Hi Glen,
This is one I have used....
https://www.scantips.com/lights/fieldofview.html
1. Input
Focal Length
2. Select
Sensor Size Option that suits or input exact size if not listed- 1 to 9 with button on left . (Usually 1,2,3 or 4 used)
3. CLICK
Compute 1-9
4.
Observe FOV results in decimal Degrees on Right Hand Side Top. There are also some field width results possible if one inputs subject distance (I don't use it. Move relevant to terrestrial and could do the same knowing the FOV with some trig)
Regarding your requirement for a 4 x 3 degree FOV, and as per the attached notated ScantipsFOV excerpt (for your case), you could also use a Full Frame camera with (
36mm x 24mm SensorAspect) with your existing
442mm Focal Length refractor/reducer and that would give you a
4.7 x 3.1 degree FOV
Or at the sort of focal length you're interested in use a 300mm lens
Best
JA