Rating of the Telescopes

Amateur  mirrors can be rated by the magnification that the image breaks down at.

As the magnification is increased from very low to upward, at some point the image will fail. Stars will no longer look like pinpoints, planetary detail will be indistinct, and the overall impression disappointing.

Unfortunately, unless the mirror is very bad, on many nights the atmosphere will limit the test, but over several nights we can get a good feel for the magnification that the image breaks down at.

Mirrors that can sustain 35x to 50x per inch of aperture rated as excellent.
Mirrors that sustain 25x to 35x per inch of aperture rated as good.
Mirrors that sustain 25x per inch of aperture are acceptable.
Mirrors that sustain 15x to 25x per inch of aperture are poor, usable only at lower powers.
Mirrors that fail at 12x per inch of aperture are plain, just not finished.

For smaller mirrors, push these numbers a little higher, for gargantuan mirrors, push the numbers a little lower.

Here are Bratislav Curcic's comments on using the star test to judge optical quality:

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 useles: '<expletive>' unlike 1, I've seen these :-)

99% of all scopes I've seen fall into '4-10' bracket.

The quality of the image is 99% due to the quality of the
optics. A good 6" will out perform a bad 10" any day, There are no
bargains to be had for a cheap price.

A short (F5 or less) optic is very difficult to make so if the price is low it will be of low quality.

Longer focal lengths are much eaiser to make accurately so these will tend to be of higher quality. You will see a better image from a long focal length telescope. Only because it is unlikely you will find a scope with a short focal length and a quality objective. Unless you pay the big bucks or spend a lot of extra time grinding your own.

If you have two equal diameter mirrors with equal RMS wave front errors
the SHORTER Focus mirror will have better resolution at all powers. In
the real world its eaiser to find a long focus quality objective so the
long focus telescopes almost always outperform the short focus ones.

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