It's the focal length ratio. The 30mm guider has a focal length of what, 150mm?100?. Your 400 mm lens has a FL of 400. Ratio about 3.5 to 4.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired