Your about to buy a newt & know enough that the better the mirror is made the better it will perform , you see a vendor offering 1/12 wave specs .
Wow you think that must be good hey !
But what does that actually mean ?
1. Is it peak to valley ?
2 . Wave length at surface plus or minus ?
3 . Or expressed as RMS ( root mean squared criterion )
Below is from Mel Bartels
Star test standard: immaterial differences between intra and extra focal images, diagonal breakout on either side of focus occurs at about the same distance, and there is no turned edge. The star test is qualitative only. Popularized by John Dobson and his protoge Bob Kestner (optician for the COBE Hubble correction optics at Tinsley Labs), this method relies on slightly defocusing a test star, first inside of focus, then outside of focus. By noting how the light from the entire mirror de-focuses, a rating in the form of: excellent, good, average, poor, or unacceptable, can be given to the mirror.
Here is Bratislav's star test scale: from 1 to 10 :
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): 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.
Are you going to be happy with a 5 or 6 ?
Then there is the secondary mirror , most mass produced can have astigmatism !
Hope to get the more knowledgeable to enter this thread then me .
Bobby .