LEARNED IT · 09/26
More magnification past the sampling limit makes the image worse, not bigger
At f/18.4 diffraction alone smears an edge over ~10 px. Barlowing the 76/700 spreads the same detail over 2.5x more pixels and adds nothing — while costing brightness and frame rate.
There is a focal ratio at which a camera has extracted everything the aperture can deliver. Past it, magnification is empty — the image gets bigger and strictly worse.
The rule of thumb: optimal focal ratio is roughly 5x the pixel size in microns. At about 1.4 um that is f/7.
| config | f/ | arcsec/px | diffraction floor |
|---|---|---|---|
| 76/350 bare | f/4.6 | 0.825 | 2.4 px |
| 76/700 bare | f/9.2 | 0.413 | 4.9 px |
| 76/700 + 2x | f/18.4 | 0.206 | 9.7 px |
Measured on a road sign at 80 m with the 76/700 + 2x: edge rise 25 px, which is 5.2 arcsec. The diffraction floor at f/18.4 is already 9.7 px. A perfect telescope could not have done better than 10 px in that configuration. The extra magnification was buying pixels, not detail — and paying for them in brightness and frame rate.
The trap is that this reverses. On 30 August the Barlow was the single best improvement available, because the f/4.6 tube was under-sampled at 1.8 px per resolution element. That advice was correct. It stopped being correct the moment the hardware changed, and nothing in the note itself would have flagged that.
Recompute the sampling every time the optical train changes. The right answer is a property of the combination, not of the accessory.