True Field of View Eyepiece Calculator
Rectilinear field-stop true-field angle, not approximate apparent-field/magnification under unknown eyepiece distortion. Model only, not an ephemeris, measured stellar identity or observational guarantee.
Description
A field stop limits the angular sky field passed by an eyepiece.
A field stop limits the angular sky field passed by an eyepiece.1
Its geometry is distinct from apparent field divided by magnification or a distortion-corrected field measurement. Use True Field of View Eyepiece Calculator to evaluate the supplied inputs under the model explained below.
When to use True Field of View Eyepiece Calculator
Use True Field of View Eyepiece Calculator to evaluate the supplied inputs under the model explained below. Establish the units, measurement scope and model assumptions before entering values.
How to use True Field of View Eyepiece Calculator and understand its results
- Establish the units, measurement scope and model assumptions before entering values.
- Enter Eyepiece field stop mm, Telescope focal mm using the field table and validation rules below, then run the tool.
- Read Field stop true field degrees; interpret the units, signs, ordering and unknown-value placeholders described below.
- Correct reported input errors and rerun. Apply successful results according to their specific meanings and check the assumptions before using them in a decision.
| Input field | Meaning | Value type | Requirement |
|---|---|---|---|
| eyepieceFieldStopMm | Eyepiece field stop mm | number | Required; constraints below |
| telescopeFocalMm | Telescope focal mm | number | Required; constraints below |
Enter nonnegative field-stop diameter and positive telescope focal length in mm. Return 2×atan(stop/(2×focal)) converted from radians to degrees. Zero stop returns zero. This is the committed geometrical convention, without distortion, vignetting or a supplied apparent-field calculation.
trueFieldDegrees=(360/π)×atan(fieldStopMm/(2×focalMm)) Use the field order, units and branch conditions described above.
| Output field | Meaning | Value type |
|---|---|---|
| fieldStopTrueFieldDegrees | Field stop true field degrees | number |
{
"input": {
"eyepieceFieldStopMm": 20,
"telescopeFocalMm": 1000
},
"output": {
"fieldStopTrueFieldDegrees": 1.1458773953669719
}
}Limitations and assumptions
Its geometry is distinct from apparent field divided by magnification or a distortion-corrected field measurement. Apply the exact scope and assumptions explained above.
All listed fields require explicit values. Unexpected fields, incorrect types, nonfinite numbers and sparse arrays fail validation. Domain-specific bounds, alignments and supported tokens are explained above; extremely large inputs or outputs can also be rejected. Input errors differ from successful false results, empty match lists and unknown-evidence placeholders.
Alternative or Complementary methods
Use Eyepiece Exit Pupil mm Calculator for a related task: Exit-pupil diameter connects aperture with angular magnification.
Use Telescope Magnification From Focal Lengths for a related task: Telescope magnification compares the telescope and eyepiece focal lengths.
References
-
True Field of View Eyepiece Calculator — tool.io
Similar or alternative tools
- Eyepiece Exit Pupil mm Calculator
Exit-pupil diameter connects aperture with angular magnification.
- Telescope Magnification From Focal Lengths
Telescope magnification compares the telescope and eyepiece focal lengths.