Thanks for the pointers. I already ordered the reducer/field flattener that goes with this scope. I think that should help the vignetting I'm seeing right now with my 2" to 1.25" adapter + t-mount adapter + t-mount to EOS adapter.
Reducers/flatteners definitely help with corner performance. They do reduce focal length, though...so, if you have an f/7.5 scope, and use a 0.85x reducer, your going to end up with a 765mm scope. Wider scopes are great for nebula and imaging larger regions of the sky, they are a lot harder to use for galaxies, clusters, planetary work, etc. For those, you want something much longer, at least 1600mm, and for planetary, it's really best to have as much focal length as you can muster (regardless of the f-ratio...3000mm, 5000mm, even 9000mm is best for planetary work.)
I'm looking to get the Celestron Advanced VX mount to go with this. Any thoughts on that? Any accessories I should also pick up in one shipment? I heard the GPS unit is nifty, as is a polar axis scope. Anything else I should look into?
Celestron is not really my first choice for mounts. They make some excellent scopes, and are they only source of hyperstar capable scopes, but their mounts are generally a bit wanting, and you pretty much always have to go to Celestron for any support. The EQ5/EQ6 range of mounts are actually better mounts, and the support community for them is truly vast. The Orion Sirius is an EQ5 type mount, where as the Orion Atlas is an EQ6 type mount. The Sirius is a step above the Celestron AVX, but closer to it in price. The Atlas is one of the most popular and best supported lower end mounts in the world. It's $1400, vs. the $1000 for the Sirius, but it is really the lowest entry mount you probably want to go with for astrophotography.
Mounts like the AVX are just too low end to really do a good job for astrophotography. Mount capacity is a critical factor, as is the intrinsic periodic error. The AVX has a max capacity of 30lb, and it's periodic error (PE) is around 30-50" peak-to-peak (P2P). The Orion Atlas, on the other hand, has a capacity of 40lb and a (PE) of around 15" P2P. The periodic error is what is going to determine the minimum size of your stars as you track across the sky for long exposures. At 30" or more, the AVX is just not going to handle exposures of more than about a minute or so without really good guiding. The Atlas can handle unguided exposures of a few minutes, and is easier to guide than the AVX.
Capacity is the next most important point. It's best not to load up a mount with more than about half it's capacity if you are doing astrophotography, unless your using a real high end mount. At 30lb, your 23lb scope is already over 2/3rds the weight capacity, where as it is barely over half for the Atlas. At 2/3rds capacity (and even more, once you throw on a camera, and even more once you throw on a guide scope and guide camera, which are really going to be essential for tight stars with any lower end mount), the AVX is going to be extremely difficult to control and guide out errors for. For the size and weight of your scope, especially with a camera and guider setup, you want a mount that is at the very least capable of handling 40lb. A mount capable of handling 60lb would be best...but that gets you into the territory of midrange mounts, which cost around $2500-3500.
So, I very highly recommend the Orion Atlas. It's a very capable mount, with a phenomenal support community. It also works with EQMOD, which is a full open source, free total replacement software driver package that lets you ditch the hand controller and control your mount entirely from a laptop (once you really get into imaging, you'll learn you also need computer control software, such as BackyardEOS if your using a Canon DSLR for imaging, PHD2 for guiding, etc.) EQMOD is more capable and more flexible than the SynScan hand controller that comes with EQ5/EQ6 type mounts. The use of EQMOD also opens up the door for improving the Atlas, which is another somewhat unique feature...there is a hypertuning mod available (which cleans up the mount and gets rid of manufacturing crap left behind in the gears, which makes the PE worse, and regreases everything with high quality synthetic grease), as well as a number of belt and worm mods. Belt and worm mods can eliminate gears, reduce backlash issues, and otherwise greatly improve the performance of your mount to midrange levels for far less cost. (NOTE: To use EQMOD, you will need to get an EQDIR cable. They are about $45, but a standard USB to Serial cable costs almost that much anyway, so it is a very worth while investment.)
You really can't go wrong with the Orion Atlas (or any other EQ6 mount, like the SkyWatcher EQ6 SynScan, which is basically the same thing, just different seller.) Either way, the Atlas/EQ6 is a much better fit given that your scope already weighs 23lb, and that you are guaranteed to need to do guiding. You can pick up the Orion 50mm mini guidescope and SSAG guider for relatively cheap, and the weight of that setup is about as small as you can get for guiding. Without guiding, even with an Atlas, the 15" p2p periodic error is going to kill your chances for doing exposures longer than a couple minutes. Average seeing is 2-3", average star size is 1.5-1.8"...without guiding, your stars will eventually be around 10-15" in size...far too large.