroPixelProcessor (APP). If you just want the steps, just scroll down to the removal steps below. The steps also apply for longitudinal aberrations.

Recently I had a clear night inbetween all the cloudy weather around Canberra. This gave me a chance to try the Carina Nebula again. Albeit at close to 100% moonglow.

After my initial editing, I have noticed in the image the stars had what I could only describe as lateral aberrations. Initially I thought they were longitudinal aberration but turns out I had my terms confused… All the cooler stars across the entire image had blue fringing towards one and the same direction, along with orange fringing going the other direction.

This suggested the problem was unlikely to be in my image train (a new Askar FRA 400 and ASI533MC). I did some reading and searching online, then concluded that they were probably lateral aberrations. Likely caused by atmospheric distortions worsening at a lower elevation viewing angle. The Carina Nebula’s elevation angle in the night sky was just over 24 degrees at the time. Previous images taken with the same setup at much higher elevation angle had no issues so that pretty much confirmed my theory.

Here’s a sample photo of the abberation in one of my calibrated light frames viewed insided APP at 400% zoom with stretching and saturation turned on:

Lateral aberration in a calibrated light frame at 400% zoom with stretching and saturation turned on. The aberrations were a lot more obvious and jarring in my final stretched and edited image.

Turns out you could ask APP to align the RGB channels in your light frames with the “Align Channels” option in the “2) CALIBRATE” to eleviate this issue.

The gotcha for me was you needed to manually save the calibrated frames and use them as light frames in order to make use of the feature. Up till now I have generally checked the option thinking APP will just work its auto-magic 🤦‍♂️ and assumed it would cache the alignment data somewhere for its integraiton workflow.

Here’s the same light frame crop at 400% zoom, calibrated with the “Align Channels” option this time:

This is a channel aligned calibrated light frame at 400% zoom.

The difference is very subtle, even at 400% zoom. There is definitely an improvement on some of the smaller stars. The stars are argueably rounder but also slightly larger in some cases. There are still some visible chromatic aberration but they are more well formed rings around the stars instead of split colours.

Left: unaligned light frame. Right: channels aligned light frame. (400% zoom with stretch and saturation)

I reached the conclusion that while the “Align Channels” option in APP helps with transverse/lateral aberrations, I still needed to rely on the defringe tools in CaptureOne, Affinity Photo and Photoshop to fix all my other chromatic aberration issues.

It is quite cumbersome that the calibrated light frames need to be saved to make use of the feature. This meant I had to monitor the file saves so I could continue the next step of my APP processing workflow. I hope in the future APP can automatically handle this for me.

Just for kicks, here is a peak at a work-in-progress edit of the Carina Nebula 🙂

Till next time. Clear skies.

Aberration Removal Workflow

  1. Load and select your light frames, calibration frames (flat, dark, darkflat, masters, etc) inside APP as usual.
  2. In the “2) CALIBRATE” tab, scroll to the bottom and check the “Align Channels” option and click the “Save Calibrated Frames” button. APP will go through the calibration process and dump each calibrated frame to its working directory.
  3. Go back to APP’s “1) LOAD” tab and clear all your images. Then load the newly generated calibrated frames as light frames and proceed with your APP workflow as normal.
    NOTE: make sure you removed/deselected your calibration frames as you won’t need them for the calibrated frames!


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By Benjamin Huang

Software developer, photographer, gamer, food junkie and music lover.

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