The focal length ratio was common when people guided manually. With auto guiding and sub-pixel guiding, the ratio of image scale is far more important. You must take into account the cameras used for guiding and imaging. Compute the image scale off your guider and imager at their respective focal lengths and make sure the ratio does not exceed about 5:1.
Thanks, very useful! Hope I understand this correctly for my gear, could you confirm this is correct?:
I currently have a the 50mm orion guidescope, it has a focal length of 162mm. In combination with the asi120MM my I got 4.77" per pixel.
My first scope is an 80/480mm APO, I used it with the asi1600MM pro, this combi gives me 1.65" per pixel - so that would be less then 1:5. Is that interpretation correct?
I also have an Edge HD 8", with a reducer, the focal length is about 1440mm, with the asi1600mm pro, I'm getting .55" per pixel. So now I'm getting 1:8.6, so the difference here is more than 1:5 and I should consider a guide scope with a longer focal length, that will get me to about 2.5" per pixel...or a camera change...(but the asi120 has a pretty small pixel size already). So my new guidescope should have a focal length of about 310mm...
I'm struggling with this...a bigger guidescope also means adding more weight to the rig...I have an EQ6R-pro so I should be fine....