Abell Catalog Builder
A free, offline desktop app for planning, imaging and logging all 86 objects in George Abell's catalog of planetary nebulae - faint, beautiful targets like the Medusa Nebula and Abell 39. A companion to the Messier, Caldwell and Sharpless builders.
Download free Browse all toolsGeorge Abell found these faint planetary nebulae on the Palomar Sky Survey plates and published 86 of them in 1966. They are large, dim and a real test of dark skies and long integrations - which makes completing them all a serious achievement.
The builder lays out Abell 1 to Abell 86 on one chart. A few entries were later shown not to be planetary nebulae (Abell 11, 32, 76 and 85), and their squares say so. Drag your photos onto the window and each one lands in the right square: name your files Abell 21.jpg, A39.png or PN A66 31.tif, or use another designation such as NGC 6742 or Sh2-274.
Right-click a photo to adjust its framing, rest your mouse on it for a 300 × 300 preview, switch the twinkling star border on or off, and export the finished chart as PNG, JPG or TIFF. Everything stays on your computer, and it runs on Windows and on both Apple Silicon and Intel Macs.
Frequently asked questions
What's new in version 1.3?
Right-click a photo to adjust its framing (move and zoom the crop, or show the whole photo). File names can now use object names as well as numbers. 16-bit and 32-bit TIFFs from Siril, PixInsight and other stacking software work, and unstretched (linear) stacks are brightened automatically. There's also Undo for removed or replaced photos, a progress bar for big batches, a Missing list you can save as a checklist, and a short welcome guide the first time the app opens.
Is it really free?
Yes. No account, no subscription and no ads. Download it and start.
Does it need the internet?
No. It runs entirely on your computer. Your photos, chart and logs never leave it - the app works from copies and never changes your original files.
Windows says "Windows protected your PC" - is it safe?
That warning appears for any app that isn't from the Microsoft Store. Click More info, then Run anyway. Some antivirus programs flag apps packaged this way by mistake; if yours removes it, restore it from its quarantine and allow it.
My Mac says it "cannot verify" the app.
The app isn't from the App Store, so macOS asks once. Click Done, open System Settings → Privacy & Security, scroll down and click Open Anyway. Pick the Apple Silicon download for M1-M4 Macs and the Intel download for older Macs.
What can I log for each object?
Right-click a photo and choose Add log / notes: dates, total exposure, sub length and count, gain or ISO, telescope or lens, camera, mount, filters, other equipment and your own notes. Each log is saved as a text file next to the photo, and a summary shows when you hover over it.
Are all 86 really planetary nebulae?
Almost. Later work showed Abell 11 is a reflection nebula, Abell 32 a flaw on the photographic plate, Abell 76 a ring galaxy and Abell 85 the supernova remnant CTB 1. They are still part of Abell's numbered list, so the chart keeps their squares and labels them.
How do photos get matched to squares?
Name the file with its Abell number - Abell 21.jpg, A39.png, PN A66 31.tif - and it goes straight to the right square. A few other designations work too: NGC 6742, NGC 7076, IC 972, IC 1454, Sh2-274, Sh2-290, Sh2-313, and the name Medusa. Anything else, drop it on the square you want.
Ready to start logging sessions?
Free, offline, and your data stays on your computer.
Version 1.3 · Updated November 2026