Perhaps I'm being over enthusiastic about current commercial optics.
Here's a scale on Mel Bartels web page. There is agreement that commercial optics are well up a notch on what they used o be.
http://www.bbastrodesigns.com/ratemirrors.html
Quote:
Here is Bratislav's 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.
|