I tried using the Baader zoom and my DSLR/planetary camera for eyepiece projection imaging, and yes, the extension tubes are essential so that you can achieve focus. Even then however, the performance is not good, it is significantly better to use the prime focus method (ie screw the camera straight into the back of the OTA).
For planetary imaging, the best method is prime focus and a barlow lens to get your focal ratio to be about 5x the pixel size of your camera. For the Nikon D3100 with a pixel size of 5 microns, this puts the optimal f/num at around f/25.
However, the Nikon DSLR system is
not the best camera to use for planetary imaging, as it is not possible to stream raw the Live View video straight to a computer, and using the inbuilt video is highly compressed making post-processing difficult. You can read about the process here and see why Canon DSLRs are preferred (it's how I started).
https://www.astropix.com/html/i_astr...esolution.html
Here's some more tips on how to take high resolution planetary images.
http://planetaryimagingtutorials.com/
Hope this helps, Andrew