agree with Jason, it could just be the normal offset mechanical alignment. However, it is also possible that the light column may be intercepting the tube on one side. If this is the case, the most likely cause is that the secondary is too far in/out. This is nigh on impossible to get right by eye with a fast Newt - you can collimate it perfectly, but the light column may still not be exactly central.
FWIW, I ended up using back projection to get that aspect right. The linked image
https://astrob.in/full/367081/0/ shows how to use a small led torch that is taped up to fit in a 1.25 adapter on the scope - doesn't need to be exact, because it is being used with a divergent beam. A bit of translucent tape increases the size of the light source, which helps a little. With a sheet of baking paper over the input aperture, back projection from the torch shows where the light column has to go (in the reverse direction) to get to the detector. A few minutes effort will quickly show up if you actually have a secondary positioning problem.
To illustrate what can go wrong, panel 3 shows a badly adjusted scope, with the shadow of the spider showing some distortion, the secondary shadow being clearly skewed and the primary light column being offset from the centre of the OTA due to poor collimation (by a lot). It is pretty easy to work out how to adjust the secondary skew, straighten the spider and then collimate the system to get back to something like panel 4 where the secondary shadow is now circular with a reasonable looking offset and the spider is straight.
However, the in/out of the secondary is still wrong as the light from the primary is still not centered. This is not adjusted by moving the primary, since that will screw up the collimation. instead the secondary is moved in/out and the the scope is recollimated to see what effect the movement has. Eventually gets to the result of panel 5 where the primary light column is reasonably close to central - not quite prefect, but this is as good as I can do with my old scope.
The primary rule for using this technique is that the results of moving the secondary can only be assessed after the system has been recollimated. ie move secondary - collimate - assess - move secondary - collimate - assess...etc.. If you don't recollimate after movement, the results are meaningless.
Hope this helps - cheers ray
edit: don't put the scope on the floor as shown in the illustration images - the collimation can be upset by the weight on the primary adjusters if they stick out - do it on the mount
also, the inherent asymmetry of a fast Newt means that the star skirts are not quite symetrical - FWIW I find that it is best to take all colour data on one side of the meridian to minimise multi-colour star skirts at high stretch