Hello,
I don't understand completely your question.
If it is about constellations, these are very important to understand and see MER resulting.
But if you observe constellations with very low MER (<10 dB), you will see in all cases scattered points and it is difficult to see the difference.
If you receive a station on 70cm or 23cm you could get 2àdB of MER and you will see the constellations as little condensed clouds.
look at my explanations here :
http://www.vivadatv.org/viewtopic.php?f=72&t=79Jean-Pierre F6DZP