Bratislav's Mirror Rating Scale:

1.   Can't find anything wrong with it, absolutely perfect: '<expletive>' Yet to see one after ~25 yrs

2.   Defects visible only in extrafocal images, and only after extensive star testing in best seeing conditions ( << 1/10 wf): 'You lucky b@$#@rd!' Can count these on fingers of one hand

3.   Extrafocal defects readily visible,  but really minor ( < 1/10 wf): 'Excellent' Best examples of  best commercial telescopes (Zeiss,AP,Tak etc) Best examples of homemade optics

4.  Extrafocal defects fairly obvious, but in focus image still essentially perfect ( 1/10 - 1/6 wf): 'Very good' Majority of current 'best commercial telescopes'; best examples of mass produced scopes

5.   Large defects visible on extrafocal images, in focus image suffers only slightly ( 1/6 - 1/4 wf): 'Good' selected examples of mass produced telescopes, most well made amateur optics; some examples of 'best commercial scopes' can still be found here

6.   In focus image visibly suffers ( ~1/4 wf): 'Acceptable' good mass produced scope, most good large/fast mirrors I've seen

7.   Image deterioration serious, clearly beyond 1/4 wavefront: 'Light bucket' majority of older generation mass produced scopes, special purpose telescopes (astrographs)

8.   It's difficult to determine when scope is in focus at all ( 1/2 - 1 wf): 'If you're happy with it ...' unfortunately, not that difficult to find !

9.   Usable only at very low magnification  ( ~1 wf): 'I don't want to have anything to do with this one'

10. Absolutely useless: '<expletive>' unlike 1, I've seen these :-)
99% of all scopes I've seen fall into '4-10' bracket