Sorry, almost forgot. Lens speed ( or F-number ).
Simply put, this is how much light the lens lets in. While the true "light throughput" is actually known as the T-number, it's usually close to the F-number on better tubes and correlates, so we just use F-numbers.
OK, best is a F-number of 1.0 or even 0.95.
F 1.4 only has half the light of a F1.0 lens.
F 2.0 has a quarter of the light of a F1.0 lens.
F 4.0 has a sixteenth of the light of a F1.0 lens.
F 8 has 1/32 of the light of a F1.0 lens.
F 16 has 1/64 of the light of a F1.0 lens.
To understand a F number - the amount of light it will let through generally, compared to F1.0 is about 1/F*F - So if you get a F3.6 lens, the light it lets through is about 1/12th.
Now, in night vision terms, a tenth of light is the different between clouded-overcast starlight and starlight... Or starlight and a little bit of moon. If you've used NV, you will understand that this is a HUGE difference.
At F2.0 you've already lost 75% of the light you could have used if you had a F1.0 lens...
So with Cascade, as with Gen3 and ALL tech, the lens quality is critical to the outcome. You can only amplify the light so much, so getting as much light to the tube as possible is important.
That is why I recommend F1.4 or faster...
Now if you don't know the F-number of the lens you are using, just make an approximations. Measure the aperture of the inlet glass lens in millimeters and divide by the focal distance of the lens -
eg, for a 50mm lens with a 50mm diameter front lens, the F-number is approximately 1.
If the inlet lens is about 25mm across, then the F-number is approximately 1.4
In practice, you really don't want to go much smaller than that and if you get a variable aperture lens, set different apertures and see how much light you lose and how much difference it makes under really dark conditions. You'll be surprised. :)