How is field of view calculated from sensor size and focal length?

Field of view in degrees equals 2 × atan(sensor dimension ÷ (2 × focal length)). The width and height are computed separately from the sensor's two physical dimensions, then converted to arcminutes for deep-sky work.

Where do I get a target's angular size?

Catalog data lists apparent size in arcminutes. For example, the Andromeda Galaxy (M31) is about 178 by 63 arcminutes, while the Ring Nebula (M57) is only about 1.4 by 1 arcminutes. Use the catalog value, not the bright core you see visually.

What fill percentage should I aim for?

Roughly 40 to 80 percent of the frame is a comfortable target so guiding drift and field rotation do not push the object off the edge. Above 85 percent the framing is very tight; well below 40 percent the object looks small and a longer focal length helps.

What does image scale in arcseconds per pixel mean?

Image scale equals 206.265 × pixel size in microns ÷ focal length in millimetres. It sets how much sky each pixel covers. Most deep-sky setups work well around 1 to 2 arcseconds per pixel under typical seeing.

Why does orientation matter?

Elongated objects like galaxies and many nebulae fit better when their long axis is aligned with the sensor's long axis. The tool checks both upright and rotated framing so you know whether to shoot landscape or rotate the camera to portrait.

Astrophotography Target Framing Calculator

A deep-sky object only photographs well when it actually fits inside your camera’s field of view with a little room to spare. This calculator combines your sensor size, focal length, and the target’s catalog angular size to tell you the field of view, how much of the frame the object fills, which way to turn the camera, and your image scale.

How it works

Field of view comes from the sensor dimensions and focal length, and image scale comes from the pixel pitch:

FOV (deg)        = 2 × atan(sensor_mm / (2 × focal_length_mm)) × 180/π
FOV (arcmin)     = FOV (deg) × 60
fill (width)     = target_width_arcmin / FOV_width_arcmin
fill (height)    = target_height_arcmin / FOV_height_arcmin
image scale ("/px) = 206.265 × pixel_size_µm / focal_length_mm

The width and height are evaluated independently, then the tool checks both the upright orientation and a 90° rotation to see which one frames the target’s long axis along the sensor’s long axis.

Example and tips

An APS-C sensor (23.5 × 15.6 mm) at 530 mm focal length gives a field of view of roughly 152 × 101 arcminutes. The Andromeda Galaxy at 178 × 63 arcminutes overfills the width in landscape but fits comfortably when the camera is rotated to portrait — exactly the kind of decision this tool surfaces. Keep your fill under about 80 percent so polar-alignment drift and field rotation do not crop the object during a long imaging session, and use the image-scale output to avoid heavy oversampling on short focal lengths.