How much faster is F5.3 compared to F7.5

[vlaiv]

14 minutes ago, Luna-tic said:

8" Edge HD with a F/7 reducer, compared to a 80mm refractor using a 1.5x Barlow (in this case a William Optic GT81 which is f/5.9  in its native focal length). Given that the imaging camera is the same for either setup (pick whatever pixel size you want), which would be the better one to image with for deep space objects?

Actual usability of either of two scopes will depend on what camera will they be paired with and on what sort of mount.

Direct comparison between the two without regard for above detail would go like this - since you are using the same pixel size on both setups (with no intent of binning I suppose) - you can simply compare F/ratios like

F/7 vs F/5.9 x 1.5 = F/8.85 (assuming that barlow indeed operates at x1.5)

Roughly speaking, image acquisition time for same SNR will be 7*7 / 8.85 * 8.85 = 49 / 78.3225 = ~0.62562 or

Edge HD will achieve same SNR in roughly 62% of imaging time.

However, I would not base choice of setup on this alone. I'd decide on type of target, then determine imaging resolution suitable for that target that can be achieved with my gear and the based on that if both scopes can manage the same - I'd choose one with larger aperture for imaging.

Say that you want to image nebulae at 3"/px - then you'll probably want to use 80mm scope without a barlow, but if you want to image galaxies at 1.5"/px - then you probably want to use Edge HD and look into binning the data from your camera.