I get great pictures of the inside of my telescope.....The image shows the secondary mirror (taking up about 40% of the picture) and the spider vangs going to the edge of the inner tube of the scope.
I have a Skywatcher 8" Quattro f/4 Newtonian and use a Canon Rebel T1i DSLR. I have taken 100's of photos since purchasing the scope a few months back. So I made the plunge and bought the Skywatcher Coma Corrector specifically made for this series of telescope. The coma corrector says it requires 55mm of back focus so I set up as follows:
T-adapter on one end of the coma corrector, inserted into my focuser and placed flat against the focuser tube edge. Other end of the coma corrector I have a 10mm spacer, attached with a Canon adapter to my Canon T1i. Data I found says the Canon has a 44mm (or 44.5mm) sensor to flange distance, so add these together and it is almost equal to the 55mm back focus I read in the Coma corrector spec's. I can mover the focuser all in, mid-way, all out and I basically get the same picture described above.
Without the coma corrector, my normal set up is a T-adapter connected to the Canon adapter, connected to the camera, no spacers. So in my mind in this case the 'backfocus' it is just the flange to sensor distance of 44mm which is built into the camera. The focuser is generally half-way of its total range of movement when I take all of my exposures.
Any help is much appreciated. I don't have any more spacers at this point to try a larger back focus value so I thought to ask first.