ngc 7331 exposure confusion

[alacant]

Hi everyone. Sorry to bring this one up again but I'm struggling. I've a 6" f5 and a 6" f8. I want to shoot ngc7331. It's small in the f5 but quite acceptable in the f8 [1]. In both telescopes, the galaxy fits onto the sensor, so here goes:

1. This is not like a camera in daylight: I am not stopping down the f5 to f8; the aperture remains the same and therefore...

2. The same amount of light from the galaxy is collected by both, so...

3. The exposure which gives a good image in the f5 will give a similar image in the f8

Am I close? Cheers, TIA and clear skies

[1] Even though it's lighter than the f5, it's gonna be a hassle mounting the f8 to be able to image with it. If it's a no-no, I'd rather use the time imaging something bigger.

[ChrisLX200]

The difference between the two is image scale, in the f/8 scope the light will be spread across more pixels - hence lower signal/per pixel, hence less bright.

ChrisH