The formula that it implements is FOV = 2 arctan (x / (2 f)), where x is the diagonal of the film. The FOV is measured across the frame’s diagonal, and is therefore smaller across the horizonal dimension, and even smaller across the vertical dimension.

The true field of view is the number of degrees your eyepiece shows you when you use it with your telescope. To calculate this, you divide the apparent field of view by the magnification.

Take the two and use this formula: (135.3x D) / L. This will give you the field of view (in arcminutes) for your telescope and imaging device. Where D is the size of the Chip, L is the focal length of your telescope.

Simply enter the telescope’s focal length, the camera’s pixel size and your sky’s seeing conditions to determine if they are a good match 🙂 A few notes: We are assuming OK seeing is between 2-4” FWHM and a resolution between 0.67” and 2” per pixel is the sweet spot.

The angular magnification M for a telescope is given by M=θ′θ=−fofe M = θ ′ θ = − f o f e , where θ is the angle subtended by an object viewed by the unaided eye, θ′ is the angle subtended by a magnified image, and fo and fe are the focal lengths of the objective and the eyepiece.

Focal length isn’t the only player on the field, either. Smaller imaging chips further restrict the field of view, while larger sensors increase it. Taking this the other direction, we can get a larger/wider field of view by either getting a camera with a larger sensor, or a telescope with a shorter focal length.

Since 1 radian = 57.3 degrees, and 1 degree = 3600 arcseconds, the resolution is 0.000000069 radians x (57.3 degrees/radian) x (3600 arcseconds/1 degree) = 0.014 arcseconds. One thousand milliarcsecond = 1 arcseconds, so the resolution is 0.014 arcsecond x (1000 milliarcsecond / arcsecond) = 14 milliarcseconds.

To bring up a field of view, select your telescope make and model, then select your camera make and model. You can select your eyepiece or binoculars by clicking ‘Mode’. Click on ‘Targets’ to choose the object you wish to view.

The scale changes according to the ‘seeing’ conditions entered. The calculator uses maths to determine its result, it is not influenced by brand or sales spiel. If the result suggests your existing telescope / camera combination is less than ideal please don’t shoot the messenger.

Loading a custom FOV display from DSS can take a few minutes to load. Help us to grow by adding additional equipment to the database. When adding an eyepiece or binocular, please don’t include the magnification or aperture details in the model, this will get added automatically.

Pixel-scale (the subject of this calculator) is arguably the most important factor to consider when choosing a camera for your telescope, but there are others: sensor size, sensitivity, dark current, read noise, anti blooming, etc. They all play a part. Please don’t base your purchase decision on pixel-scale alone.