I think that afocal is a great way to allow people that don't have fast scopes and insufficient in-travel to use a reducer to get a brighter view but I too think that in this test, the prime focus image actually presented a view that was brighter than the afocal view with the 27mm Pan and that kind of supports my only experiment with afocal.
Take a look at the limiting magnitude. In particular, look at the prime focus view, and go to the Swan's tail, then go up from the tail to about half way to the top of the frame. Here, you will see a string of one brighter and four fainter stars in a line. Also, if you look you will see that there is a little star slightly to the left of the brighter star and the first dim star in the chain.
Now, look at the same chain in the 27mm afocal picture. Here, the stars are much less sharp and just don't show with the same authority as in the prime picture. Note also that the little faint star (which should now be to the right of the string) is almost gone.
Another area is the swan's beak area. The sharpest view (and I think the best view) of this area is the one shown at prime focus. (not counting the 7nm filter view, which of course we don't have anything to compare with).
And of course the nebula is simply better in the afocal view vs the 27mm Pan and I think not any worse than the view in the 55mm pan due to the reduced scale in that eyepiece. Now, I do have to say that the 41mm Pan image is I think the best of the afocal views, but the real star here is the 7nm filter.
I looked over all of these pictures, and I have to say this. I can see everything in the prime focus image that is shown in any other of the unfiltered images (except of course what does not fit into the field) and I actually think that it is one of the most pleasing views.
When I tried afocal, the difference at the eyepiece was just not what I had expected I had expected the afocal view to have brightness closer to to the view in my f/2.8 scope and it was much closer to the f/4.9 view (though effective was f/3.2) My target was the Crescent Nebula, and I was just disappointed that the result was not better than it was. It was brighter though, so I am not disputing that afocal works, but the question then is how well does it work. Brighter is brighter though,and in my experiment, the view was slightly brighter, and because of that, I think afocal has a place in NV astronomy. I think a 55mm Plossl would have made it worth it, but for me, the 42mm eyepiece simply did not offer a brightness increase I had hoped for. As with above, I do not want to loose to much image scale because then the views would not be so much larger than in my 6" f/2.8.
I love the comparison though, and I do encourage others to do some afocal work as well. I think this is a great contribution and I am eager to hear/see yours and others work and opinions, and I appreciate your and Peter's efforts to help us better understand afocal result and expectation.
Edited by Eddgie, 18 October 2017 - 11:38 AM.