Let me start by saying I probably did all this the hard way. I should have planned ahead better. I should have read more before I started rather than after. I spent a lot of time trying things that didn't work, until I found the things that did. I like to think it was the road less traveled, but I suspect many of us all go through the same thing.
There are many telescopes that are better suited for astrophotography.
There are even Newtonians that are designed for this.
But that wasn't what I started with. I started with a 10" f4.7 Dob, that I later moved to an equatorial mount.
When I started down this path everyone warned me that there were issues using a Newtonian telescope for astrophotography.
1. They don't have enough focus travel for DSLRs.
2. The large tubes create a lot of sail area and cause problems every time the wind blows.
3. you can never balance the off center weight of the camera correctly.
4. There is too much coma on the flat sensor of the camera.
Honestly there was a LOT of other stuff people told me, but most of it ended up being more myth and urban legend than practical advice.
The 4 listed above were the ones that had the most truth.
The wind and sail area is something you just have to deal with. I've seen some of the more serious people build portable wind breaks out of PVC and tarps. I plan to do this as well one of these days. I just haven't gotten around to it yet.
So, my solution for the time being is to always setup down wind of my jeep. It makes a decent wind break and I don't have the additional setup.
The issue of balance is largely exaggerated by the naysayers. Yes, Make sure you have a mount that can carry the weight of your scope and gear. Yes, you will need to balance your gear every time you set up. But If you are planning to do astrophotography, You are better off to be balanced with your camera in place and switch to an eye piece as needed than to balance for your eye piece and then switch to a camera. The weight of both cameras and eye pieces vary greatly. Setup for what you plan to use.
Coma is not just a Newtonian issue. It affects all types of telescopes. It's more common with lower cost telescopes and is more pronounced on scopes with a fast f stop(low focal ratio). Since many Dobsonians and Newtonians fall under this description they get a bad rap. This is why fast scopes with low coma are very expensive. If you zoom in on some of the stars in the corners of my images you see that they aren't as round as the same size stars in the center of the same images. This is coma. My scope simply wasn't designed to be used as an astrograph. It can be used, but I know and accept the limitations of my equipment. A number of companies sell coma correctors. Someday I'll invest in one, but it's like the wind break. I just haven't yet.
The focus issue is one that required the most effort to work around. DSLRs locate the sensor further aft than your lens would have been by a considerable amount. This is to make room for the mirror mechanism. The stock focuser on many Newtonian scopes just doesn't have the travel to bring it in. The trick is to reduce the distance between the camera and the telescope mirror.
The first thing you'll need is a T-ring and adapter. The T-Ring is something that will be specific to your DSLR and attaches where your camera lens normally would. There are lots of after market T-rings. Try to pick one that is as thin as possible. Remember we're trying to reduce the distance to the sensor and every little bit helps. The adapter I use is a 2". For some reason they insist on adding an extension to the 1 1/4" adapters. For my application, it's counter productive. If you're using a filter, the inside of the adapter should be threaded.
Second & third frame:
On my 10" f4.7 I replaced the stock focuser with one with a lower profile. It has an extension for using an eye piece. This moves the camera most of the way to where it needed to be, but it was still a little short.
To make up the last bit a friend suggested changing out the hardware in the mirror cell. FYI, Ace Hardware carries the correct metric sizes. Be careful with the thread length on the remaining thumb nut. You want to be sure to keep a few threads engaged or risk having your mirror come lose.
So for those that are struggling to get that first image, I hope this helps.