Here’s a concise review of the Astro+FOV+Calculator+Hot app (assumption: you mean an astronomy field-of-view/calculator tool named “Astro FOV Calculator” or similar). If that’s incorrect, I’ll assume this app variant and give a general review covering features, usability, accuracy, pros/cons, and recommendation.

For camera sensors, the calculation changes from circles to a rectangular grid using trigonometry: Field of view calculator for imaging? - Cloudy Nights

Separate calculations are required for both the horizontal and vertical axes. 2. Image Scale (Pixel Scale)

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Select a celestial object (e.g., M31, M42) to see how it fits in the field.

: Input the native focal length of your telescope (e.g., Focal Reducers/Barlows : Include any correctors. A reducer drops a scope down to , widening your FOV. 2. Choose Your Camera Sensor

The final calculation confirms that field of view is strictly inversely proportional to your telescope's focal length.