Building Wallace, a 6-inch f/2.8 grab-and-go telescope, and hiking it to a cliff

An amateur optician documents Wallace, a 153mm ultra-wide-field telescope he ground and built himself, and pairs the technical writeup with an unusual photo-and-video diary of carrying it on a hot evening hike to a clifftop for observing. The framing is less about specs than about a question the maker keeps returning to: how do you share an experience — a deep-sky view, a trail run — that is fundamentally personal and can’t really be transmitted? His answer is an ‘ambient impression’ of forest sounds, wind, and setup-from-a-backpack footage rather than polished videography.

The optics are the heart of it. The f/2.84 focal ratio was partly an accident — he misread a shim thickness while grinding the mirror on a sine table and overshot his f/3 target. The primary took two attempts after a deep scratch ruined the first, and the finished mirror keeps a small central hole that the secondary fully masks, landing at 0.92 Strehl. Because such a fast mirror produces heavy coma, correction is non-negotiable; he uses a cheap GSO four-element corrector (designed by Roger Ceragioli, bought for 65 euros) that performs well once spaced far beyond its incorrect factory backfocus, around 79mm. The two-module dovetail build nods to the Coulter CT-100 and is 3D-printed in ASA, with carbon rods and a simple six-point mirror cell.

The project is fully open: the print files are free on Printables, and the build is discussed across French and English amateur-astronomy forums. A lighter successor is already underway, using a thin meniscus blank that cools fast in the field — the maker’s main grab-and-go priority — and this time he’s chasing 0.95 Strehl ‘for the beauty of the craft.’