Exposure calculation with various OTAs

[mwg3656]

I have loaded(cloned) 3 different Imaging Systems each a different OTA and I get results that are counter-intuitive.
Exposures were set at 15 SNR, all use the same ZWO 294C pro (gain 200, Bin x 1) camera with no filter and all targets are M33. Only exposure time is provided (set up time not included). System listed below:
1. Celestron 11" RASA,           279mm aperture and 620mm FL,  f/2.2 results in exposure time of 158min.
2. Celestron C8 w/x0.63 reducer, 203mm aperture and 1280mm FL, f/6.3 results in exposure time of 94min.(note: FL is 2032mm x 0.63, f/10 x 0.63)
3. Meade series 6000 quad APO,    70mm aperture and 350mm FL,  f/5   results in exposure time of 30min.

This results are different that I would of thought. I would have thought that the exposure time would have been a function of f-stop, so system #1 would have the least time, system #3 in the middle and system #2 would have the longest time.

How is the exposure calculated when the OTA changes? Is there a parameter I need to double check?

[mwg3656]

I found the problem and solved it.

Systems #2 and #3 were at a location with a sky of 21.9 mag/arcsec2 and system #1 was at a location with a sky of 19.6 mag/arcsec2. This difference in sky brightness would have a significant effect on SNR calculations.