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Telescope + camera field of view and sampling calculator

Before pairing a camera with a telescope, two numbers decide everything: the patch of sky your sensor actually covers, and the sampling — how much sky each pixel sees. This tool works both out from focal length, sensor size and pixel size.

Engraved near the focuser or on the tube label (e.g. “D=150 F=750”).

1 with no accessory; “2” for a 2× Barlow; “0.8” for a 0.8x reducer.

Camera spec sheet. APS-C: 23.5 × 15.6 mm; full frame: 36 × 24 mm.

Same spec sheet, second dimension.

Spec sheet, “pixel size” or “pixel pitch” line (e.g. 3.8 µm).

Field of view — width

1.80°

Field of view — height
1.19°
Sampling
1.05″/pixel

Well suited to deep sky under typical seeing (2 to 4″).

The method

The sensor is a small window placed at the telescope’s focal plane: the longer the focal length, the narrower the patch of sky that window cuts out. The field comes from exact geometry (arctangent), not the small-angle shortcut that drifts beyond a few degrees — which is exactly the case with camera lenses.

Sampling converts one pixel into an angle on the sky: 206,265 arcseconds per radian, times pixel size, divided by focal length. That figure tells you whether your setup is built for deep sky or planetary work.

A Barlow multiplies the effective focal length, a reducer divides it: the factor applies before everything else.

In the formula: F is focal length (mm), k the amplification factor, d the sensor dimension (mm), p the pixel size (µm).

field (°) = 2 × arctan(d / (2 × F × k)) · sampling (″/px) = 206.265 × p / (F × k)

Good to know

  • A reducer only delivers its nominal factor at its design back-focus distance: mounted too close or too far from the sensor, the real factor drifts.
  • Sampling finer than the seeing buys nothing: below 0.6″/pixel in long exposure you are recording blur. Aim for roughly a third to half of your local seeing (2 to 4″ at most sites).
  • The computed field is geometric: the telescope’s fully illuminated circle can vignette the corners of a large sensor well before that limit.
  • 2× binning doubles the sampling: a simple way to rescue a focal length too long for your sky.
  • Framing landmarks: the Moon spans about 0.5°, the Andromeda galaxy about 3°.

FAQ

What is a good sampling value for deep sky?

Between 1 and 2″ per pixel you are comfortable under typical 2–4″ seeing. Below 0.6″/pixel, the extra detail is eaten by turbulence in long exposures; above 2.5″/pixel stars start looking blocky, which remains acceptable for wide-field work.

Why does planetary imaging deliberately oversample?

Planetary imagers shoot thousands of very short frames and keep only those captured during moments of steady air (lucky imaging). The limit is no longer average seeing but the optics’ resolving power: 0.1 to 0.25″/pixel with a Barlow is the target, which this tool flags as “oversampled” — that is expected.

Does a smaller sensor magnify the image?

No. At the same focal length, the object is exactly the same size on the sensor; a smaller sensor simply frames tighter around it. The apparent “zoom” is cropping, not added detail — detail depends on sampling and seeing.

Can my sensor be too large for my telescope?

Yes, in two ways: the fully illuminated circle (corners receive less light — vignetting) and optical correction (Newtonian coma, refractor field curvature), which degrade stars far from the axis. A matched corrector often fixes the second problem, not the first.

Will the whole Moon fit on my sensor?

The Moon spans about 0.5°. Compare with the displayed field: on APS-C it fits whole up to roughly 2,500 mm of focal length; on a small planetary sensor you often need to stay under 1,000 mm or shoot a mosaic.

Degrees, arcminutes, arcseconds: how do they relate?

One degree is 60 arcminutes, one arcminute is 60 arcseconds. A sensor’s field is counted in degrees or arcminutes; sampling is counted in arcseconds per pixel, since one pixel only sees a tiny patch of sky.