RigTonightHow to calculate your telescope's field of view, and what will fit in it
Before you buy a telescope or plan a target, you want to know one thing: will it fit in the frame? That depends on just two numbers, the focal length and the size of your camera's sensor.
The formula
For each side of the sensor:
field of view (degrees) = 2 × atan(sensor size ÷ (2 × focal length))
For the small angles in astrophotography, a simpler version is almost exact:
field of view (arcminutes) ≈ 3438 × sensor size (mm) ÷ focal length (mm)
Use the effective focal length: multiply by your reducer's factor if you use one (a 0.8× reducer on a 500 mm scope gives 400 mm). One degree is 60 arcminutes, and the full moon is about 31′ across, which makes a handy yardstick.
Worked examples
An APS-C sensor (23.5 × 15.7 mm, as in the Player One Poseidon-C Pro or most DSLRs):
| Focal length | Field of view | Good for |
|---|---|---|
| 135 mm (camera lens) | 10° × 6.7° | Whole constellations, the Milky Way in Cygnus |
| 250 mm (a 50 mm f/5 refractor) | 5.4° × 3.6° | Big nebula complexes, Andromeda with room |
| 400 mm (FRA400) | 3.4° × 2.2° | Andromeda tight, Orion Nebula, Pleiades |
| 600 mm (80 mm f/7.5) | 2.2° × 1.5° | Rosette, Heart, Triangulum Galaxy |
| 1422 mm (EdgeHD 8 + 0.7×) | 57′ × 38′ | Larger galaxies, the Eagle, globular clusters |
| 2032 mm (EdgeHD 8) | 40′ × 27′ | Small galaxies and planetary nebulae |
A smaller sensor gives a smaller field at the same focal length. An IMX533 camera’s 11.3 mm square sensor (such as the Player One Ares-C Pro) at 400 mm covers 1.6° × 1.6°.
How big are the famous targets?
| Target | Size |
|---|---|
| Andromeda Galaxy (M31) | 190′ × 60′ (about 3° × 1°) |
| Heart Nebula | 150′ |
| North America Nebula | 120′ × 100′ |
| Pleiades (M45) | 110′ |
| Orion Nebula (M42) | 85′ × 60′ |
| Rosette Nebula | 80′ |
| Triangulum Galaxy (M33) | 70′ × 42′ |
| Eagle Nebula (M16) | 35′ × 28′ |
| Pinwheel Galaxy (M101) | 29′ × 27′ |
| Whirlpool Galaxy (M51) | 11′ × 7′ |
| Dumbbell Nebula (M27) | 8′ × 6′ |
| Ring Nebula (M57) | 1.4′ × 1′ |
How much of the frame should a target fill?
There's no single right answer, but a target looks best when it fills somewhere between a quarter and most of the frame:
- Too big: it spills off the edges. You need a shorter focal length, a reducer, or a mosaic of several panels.
- About right: the target with some space around it, which also helps when you crop out the edges after stacking.
- Too small: the 11′ Whirlpool Galaxy at 250 mm, with 3.76 µm pixels, spans only about 210 pixels. You can crop, but a longer focal length will show far more detail.
Field of view isn't the whole story
Focal length also sets your image scale, how many arcseconds of sky land on each pixel. Push the focal length too far for your camera and seeing and you'll be oversampled; too short and stars look blocky. Image scale and sampling explains the balance.
Let RigTonight do the maths
Pick your telescope, any reducer and your camera in the rig builder and it shows your field of view and image scale straight away. The Targets tab then marks which of 520 objects fill your frame well, which are too small and which need a mosaic, and the sky preview draws your exact frame on each one.