Ideal Exposures

[mickw]

I'm trying to understand exposure lengths and their bearing on subjects.  I've made a few assumptions and even resorted to maths (don't want to do that too often)

Example
Marks M81-M82
Scope f2.8 subs 5 mins
Me using similar camera but scope of f6.6
Would the assumption be correct that I would need a sub length of 12.5 mins (approx 2.5 stop difference in scope f no.) to achieve similar data acquisition ?
Similarly me using a scope of f15 would need 24 min subs (approx 4.75 stops difference)

Generally everyone does 5 min subs at various quantities with any kit - exception being M42 with its associated bright bits.
I appreciate that more subs the better to improve SNR

[Fay]

My ED80 is F7.5, I think F6.6 with focal reducer. I usually do at least 300 sec & prefer 600 secs. Depends on subject. eg globular cluster, perhaps around 120-180 secs would not need so much as a nebulae.

Also Ha filter would need more than a L

I have generalised

[mickw]

OK that makes sense with the glob. as they are just points of light, which also explains why my M44 was pretty boring.
So I would expect to do longer (180-240 or 300) to get the nebulosity around M45.

[Fay]

Yes at least 300 secs