Quote:
Originally Posted by stargate101
P.S. John , I dont mean to sound antagonistic so please to take offense however Synta(Skywatcher) use Pyrex glass in thier mirrors not BK7 glass which is inferior and cannot achieve the same high Lambda with thier coating.
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Hi James,
No offense taken but your statement is
100% INCORRECT. Unfortunately your perception is a misconception many people have. Mainly due to some good marketing by Skywatcher (Synta) and Orion.
Without trying to get too technical the major difference between Pyrex and BK7 and for that matter plate, float or soda lime glass, is in the thermal effects; not in the optical quality in any way. Pyrex has greater thermal conductivity and a lower co efficient of thermal expansion than BK7, which means it cools faster and expands and contracts slightly less than BK7, for a given temperature change. The advantage of this is that the pyrex mirror will cool faster and will deliver its best images a touch quicker than a BK7 mirror. However, with a mirror of only 10" aperture the difference is very marginal. With a 14" and larger mirror it makes a pretty big difference in mirror stabilisation time. In terms of optical quality, a good optician can make a mirror equally as good from BK7 as he can from pyrex. I skilled optician when working with larger size mirrors may have to wait a little longer for the glass to cool properly before testing a BK7 mirror, that is all. If he waits he can make it just as good. Again however with 10" mirrors it is really a moot point. With a big mirror it matters a lot more because of the additional glass mass.
I have used in excess of 30 different samples of GSO scopes from 6" to 16" aperture and in excess of a dozen different Synta (Skywatcher) scopes from 8" to 12" aperture and I can assure you that on average from my analysis, the GSO optics are at least as good, if not a fraction better than the Synta optics. To be honest the early Synta Optics from 8 or 10 years ago were pretty hit and miss. The more recent Synta optics are generally very good, due in the main part to the quality control introduced in their manufacturing techniques by Orion USA. You should note that GSO produced the Orion dobs long before Synta (about 2000 to 2004 from memory) and had the benefit of Orion quality control long before Synta. In other words GSO have been getting it right, long before Synta ever got it right.
Why you likely perceived that the 12" Skywatcher scope outperformed the 16" GSO scope on the night you tried them is due to one of 3 factors, or a combination of 2, or all 3.
1) The 16" scope was not collimated properly.
2) The 16" scope had not cooled properly.
3) The seeing conditions were not good enough to support 16" of aperture, but were able to support 12" of aperture.
Some things you need to be aware of in regard to the above.
1) A 16" scope takes a lot longer to cool and stabilise optically than a 12" scope.
2) The image quality in a 16" scope is significantly more affected by poor seeing than a 12" scope
3) A "fair" 16" scope will outperform a "very good" 12" scope, given good thermal stabilisation and seeing conditions good enough to support 16" of aperture and good collimation.
Another possibility of course and fairly unlikely IMO, is that the 16" scope was one of the rare lemons to slip through the cracks.
FWIW, I have some exquisite telescopes up to 30" aperture at my disposal and I have no issues using a 10" scope with a BK7 mirror. In fact, I own one that has an absolutely outstanding GSO BK7 mirror in it
Cheers,
John B