Focal length and field of view

Side view: lens and 24 mm sensor

What the sensor records (same sky, same sensor)

Mirror separation d1100 mm

Focus behind primary b300 mm

Detector size (square)30 mm

Magnification m–

Effective focal length–

Plate scale–

Field of view (side)–

Sky area vs prime focus–

Secondary / primary ⌀–

Focus shift per mm despace–

The convex secondary intercepts the beam before the prime focus and slows its convergence. With the prime focus as a virtual object:

p = f₁ − d, s = d + b
m = s / p, F = m · f₁

Extending the final ray back to the incoming ray's height places an equivalent single lens a distance F in front of the focus. Plate scale is 206265 / F and FOV ≈ detector size × plate scale, so each side of the field shrinks by 1/m and the area by 1/m².

Larger m puts the secondary nearer the prime focus, where the beam is narrower, so the secondary is smaller.
Focus moves by about m² + 1 times any secondary motion: at m ≈ 4, 10 µm of despace shifts focus by about 170 µm.
These are paraxial numbers. Coma and field curvature set the usable field; mirror conics or a corrector address those without changing F.

The real telescope, to scale (500 mm f/3 primary, f₁ = 1500 mm)

Unfolded: equivalent single lens at distance F from the focus (vertical scale exaggerated)

Same detector, same sky: what it records at prime focus and behind the secondary