Hi Fernando , a scope at f/4 is very , very critical of any misalignment , so you probably need to do a star test alignment ( the most sensitive test there is ) , eg place a reasonabley bright star in the center of the field of view and dis-focus it until it gets big enough for you to see the secondary and spiders in the blob ,
, this will instantly tell you if its a collumation issue .If it is remember tiny movements .
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Astigmatisim is a problem if its built into the mirror , there are no fixes other than returning the OTA and trying another , sad but true
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Try loosening the 3 clips holding the primary mirror , it might have pinched optics? , they should only just hold the mirror , thats all .
Keep us informed as to how it goes and good luck.
Brian.