While the concept of etendue comes at work in relation to microscopy illumination systems, I only recalled that it could be helpful in comparing astro imaging configurations when I read a Celestron white paper on their RASA astrographs, and again when CN poster rockstarbill posted the "Performance = aperture^2 * scale^2 * QE" formula, here: https://www.cloudyni...x455/?p=9639554.
In that post, and among the systems rsb chose to compare, the FSQ106 with the 0.72x reducer won, with an "Imaging Etendue" (QE factored in) of 37,064. Via private message, rsb shared a link to this thread, and the cool Google Sheets calculator by the OP. Thanks to you both! With such a nice tool, I plugged in one my Sigma Art lenses, the 105 mm f/1.4, to see how it would score (with my Canon 6D, for which I guestimated a QE of 50%). Wow, even stopped down a bit to f/2.0 to reduce vignetting, the imaging etendue was surprisingly high: 168,734! I then entered values for some of my other Sigma Art lenses, all at f/2.0. That got my attention: their scores were all an identical 168,734, irrespective of their very different focal lengths/diameters! That shouldn't actually have been much of a surprise, though, and certainly won't be to the more technical posters here.
On reflection, though, the "Performance = aperture^2 * scale^2 * QE formula" formula might suggest to those without optical training that the first term, aperture^2, is more valuable than it really is. This lead me to reformulate the basic etendue formula in terms that are perhaps more germane to astro-imagers.
This one is:
Imaging Etendue equals 424.36 * QE * pixel size^2 / focal ratio^2
For reflectors, you also have to multiply by 1 - secondary/primary ratio^2.
This and the QE factor aren't in the formal etendue calculation, but they apply for our purposes, hence the term "Imaging Etendue".
Pixel size is in um. The normalized version (QE = 100, pixel size 1 um, focal ratio 1.0) has an imaging etendue of 42,436.
From this perspective, imaging throughput on extended objects is all about high QE, larger pixels, and faster optics. Obviously, you can easily get into trouble here with an undersampled image, and similarly, the "right" field of view is completely target dependent. But I at least find this reformulation to be helpful.
Edit: Going back through the thread in detail, I see where jhayes tucson had already expressed it this way, albeit in longer form: https://www.cloudyni...ator/?p=9349503
All the best,
Edited by Coconuts, 14 September 2019 - 09:40 AM.