Astro Pixel Processor is a feature-rich software package that simplifies the complex task of processing astrophotography images. In this step-by-step guide, we’ll walk through the process of turning a set of Slooh LRGB image files into a stunning final photo using Astro Pixel Processor’s intuitive interface and powerful tools.

I recently added Astro Pixel Processor to my image processing software suite. This article describes, step-by-step, how to process a Slooh LRGB image using Astro Pixel Processor.

Sample Observation

I have made a sample observation of the Trifid Nebula, or Messier 20, available for download on the Samples page or download from here directly. Although I took the observation using another remote telescope service, it is similar to what you would get from the Canary One telescope – the observation ZIP file includes three luminance and one image each using the red, green, and blue filters.

Result

This image is the result of following the directions in this article:

I have slightly embellished the image with some star diffraction spikes, another article demonstrates how to create this effect for the stars in your own images.

Acquiring Astro Pixel Processor

Astro Pixel Processor offers various licenses including a free 30-day trial license. If you don’t want to commit to a long-term purchase of the software, you can get the Renter’s License, which currently costs 60 euros and allows you to use the software for one year; you are entitled to all updates throughout the year. Alternatively, you can buy an Owner’s License for 165 euros which also entitles you to updates; however, major releases may require an upgrade fee.

Download, select a license to use, and activate your software; you can use it offline once it has been activated.

Processing a Slooh Observation using APP

Start by unzipping the sample observation; place the files in a folder that you have access to read, write, and delete files.

Use the following directions to process the observation:

  1. Create a new folder called APP under the folder where you unzipped your files; this folder is your working folder and it is where APP saves all of its files. Use a different folder for each project or subject of observation to keep the files separated from one another.
  2. Start Astro Pixel Processor.
  3. Click the large button on the left side of the screen to set your working directory; select the folder you created in step one.
  4. Load the observation’s files into APP:Click the tab labeled 1) LOAD
  5. Select the folder where you unzipped your files
  6. Highlight all of the FIT files from the observation and click the Open button
  7. A new window will pop up, asking which channel you want to assign the frames. Slooh and most other remote telescope services include the FILTER keyword and value in the FITS file header, so you do not need to assign a channel manually. Ensure that the last option on the page is selected, it is labeled “apply FILTER header tag or assign RGB/MONO” and click Ok
  8. APP loads all of your files and assigns the appropriate color to each channel; you can conform that the settings are correct by reviewing the file listing at the bottom of the APP window: Luminance is shown in white, and red, blue, and green are shown in their respective colors.
  9. You can use the default options for all of the tabs numbered from two to five; you can browse through the settings and hover your mouse over the options to read the tooltip documentation; this is a straightforward integration so you don’t need to change anything.
  10. Select the tab labeled 6) INTEGRATE
  11. Under the first option, Multi-Channel/Filter options, select the ‘integrate per channel’ optionThis setting integrates each channel separately. If you select from the other two options, you end up with a greyscale combined image instead of an RGB image.
  12. For the ‘integrate’ option, select ‘average’.
  13. For the ‘weights’ option, select ‘quality’.
  14. Under ‘outlier rejection’, select ‘sigma rejection’ for the ‘filter option.
  15. Under ‘local normalization correction’, select ‘no LNC’ for the LNC degree option.
  16. Click the Integrate button at the bottom of the tab’s options
  17. Enter ‘Trifid Nebula’ in the window that pops up, asking for the name of the astronomical object.
  18. APP processes your images; this could take some time to complete.Once APP finishes, it will open one of the integrated images and display it near the center of the screen.
  19. We’ll now combine the L, R, G, and B integrated files into a single RGB file.Select the TOOLS tab
  20. Click the ‘combine RGB’ button; the screen will clear and this is expected to happen
  21. Click the ‘add channel’ button
  22. Select all of the files in the folder and click OpenAPP created these files and named them using your base name in addition to the color of the filter in use for the file. As before, the files are already tagged with the channel in use, so you don’t have to select them individually or manually.
  23. A window will pop up for each channel in the observation – you can simply click Ok to accept the default since APP derives the channel from the files’ metadata.
  24. Once you have cleared the popup window, your screen will go blank because APP is waiting for you to make a selection:Click the button labeled ‘(re-)calculate’
  25. APP displays a temporary image near the center of the screen; this image is for display purposes only.
  26. Under the ‘normalize’ option, select ‘add-scale’ and click the button labeled ‘(re-)calculate’You can experiment with the various methods and select the one you like the most.
  27. Click the ‘Save’ button on the screen’s left side and accept the default options in the Save Options box that pops up. Note the name of the file that APP is saving.
  28. Click the ‘cancel’ button to return to the TOOLS tab
  29. APP has saved the integrated RGB file and added it to the listing of files at the bottom of the screen. Double-click the file named ‘Trifid_Nebula-LRGB_1.fits’ and APP will display the file near the center of the screen.
  30. Next, we’ll calibrate the background; click the ‘calibrate background’ button
  31. Click Yes to continue
  32. APP displays your image near the center of the screen; it is waiting for you to select at least four regions that contain background and stars only. Using your mouse, click and drag over an area of stars and background – do not include any nebulosity in your selections. Here’s an example of where to place the samples:


