Astrophotography Field of View Calculator: Will It Fit?

Before you spend a clear night on a target, it is worth knowing whether it will land in the frame at all. Enter your telescope or lens and your camera, and this works out the exact patch of sky the sensor sees, the arcseconds each pixel covers, and how thirteen popular targets would frame up. The mistake that wastes most first sessions: the Andromeda Galaxy is nearly six times wider than the full Moon, so a long focal length crops it rather than showing it.

What the two numbers actually tell you

The field of view is geometry: sensor size divided by focal length. Shorten the focal length or use a bigger sensor and you see more sky. The image scale, in arcseconds per pixel, is the one that decides how much fine detail you can record, and it depends on pixel size rather than megapixels.

British seeing blurs a star into a blob roughly 2 to 4 arcseconds across on most nights. You want about two to three pixels across that blob, so an image scale near one third to one half of the seeing figure is the sweet spot. Go far below it and you are spreading the same photons over more pixels for no extra detail, with noisier subframes and longer downloads. Go far above it and stars turn into small squares. That rule is for deep sky exposures only: planetary and lunar imagers deliberately oversample and then throw away the worst frames.

Rough guide by target

• Big nebulae and the Andromeda Galaxy want a wide frame: a short refractor around 250mm to 400mm focal length, or a 135mm camera lens.
• Most Messier galaxies and planetary nebulae suit 600mm to 1000mm, which is where an f/5 newtonian or a small SCT with a reducer sits.
• Planets, the lunar surface and double stars need the opposite extreme: 2000mm and up, a small planetary camera and lucky imaging from video, not long exposures.

If a target comes out too big for the frame, the answers are a focal reducer, a shorter telescope, or a mosaic of two or more panels. If it comes out tiny, resist the urge to crop hard at capture time; you cannot add detail the image scale never recorded.