Star Drift Field of View Calculator
Time a star drifting across your eyepiece field, enter its declination, and get a measured true field rather than an estimate from catalog specifications. Three timings also show whether the result is repeatable enough to trust.
Measured true field (degrees)1
- Mean full-crossing time (seconds)
- 240
- Trial spread (seconds)
- 4
- Trial spread percentage
- 1.67%
- Measured true field (arcminutes)
- 60.16
- Magnification
- 48
- Effective field-stop diameter (mm)
- 21
- Effective apparent field (degrees)
- 48.13
- Rated-spec FOV difference
- -3.74%
Assumes the star crossed the center of the field with tracking, guiding, and slow-motion input switched off.
The field-stop and apparent-field values are inferred from the measured field; eyepiece distortion can make both differ from physical specifications.
The field-stop calculation uses a small-angle approximation, which becomes less exact for very wide fields.
How to use this calculator
- Choose a reasonably bright star near the celestial equator and look up its catalog declination.
- Center the star, switch off tracking, guiding, and slow-motion input, then time its drift across the field three times.
- Choose edge to edge for complete crossings, or center to edge when each recorded time covers half the diameter.
- Enter the system’s effective focal length, eyepiece focal length, and the eyepiece’s rated apparent field.
- Repeat the test if the trial-spread warning appears or if the star did not pass through the center.
How a star-drift field test works
With the drive switched off, a star appears to move through the eyepiece because Earth rotates. A star on the celestial equator crosses about one degree in four minutes. Stars farther north or south drift more slowly across the field, so a useful drift measurement needs a declination correction.
Let t1, t2, and t3 be the recorded times in seconds. Their mean is multiplied by k, where k = 1 for an edge-to-edge timing and k = 2 for a center-to-edge timing. The full-diameter time is t = k(t1 + t2 + t3) / 3.
The measured true field is:
theta = t × (360 / 86164.0905) × cos(delta)
- theta is the true field in degrees.
- delta is the star’s declination in degrees.
360 / 86164.0905is the sidereal angular rate in degrees per second.
The calculator also reports arcminutes, which are often more convenient for comparing eyepieces. The range of the three recorded times, divided by their mean, is the trial-spread percentage. It is a practical check on stopwatch timing and whether the path was truly centered.
Turning the measurement into eyepiece figures
Magnification is M = F / fe, where F is the effective telescope focal length and fe is eyepiece focal length. From the measured field, the calculator estimates an effective field-stop diameter with D = F × theta × pi / 180. It also gives an approximate apparent field, Aeff = M × theta, and compares the measurement with the common catalog estimate Ar / M.
Use the effective focal length of the whole system. A Barlow, reducer, or corrector changes both magnification and the inferred field stop. A measured field that differs from apparent field divided by magnification is not automatically an error: apparent-field ratings, distortion, vignetting, and the location of the physical field stop all affect that comparison.
What the timing cannot correct
The star must follow a diameter through the center. An off-center chord is shorter than the diameter and returns a field that is too small. Tracking left on, mount movement, reaction time, or an incorrect declination can be larger errors than the arithmetic. Near either celestial pole, the cosine correction makes small timing and centering errors much more important; a star near declination 0° is usually the easier choice.
The field-stop result is an optical inference, not a caliper measurement. The small-angle conversion is least exact for very wide fields, and a camera or finderscope may need its own effective focal length and a field dimension chosen deliberately.
Worked example
Suppose three edge-to-edge crossings of a star at declination 0° take 240, 242, and 238 seconds through a 1200 mm telescope with a 25 mm eyepiece. The mean full-crossing time is 240 seconds and the trial spread is 4 seconds, or 1.67% of the mean.
The sidereal calculation gives 240 × (360 / 86164.0905) × cos(0) = 1.0027°, or 60.16 arcminutes. Magnification is 1200 / 25 = 48x. The inferred field stop is 21.00 mm and the approximate apparent field is 48.13°. Against a rated 50° apparent field, the predicted true field is 50 / 48 = 1.0417°, so the drift result is 3.74% lower.
Common questions
How do I perform an eyepiece star-drift test?
Pick a reasonably bright star, center it in the field, switch tracking off, and start timing as it reaches one edge. Stop when it reaches the opposite edge, then repeat at least twice. A star near the celestial equator keeps the correction small and is easier to time.
Why must the star cross through the center of the field?
A centered path is a diameter of the field. Any path above or below the center is a shorter chord, so its drift time produces a field that is too small. Recenter before every trial rather than trying to correct an off-center timing later.
Why does star declination change the drift rate, and which stars work well?
The apparent east-west drift is proportional to the cosine of declination, so it slows away from the celestial equator. Stars with declinations near 0° need little correction and give more robust timings. The calculator accepts either positive or negative declination, but warns above 80° from the equator because the correction magnifies errors.
Why can measured FOV differ from AFOV divided by magnification?
Apparent field divided by magnification is a useful estimate, not a direct measurement. Eyepiece distortion, rounded catalog values, vignetting, and the actual field-stop position can all create a difference. A centered drift test measures the part of the sky you can actually see in that optical setup.
Can I measure a finderscope or camera frame with the same method?
Yes, provided you can time a star across a clearly defined horizontal field dimension with all tracking off. For a camera, use the effective focal length and treat the result as the measured dimension along the drift path. A rectangular sensor has different horizontal and vertical fields, so do not assume one timing describes both.
How many drift trials should I time, and do Barlows or reducers matter?
Three trials are enough to reveal a noticeably off-center path or stopwatch slip; more are useful when the spread is high. Enter the effective focal length with any Barlow, reducer, or corrector in place. Those accessories change magnification and the inferred field-stop diameter even though the drift measurement itself still gives the true field.