The reason why secondary mirror focusing works is because the intermirror spacing changes the focal length. If the correct backfocus is known and the CCD is placed there, you will get well corrected and focused images when the spacing is correct.
The main issue I had with DSI is it is too subjective and you need calm air to do it. Couple that with focuser tilt when moving the primary on the GSO scopes, you end up not really sure where you end up and spend a great deal of time adjusting both the front and back on each step.
For reference I have an 8" RC steel tube, and no tilt-plate accessory.
My method in principle aims to align each main component in such a way that the mechanical interplay is no-longer a source of error. It can easily be done any time of day. I have verified the results by Star Tests, Howie-Glatter with holographic rings and a Ronchi test. All passed muster.
Step 0: The Setup
I like to position a DSLR behind the telescope on a Tripod with a zoom lens. The camera is connected to a laptop with me at the telescope so I can see the live view and control the focus while making adjustments. At no steps in the process does perfect alignment of the camera affect the judgment of adjustments needed in a significant way.
Step 1: Align the Secondary:
Traditionally you might think a cheshire eyepiece will work. But that is susceptible to focuser tilt error.
The solution is a collimation cap placed directly at the rear of the telescope with the focuser removed. I have two versions in the picture below. On the Right, my original foil on card stock and a 2mm peephole. On the left a 3D printed disk with the Howie Glatter laser 1mm knife-edge aperture mounted into it I made more recently.
Shine a lamp into the front of the telescope with paper to diffuse it to illuminate the interior. Next set the camera, zoom so the FOV covers the wide opening of the rear of the telescope. Adjust the exposure and focus on the center mark of the secondary. (Use live view zoom here, You will need the wider FOV for step 2.) Install the collimation cap and carefully reposition the camera to align it down the axis of the scope. Looking through the peephole with the correct zoom and focus you will see the center donut mark on the secondary and the shadow of the peephole.
Step 2: Align the Primary
Next remove the collimation cap. Adjust the exposure and focus. You should see the internal mirror surfaces, two views of the spider vanes and concentric rings of the internal baffles. Do not attempt to make the rings concentric. Any error on the camera's position will translate to an error of concentricity. Instead the goal is to align the two reflected views of the spider vanes. This view is true regardless of camera offaxis error.
When adjusting the primary, ONLY adjust 2 of the 3 screws. Reserve the 3rd for a final, minute adjustment if necessary. This way you limit the chance of changing the inter-mirror spacing.
Once the spider vanes are all perfectly in line. Congratulations! The telescope is now perfectly optically aligned!
Step 3: Focuser Tilt
Reinstall the focuser assembly. This step greatly benefits from having a tilt plate for adjustment. But there is play in the tilt of the stock crayford focuser before you lock the thread ring down.
At this point a laser collimator is useful. Take for granted that the secondary mirror is already perfectly adjusted, the laser should return directly when the focuser tilt is set to hit the center of the secondary mirror.
Step 1 aligned secondary without concern for focuser misalignment.
Step 2 Optically aligned the primary with internal reflections
Step 3 Focuser tilt set without moving the primary or secondary mirrors.
Step 4: Optional spacing tuning.
As I mentioned before intermirror spacing is very important. If it is incorrect the focal length will change dramatically And coma will appear due to spherical aberrations.
When the mirrors are Too Close there will be Undercorrection. Too Far will introduce Overcorrection.
Focal length will shift by a factor of 1 to 10.
Example: Decreasing the distance between the mirrors by 1mm will increase the focal length by ~10mm
If you see coma and want to do Step 4, there are 2 acceptable ways to adjust the mirror spacing.
1: Adjust the center screw on the secondary mirror and re-do the secondary tilt adjustment.
2: Use the primary collimation screws to move the entire primary and realign the reflections.
3: There is a threaded locking ring on the secondary mirror baffle that may adjust the distance. But it might move the mirror off-center, so I would not recommend it.
All in all when you get used to this workflow it is very quick to do. Should take under an hour.