I use the diffraction patterns produced by a Bahtinov Mask to check my sensor alignment.
I usually take a long exposure and move the mount by jogging 30' arc over the range +- 1,5 degrees. Both horizontally and vertically. Of course the Bahtinov Mask needs to be rotated by 90 degrees for each direction.
This is a stack of six exposures with the maximum pixel value ending up in the final stack. 3MB
http://d1355990.i49.quadrahosting.co.../AlignBagt.jpg
The centre pattern is two Bahtinov diffraction patterns at 90 degrees to each other superimposed. They indicate near to perfect focus at the centre.
The other four are the Bahtinov diffraction patterns 1.5 degrees away from centre.
For perfect alignment ie orthogonality of the sensor to the optic axis the four satellite patterns should be identical. They differ to the central patterns due to slight field curvature. This is what is expected in a field of 3.5x3.5 degrees at F3.
All these six exposures were taken without changing focus.
Bert