Lovely image, as usual. I'm assuming that it was taken with the Tak 90 at f/4.5 with a resultant 3.97 arcseconds per pixel as that's the widest field of view you mention in your book. If I opt, as I hope to be able to do, for the KAF-6303E and its 9μm pixels then I'd be getting about 1.9 arcseconds per pixel. The pixel count is almost identical to that of the SXV-M25C so I'd have to mosaic 4, or more realistically 5 following the advice in your book, images to get the same sky coverage. If the seeing was good enough that mosaic should have twice the linear resolution, of course, but that is still potentially a lot of extra time at the telescope.
But, and you knew I was going to offer a but didn't you
, the TEC 140 captures photons at about 2.4 times the rate of the Tak 90. Those photons are spread more thinly (1.9 arcseconds per pixel compared with 3.97 arcseconds per pixel) by a factor of about four which would imply that the Tak 90 imaging system would be capturing photons at each pixel at about 1.6 times the rate of my intended TEC 140 system. That would certainly seem to be a win for the Tak 90 when going really deep when the desired signal is only just creeping above the noise from the sensor though maybe there's a little compensation from the slightly higher QE of the KAF-6303E and, of course, the fact that it can use all of its pixels when narrowband imaging while the Bayer matrix of the Sony ICX453AQ chip is, when Hα imaging, only using one in four of its pixels
I think the problem with so many discussions about the "f-ratio myth" is that one never ends up comparing like with like. I believe I can argue quite successfully that if one put some mythical sensor with 18μm pixels and a 6MP count behind the TEC 140mm (objective diameter) f/7 'scope then it would, to a first approximation, outperform the Tak 90 plus a monochrome (non Bayer) equivalent to the SXV-M25C by that factor of 2.4 per pixel
, as implied by the different objective lens diameters, as each pixel on the two systems is seeing the same area of sky. But 18μm pixels and a 6MP pixel count would not only be horrendously expensive in it's own right but it would need 79mm filters!!! But it would
image at 2.4 times the rate of the Tak 90 at f/4.5 despite only being at f/7.
My aim in putting together my new system is to achieve about a 2 arcsecond per pixel resolution. Within the constraints of a reasonably high pixel count and 50mm filters that means employing a sensor with pixels of between 5.4μm and 9μm if I'm reading the various spec sheets correctly. That implies a focal length of between 557mm for 5.4μm pixels and 928mm for 9μm pixels. After that the choice would be to go for the largest aperture I can afford (at good quality). The only downside of my final choice of the TEC 140 f/7 with it's 980mm focal length is that, after discussion with Yuri at TEC, I know that it is not really compatible with focal length reducers but a 0.8x reducer isn't going to make such a huge difference anyway.
But hey, this is all theorising on my part and all the theory in the world can be disproved by practical experiment. Maybe I'll be eating my words (or should that be "eating my keyboard"
) in about six months time. I'm particularly interested in not only imaging the various emission lines (the usual suspects) but also imaging at the adjacent narrowband continuum wavelengths where I expect only stars to be visible thus gaining significantly from the 140mm aperture. The aim is then to combine that data in various ways during PP and hopefully achieve distinctiveness
in the process. This will mean lots of time spent on relatively few objects so I'm going to have fun exploring the universe however it goes even if I can't go incredibly deep.
P.S. I hope you'll forgive my rather laboured approach above and I certainly hope I've made no basic errors in logic or, indeed, "sums". I know that with your academic background this is is "Basic Optics 101" to you but I have to go a little more slowly. f/7 compared to your own f/4.5.