What role does focal ratio play in observing at different magnifications?
Telescopes with high focal ratios produce great planetary images as they “maximize” magnification of even the most powerful eyepieces. However, you need to be careful not to magnify the object beyond the realistic range for the sky conditions since magnification decreases surface brightness. While telescopes with high focal ratios are primarily planetary telescopes, those with lower focal ratios allow us to view deep sky objects better. These ratios “minimize” magnification of even the least powerful eyepieces and, with sufficient aperture can produce bright, wide images of deep sky objects.
