Thanks for the positive feedback, everyone.
Some more details and background:
A longstanding photography goal of mine has been to assemble a truly portable (everything fits in one large camera backpack that can easily be an airline carry-on) but capable astrophotography package. I'm still refining the package but here are some of the components I'm leaning towards after quite a bit of testing:
1) Nikon D810. The resolution and dynamic range at low ISOs ensures that the sensor won't be a bottleneck for IQ. Of course light sensitivity is not the best, but that's what #3 is for.
2) Either the Sigma 150-600S or the Nikon 300/2.8 + TC20eiii. The Sigma is probably sharper at 600 mm f/6.3 than the Nikon combination at 600 mm f/5.6 or f/6.3, BUT I suspect the Sigma has more coma at the same apertures than the Nikon set up, and I can't easily fix coma in post. More on this point later when I try to shoot the same nebula with the Nikon combination (weather has prevented me from doing so recently). The Sigma TC-2001 teleconverter makes an amazing 1200 mm focal length lens, but I'm not convinced the resulting image has any more information in it, and 1200 mm introduces a lot of challenges with respect to tracking, wind sensitivity, and tiny aperture. It's possible the TC-1401 will be better (840 mm); will test later.
3) The iOptron SkyGUIDER (NOT SkyTracker) equitorial mount to cancel the earth's rotation and thereby enable very long exposures. With care, this mount can support the D810+ either of the lens combinations in #2. In fact the motor is the same as iOptron's ZEQ25 mount (~$900), which is rated for 27 lbs, even though the SkyGuider is rated for only 11 lbs + 7.7-lb counterweight. I've made slight modifications of the mount to improve practical use in the field and more robust interfacing with the ball head on which the camera is mounted. The iOptron massive battery counter-weight is perfect, and serves the dual purpose of counter-balancing your camera+lens, and powering all the equipment, including your laptop if you make the proper adapter cable.
4) A 50 mm Orion guide scope, the Orion Starshoot Autoguider CCD, and a tiny laptop running the freeware PHD2 (I use a MacBook Air 11"). This combination images any star near your target and does real-time correction on the equatorial mount to make the tracking as accurate as possible. It can make the precision of the equitorial mount absurdly good if properly set up, which is crucial because a key to good astrophotography is long exposure times. You cannot get very sharp images over long exposures (>10 s) that are up to the potential of #1 and #2 through either a hand-guided mount, or even through a simple equitorial mount that is not computer-corrected. I've tried.
5) A sturdy tripod and two ball heads. I use the Sirui M3204x, which is superb, especially for $370, rather than a traditional short, heavy astrotripod, and the Sirui K-30x ballhead, which is also excellent and up to the challenge of keeping a 600 mm lens pointed at an object many lightyears away for minutes at a time. The second ball head is less important-- this is what you mount the relatively light guiding scope to. It's important that your whole rig is as stiff as possible. I also found that putting Celestron vibration suppression pads under the legs of the tripod seems to improve IQ as well. When you are shooting stars at 600+ mm focal lengths, even walking near your tripod can be a problem.
Aiming was not that difficult, thanks to the zoom lens. You can aim by eye, zoom out to 150 mm, take a test shot, then relate the brightest live-view stars to the map produced by your test shot as you zoom to 600 mm. It also helps to have StarWalk or a similar iPhone app handy to orient you if you aren't familiar with the night sky and the constellations.
I focused manually using live view on any bright star, assuming that an object 1,400 light-years away versus 5,000 light-years away is more or else at the same distance
Incidentally, on my Sigma 150-600 S the optimal focus setting is well before the "infinity" marker on the barrel.
No filters or TCs for the Orion nebula shot. Stacking and post processing was done in Nebulosity and Photoshop, only to crop and adjust contrast and gamma curves (not to add star diffraction spikes, which are natural in this image. No noise reduction, which was naturally pretty low thanks to stacking more than a dozen 30-second exposures at ISO 250.
Overall I'm pleased with how this first deep-sky object I've ever photographed turned out, and I'm hopeful that better results will be forthcoming as I learn and test more. All of the above fits in a backpack with room to spare, which will be great for trips to less light-polluted areas. I should also note that beyond the equipment that many serious amateur and pro photographers already have, everything above (mount, scope, auto guider, battery) costs less than ~$1,000 total, which makes it relatively inexpensive to get into astrophotography. Plus you get to use those big lenses for something very different!
