Don and Curtis and ccs_hello and bwa,
In general, I have ruled out all small (1/3 and 1/2) format, large pixel sensors. With the tiny FOV, they work OK for close-ups of smaller DSOs but not so well for the larger, extended objects. I happen to like nice star fields surrounding the objects. Lodestars and Mallincams do not do stars well, at least for my tastes. Yep, that's right, I like pretty pictures. Nope, I don't want to spend a lot of time with astrophotography. Yep, I do want almost immediate results. Nope, I am not happy with the current state of EAA. Yep, I am stuck somewhere in the middle, between the current state of EAA and astrophotography. The Sony A7S is breaching this area, but like everything else, it doesn't do everything well. With that huge sensor area, it is fine for the larger, extended objects using relatively small, fast optics. But to maintain a relatively fast focal ratio to compliment the high sensitivity of those large pixels, it requires the larger apertures and/or longer focal lengths to do well with the smaller DSOs.
Here is an example of what I am talking about. Going back to jdbastro's A7S images, here are two examples that show the extremes.
First one is the Ring Nebula, a tiny DSO that requires either significant magnification or an equally tiny image sensor. Jdbastro uses a 10 inch OTA at a slow f/12 focal ratio to obtain sufficient magnification. The APS-C crop field of view is small at roughly 27 minutes x 18 minutes at 0.6 arcseconds per pixel, maybe a bit oversampled for EAA tastes. But this fine image scale will allow some more cropping (enlarging) before the image becomes pixilated. Very pretty, but still tiny and more stars than even I need. Fortunately, the 1.5 arcminute diameter Ring is bright enough, even at the slow focal ratio, to complete the image in 30 seconds.
The second example shows the other extreme. The APS-C cropped image contains both the Lagoon and Trifid Nebulae using a smallish, 3.5 inch OTA at a fast f/4.5. The field of view is huge at roughly 3.3 degrees x 2.2 degrees at a rather undersampled 4.3 arcseconds per pixel. The two nebulae are tiny in the image and would not fare very well with much more cropping (enlarging) before the image would get quite pixilated.
And here are the results. The Sony A7S does well on smaller DSOs as long as the optics are quite large, the larger the better to get the necessary magnification to fill up that huge sensor. At the opposite end, the Sony A7S does well with smaller, faster optics as long as the subject matter is large, extended DSOs surrounded by large star fields. Otherwise, plan on longer exposures to offset the slower focal ratios that will be required with smaller optics. The Sony A7S is ideally suited for very large optics that are also very fast... in other words, expensive glass and mounts.
When I get time, I intend to start one or more threads to discuss what I think is the Goldilocks zone for EAA systems, at least for me. Think medium… medium size sensors with medium size pixels and medium size optics with medium size mounts, all at a price/performance ratio that I can tolerate.
Edited by chasing photons, 28 June 2015 - 03:00 PM.