Well, with such a replace-the-camera then visually-focus-on-that-contraption instead device it all depends on you being able to replace your camera sensor and put this device into the exact same place. And that is where the problem lies.
So this is a part for an astro-imagers DIY for an SLR equivalent, uses the very same approach and visual aids otherwise built into the focusing screen.
With film, you had no other choice, unless you're OK with waiting hours to weeks for the actual image to be available between focusing iterations. And spatial resolution was quite limited, so absolute focus plane errrors mattered a lot less.
But modern camera sensors have over 10x that spatial resolution, so need that much more precision. Plus, with any electro-optical sensor you can get an instant view taken at the truely same location as your imaging sensor. Easily magnified and with a sensitivity many times higher than a human eye. When that kind of focusing became available, it was a truely disruptive technology. Which means: made the old ways completely obsolete in an instant with no reason to ever go back.
Edited by triplemon, 22 April 2025 - 12:18 PM.