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Astrophotographers often return to the same targets to rephotograph them using different equipment.

As a result, we often end up with multiple sets of frames captured with a range of focal lengths or camera sensors.

Combining this data can produce stunning images that can be cropped in tightly to reveal intricate features.

One program that does this well is Astro Pixel Processor (APP), a powerful tool that easily blends multi-focal-length data in a few simple steps.

Here I’ll explain how to use APP to produce a high-resolution image, using my Whirlpool Galaxy image (above), made using data from two different imaging setups, including a triplet refractor and a Newtonian reflector.

Getting started

Before opening APP, I went through each dataset of images and removed any frames containing plane trails, clouds or other unwanted elements.

I also took dark and flat calibration images for each and saved the datasets into two folders.

When you first open APP, a window pops up asking you to select a working directory, the location where you have saved your data.

If for any reason this does not pop up, click the ‘Directory’ button (highlighted, image below) and navigate to your image folders.

Beneath the working directory button is a series of tabs numbered 0) to 6), which will prepare your datasets for stacking.

The ‘1) Load’ tab is where to load your light frames and calibration frames. First, ensure the three tick boxes are selected.

Next, click the ‘Light’ button underneath and navigate to the folder that contains the light frames from your first dataset, select all and click ‘Ok’.

A pop-up will ask what channel your light frames should be assigned to. As mine were colour images,
I selected ‘RGB’.

Create your masters

Another pop-up will appear, asking what session to assign the files to. As these are the light frames from your first dataset, tick ‘session 1’ (see image above).

Next, upload any calibration frames for dataset 1 by selecting the ‘Flat’ and ‘Dark’ buttons and repeating the above steps for the calibration files.

Repeat this step for the light and calibration frames from dataset 2; this time select the ‘session 2’ option. 

Clicking on the ‘2) Calibration’ tab, leave all settings as default. If you’re using master calibration frames, click on ‘(Re-)assign Masters to Lights’.

If not, click ‘create Masters & assign to Lights’. Next, head to ‘3) Analyse stars’.

This is where APP will review each light frame for its star quality. Simply scroll down and click ‘Analyse’, then wait for the script to finish.

Next, click ‘4) Register’. The following steps enable the multiple datasets to register correctly (see image below): tick ‘flip descriptors in X/Y’ and ‘use dynamic distortion correction’.

As this data is from different camera and telescope setups, be sure to untick the toggle box for ‘same camera and optics’ before clicking ‘start registration’.

On the ‘5) Normalize’ tab, access the advanced normalisation function by selecting ‘advanced’ from the dropdown menu.

Leave all other options as default, click ‘normalize lights’ and wait to process. 

Our last stop is ‘6) Integrate’ (see image below). We suggest adjusting the slider to 95%, so that APP picks the best 95% of the light frames for stacking.

Other things to change in this tab include setting integrate to ‘average’, weights to ‘quality’, composition to ‘full’ and LNC to ‘no LNC’.

Ensure MBB is ticked; typically you want to set this at 5–15%; 5% is generally fine.

Tick ‘No under/over shoot’ and, finally, click ‘integrate’. A window will pop up, allowing you to give the stacked image a name, after which click ‘Ok’.

Note, the stacking process takes a while! 

Once complete, save your stacked image in your working directory, ready to crop out any remaining stacking artefacts and continue editing.

My final image of the Whirlpool Galaxy at the top of this article shows how far I could crop and how many additional details could be teased from my multi-setup image.

3 quick tips

1. Organise your light and calibration files into separate folders for each dataset.
2. Pre-calibrate’ your light frames in APP beforehand to speed up the stacking process.
3. Experiment with different settings in the ‘Integrate’ tab to see how they impact your stacked image.

If you’re an astrophotographer, share your images with us by emailing contactus@skyatnightmagazine.com.

This guide appeared in the February 2025 issue of BBC Sky at Night Magazine

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