The double defraction spikes indicate that the spider vanes in one direction aren't quite aligned together (I have the same with my 12.5" dob). The below diagram (loose use of the word
) may help you understand. It is a top view of your tube looking down on the spider vanes:
ie
..............|
..............|
....(------O______) <- notice the offset of opposing vanes
..............|
..............|
..............^ -Fine this way
If you can't understand the diagram I get back to me. To see if this is the thing that is creating your double diffraction spike you can just look along the line of the spider vane and you should be able to pick the miss-alignment. If it is...how do you fix it? If you spider is completely adjustable you should be able to "twist it out" through adjustments, however if the spider vanes are fixed (like my dob
) then you just have to live with them or replace your spider (as I eventually will....one day
)