  33. Click the button labeled ‘(RE-)CALCULATE’
  34. Click the Ok & SAVE button, note the name of the file that APP is saving, and click Ok
  35. When you return to the TOOLS tab, APP will display the last file you had open – this is not the one you just worked on. From the file listing at the bottom of the APP window, double-click the file named ‘Trifid_Nebula-LRGB_1-cbg.fits’
  36. Next, we’ll remove the light pollution from the image – the light pollution appears in the corners of the image and throughout by muting the colors a little.Click the button labeled ‘remove light pollution’
  37. Click Yes to continue.
  38. Place samples on the background; here’s an example of where to place the samples:



  39. Click the button labeled ‘(RE-)CALCULATE’
  40. Click the Ok & SAVE button, note the name of the file that APP is saving, and click Ok
  41. When you return to the TOOLS tab, APP will display the last file you had open – this is not the one you just worked on. From the file listing at the bottom of the APP window, double-click the file named ‘Trifid_Nebula-LRGB_1-cbg-lpc-cbg.fits’
  42. Now we’ll modify the image’s stretch and saturation before saving the final version. On the right side of the screen , locate the stretch selection dropdown – it is under the options for DDP, auto, and saturation (it is beside the red box in the following screenshot):


  43. Try the various options; the display updates whenever you make a selection. Find one that you prefer – you are looking for a balance between the image’s light and dark areas; I selected ‘10% BG, 3 sigma, 2,5% base’
  44. Put a checkmark in the box next to ‘saturation’
  45. Click the ‘save’ button, note the name of the file that APP is saving, and click Ok.

We are finished with APP and will now make our final adjustments in Photopea.com (or Photoshop if you have it installed on your system; Photopea.com is nearly identical to Photoshop)

  1. Open your browser and point it to Photopea.com
  2. From the Photopea.com File menu, select Open, and select the APP working folder from step one in the previous section.
  3. Select the file named ‘Trifid_Nebula-LRGB_1-cbg-lpc-cbg-St.tiff’
  4. From the Edit menu (ensure you pick Edit from Photopea.com in your browser), select Transform, and select the first Rotate 90 deg
  5. Now we’ll crop the image to exclude some of the undesirable regions. Press the M button on your keyboard, or click the third icon from the top-left
  6. Click and drag to surround the area you wish to keep
  7. From the menu, select Image – Crop
  8. From the File menu, select ‘Save as PSD’, accept the default filename, and click Save
  9. From the File menu, select Export As, then select PNG and click Save

Photopea.com has saved your final image in your browser’s Downloads folder.

We saved the image as a PSD, which is the native Photoshop/Photopea.com format and it has high resolution so you can save it in another format later if you like and still have access to the original.

Conclusion

Astro Pixel Processor is a powerful and user-friendly tool for processing astrophotography images. By following the step-by-step guide in this article, you can easily process your Slooh observations and achieve stunning results. The key steps include loading your files, integrating the channels, combining the LRGB images, calibrating the background, removing light pollution, and making final adjustments in Photopea or Photoshop.

Remember to experiment with different settings and options to find the best combination that suits your image. Don’t be afraid to try out various stretch and saturation settings to bring out the details and colors in your astrophotograph.

With practice and patience, you’ll soon be producing breathtaking images of celestial objects using Astro Pixel Processor and your Slooh observations.