A free, self-contained observation log for the Messier catalog — built for the field, designed for the Astronomical League, and distributed as open-source software.
The Messier Marathon Observer's Log is a browser-based tool created by Aaron Henderson for fellow amateur astronomers observing Charles Messier's catalog of 110 deep-sky objects. It requires no installation, no account, and no internet connection after the page has loaded once. All observation data is stored locally in the browser — nothing is ever sent to a server.
The log is designed around two use cases: a casual Messier checklist for observers who want a fast, red-light-friendly way to tick objects off during a marathon night, and a full AL certificate record for observers pursuing the Astronomical League Messier Club observing award. Both use cases share the same interface — the full observation record forms are simply available for those who need them.
The project was developed with the Skies of the Roanoke Star Party (SRSP) 2026 in mind, and was tested by field observers before its first public release.
Night vision mode — deep black background with red-spectrum text to preserve dark adaptation; toggles to a warm parchment day mode for planning indoors. All emoji UI icons automatically switch to short ASCII labels in night mode so no bright or coloured symbols appear at the eyepiece. Object photos in the lightbox are automatically red-filtered.
110 objects in marathon order — west to east, beginning with objects that set first after dusk and ending with those that rise last before dawn. Click the Object column header to toggle between marathon sequence and Messier number order.
Angular size column — apparent dimensions in arcminutes for every object, sourced from the SEDS marathon data, displayed alongside magnitude and coordinates.
Monospace type symbols — each object type is represented by a distinct ASCII symbol (@ spiral, ◎ globular, ~ diffuse nebula, ○ planetary nebula, etc.) that reads clearly in red-only night mode where colour alone cannot differentiate types. A matching legend appears below the toolbar.
Full AL observation records — per-object forms capturing date, time, site, seeing (Antoniadi scale), transparency, limiting magnitude, equipment, written eyepiece description, and sketch notes. The ✎ button is outlined when no record exists and fills solid red when one is saved — giving at-a-glance status across the whole table.
Session Setup — set location (with GPS) and equipment once per night; values auto-fill every record opened during the session. A pulsing Session active indicator appears in the toolbar while defaults are set.
Themed confirmation dialogs — all destructive actions (reset data, clear session, import records) use a custom modal that follows the current night/day theme, so no browser-native white dialog ever appears at the telescope.
In-browser PDF report — generates a formatted multi-page submission document with cover page, observation index, and one full page per record, entirely in the browser with no internet required.
CSV and JSON export/import — back up and restore all data; move between devices; merge records without overwriting existing entries.
Image lightbox — click any Messier number to see the object photo (red-filtered in night mode), full catalog data including angular size, a finder chart link with red-filter warning, and a Stellarium Web link.
Works offline — the full tool functions without network access after first load.
| Version | 1.0 — SRSP 2026 Edition |
| Status | Active development — feedback welcome |
| Repository | github.com/thg4web/mmol |
| License | GPLv3 — free to use, share, and modify |
| AL Program | astroleague.org — Messier Observing Program |
| Sky Atlas Ref | Pocket Sky Atlas, Roger Sinnott (Sky & Telescope) |
| Object Data | SEDS Messier Database (Hartmut Frommert) |
Object photographs courtesy of the SEDS Messier Catalog. Marathon sequence and angular size data from Hartmut Frommert's SEDS marathon tables. Sky simulation links provided by Stellarium Web. Geolocation reverse-geocoding via OpenStreetMap Nominatim. In-browser PDF generation via jsPDF and jspdf-autotable. Typography: Cinzel, Share Tech Mono, and Crimson Pro via Google Fonts (self-hosted).
Special thanks to the field testers who observed with early versions of this log and provided the feedback that shaped it into what it is today. Clear skies.
Questions, feedback, or bug reports: mmol [at] thgnetworks [dot] com