Can we come up with better metric than hours?

Maxim:
1 hour on f2 ≠ 1 hour on f7
1 hour of 120 30sec exposures ≠ 1 hour of 6 10min exposures
1 hour on OSC ≠ 1 hour on Mono
etc…

Should we maybe come up with something better than hours? 🙃

I think I get your point, but I'm not sure there is a good way to solve this. You would also need to somehow factor in bortle scale, sensor sensitivity, moon position and filter bandwidth, and maybe other things.

Stating total exposure in hours is probably the best common denominator that makes sense.

D. Jung:
Stating total exposure in hours is probably the best common denominator that makes sense.

It’s definitely the best to show the effort, but  if we imagine the statement “1000h image of M31 taken with f15 refractor”, here I’m automatically skeptical about the relevance of the hours to the final result. That’s why it’d be cool to have some other units, similar to golf handicap or something 😅 but as you said there are probably too many factors to consider

On the same topic, I do have further (fun) KPI suggestions: 

• "cost of equipment" or "actual vs known to wife cost"
• "Peak blood pressure during setup"
• "Number of total satellite streaks in subs"
• "Hours of lost sleep"
• "Time in month waited for clear night to take image"

Maxim:
1 hour on f2 ≠ 1 hour on f7
1 hour of 120 30sec exposures ≠ 1 hour of 6 10min exposures
1 hour on OSC ≠ 1 hour on Mono
etc…

Should we maybe come up with something better than hours? 🙃

Better than hours for what?  Are you trying to compute signal strength?  If so, perhaps you should learn more about radiometry.   You could then compute the signal strength in Watts/m^2 and then multiply that by the area of your pixel times the responsivity of the sensor in photo-electrons/Watt to arrive at photo-electrons from each sensor, which will of course vary over the field depending on the irradiance distribution of the object.

I'm afraid that you'll never get a good answer without first asking a good question.  What is it that you are trying to improve?

John

Thanks for all the great answers! I guess the signal strength or SNR is closest.
I think I was referring mostly to the hours multiplied by sort of "system potential", so the final unit reflects time, equipment, conditions, rather than SNR of the final image.

If you want to compare photons to photons with some account for sky noise between setups for broadband images, it can be approximated by the following: 

S - Score
p = pixel size in microns
F = F ratio
T = total integration time
SQM = sky brightness in (magnitude / arcseonds^2) assuming a limit of 22. 

S = (p/F)^2 * T * 2.512^(22-SQM)

Aiming for a score of 1 should provide a relatively noise-free image if not stretched over aggressively. For dark nebula with high contrast 2+, and for faint tenuous IFN, 5+.

This assumes that between setups the same QE (quantum efficiency), filter bandwidth and transmission, and optical transmission/reflectivity. 

Astrobin can have all of this data if someone provides all of the gear they used, as well as an SQM estimate. SQM could also be approximated by Bortle and average moon % as well. 


To account for mono LRGB imaging you could use the luminance as the base integration time + a synthetic luminance integration time. Maybe the following:

T_eff = effective integration time
T_lum = luminance integration time
T_R = Red filter integration time
T_G = Green filter integration time
T_B = Blue filter integration time

T_eff = T_lum + (T_R + T_G + T_B)/3


An idea for color richness:

If you care about color richness you could use (T_R + T_G + T_B)/3 for integration time, and for color cameras it would just be (T / 1.2). I'm using 1.2 as an approximation for the Bayer matrix channel spillover (i.e. where the red filter picks up some 'blue light' or the blue filter picks up some 'red light'). Mono imaging with RGB filters with distinct cutoffs provides more color distinction and richness over a color camera. 


Narrowband: 
If someone can think of a way to include how narrowband can improve contrast and boost the score as a term I'm all ears. My initial attempts at that seem incredibly complicated since the QE profile of cameras, the spectrum of the sky, the filter bandwidth, and the f-ratio of the system need to be taken into account.