Peter wanted to take his new (to him) Lockwood Dob out last night to try it out and invited me to join him, which I happily did.
I am hoping Peter will do a writeup and I don't want to steal his glory on the new scope so I will not go into great detail but there is something I came away with.
Sadly, Peter could not reach focus with his PVS-7 and my Mod 3 was also about 20mm short. This meant we were forced to do all observing afocally, and this turns out to have been a very good thing. When we looked at the Veil for example, we were using the 41mm Panoptic and I was just floored that even though is scope is f/4, the image was just as bright a I could get using my 6" f/2.8 scope.
I guess that I did not realize it and maybe my logic on this is off, but using the Mod 3 at 1x and peering into the eyepiece gave an image scale that was about .7 times the scale would be if the NVD was used afocally. This means the energy concentration at the photocatode would then be the same as if we were using a focal reducer (which essentially does the same thing, which is to say that it squeezed the image into a smaller area on the photocathode so more photons striking a given area makes a brighter image).
If this is correct, then the 41mm Pan would have given the same image scale as he would get using a .73x focal reducer, and that would result in the same brightness at the photocatode as I get using the Boren Simon f/2.8. Does that sound right?
This is what I saw. I will not steal Peter's thunder by talking about every target we viewed, but when we were viewing the Veil, I was quite surprised at how much brighter it was than in my f/5 dob. f/4 Is going to be brighter, but not to the difference we could see last night. Again, the Veil seemed far brighter than it should be for f/4 and pretty much equally bright as it was in my f/2.8 scope. The veil was stunning! There are no other words. The amount of detail was easily the very best I have ever seen and just like with the Boren Simon, when we pushed off to areas around the primary nebula, there were other significant features to be found. In Arigua, we saw a nebula that did not show in charts (again, much like when using the Boren Simon) much the same way I do with the f/2.8 scope.
I did this comparison on several nebula, and in all cases but one I felt that the afocal view in the f/4 with the 41mm Pan was delivering an image that was about as bright as I was getting at f/2.8.
If this is correct it would mean that while there is little value going afocally once you get an eyepiece that is shorter in focal length than the 27mm eyepeice gives at prime focus, by going to longer focal lengths, you are getting the same function as you would get with a reducer, which is to concentrate the photons into a smaller area on the photocathode. Once you get shorter than 27mm eyepeice of the device, you now dim the image in the same way that you would if you Barlowed.
As a kind of confirmation to me, we did put in a 22mm Nagler, and here, it was like looking though my f/5 scope. It had lost enough brightness that I felt we stepped back a notch.
I was quite floored at how bright the images were, and if this really does give the same benefit as a focal reducer, then that would mean that by using a 55mm Plossl and afocal projection, I would get the same image scale in my 12" dob as if I used a .5x reducer and this would suggest that I would get the same brightess as an f/2.5 instrument would.
Is my understanding off here? While I dislike the mechanics of Afocal projection, If I could the same benefit of a focal reducer without having to cut the trusses on the dob, I would be interested in trying.
(There was one place where the 16" used afocally did not equal the Boren Simon for brightness and it was an oddity. We looked at the Crab Nebula, M1. Peter wanted to see it and since I had been doing comparisons, I slewed the scope over to it. Now as luck would have it, the previous target was stellar and I had the filter wheel turned to an empty slot. When the scope stopped, BANG, there was the Crab Nebula. I have looked for it in the past, but always with an H-alpha filter, and it has been disappointing because I was never able to see it. Of course not having the filter in place I thought it would be better with the filter, but clicking in the 12nm dimmed it considerably, and the 7nm just about estinguished it. Peter brought the 16" to bear and for this one target, I would say that the 6" showed it as being brighter, but this could have simply been something to do with the very small size which probably must made it seem more intense than it was. Otherwise, when I compared the different nebula in the 16" with the 41 Pan used afocally, I think the image was just as bright as I was getting at f/2.8)
Anyway, if my logic is off here someone will help me understand why, but if not, I would say that afocal has a major role to play if there is no way to get a focal reducer to work with a scope. For eyepeices shorter than 27mm, this logic (if correct) would suggest that you get much of the same benefit.
I am eager to hear arguments to this because if this is correct, I am going to find a used 55mm Plossl and set up an afocal rig for it. I would still do most of my observing at prime focus, but I could see myself using this in the 12" dob if it gave the equivalent of a .5x reducer.
Not great conditions, but we still had a wonderful session. 70% Moon and so-so transparency, but that did not stop us from getting one of the best views of the Veil that I could ever have hoped to have seen. Fantastic.. I envy Peter's new scope but I will let him tell you about that.. LOL.
Edited by Eddgie, 01 October 2017 - 10:58 AM.