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Your telescope focal length in mm = 18 x 25.4 x 4.5 = 2057.4mm
Field of view (deg) = linear dimension of sensor x 57.3/focal length of telescope.
E.g. if you had full frame DSLR (36m x 24m sensor) your field of view would be 36 x57.3/2057.4 = 1 deg on long axis and 24 x57.3/2057.4 = 0.67 deg on short axis. You can work out diagonal using Pythagoras. Sqrt(1x1+0.67x0.67)=1.2 deg.
If you don’t have the sensor dimensions but do have the number of pixels and pixel size then first multiply these out to get the linear dimension.
E.g. 6 million x 4 million pixels of 2.2 microns each gives 6000000x2.2/1000000= 13.2mm x 4000000x2.2/1000000= 8.8mm
Field of view = 13.2x57.3/2057.4=0.37 deg by 8.8x57.3/2057.4=0.25 deg. Not enough for a full lunar disk so either use a bigger sensor or shorter focal length telescope (for example).
The 57.3=number of degrees in a Radian. A Radian = The length of the radius of a circle around it’s circumference. Don’t really need to know any of that only the formula.
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