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Originally Posted by iceman
ditto AN, I guess it just depends what you're comparing it to. All of the GSO optics in the 10" and 12" models are f/5, which would make comparing against your f/7 quite difficult.
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Originally Posted by argonavis
I don't see why. Provided magnification is held constant, there should be no difference in optical quality between an f5 and an f7.
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AN, there can be a difference between an F5 and an F7 newtonian scope given equal optical quality, particularly if the F5 scope is a tube scope and not a truss scope.
With a tubed scope you need a larger secondary mirror due to the necessity of the tube to have a larger diameter than the Upper Cage Assembly on a truss scope. This is necessary to alleviate tube currents in the tubed scope which are not an issue in the truss scope. In addition to this an F7 scope regardless of design will always have a smaller secondary than an F5 scope. The larger secondary on the F5 scope can lead to a loss of contrast, particularly if it is over 20% by diameter.
An F7 scope will have a much larger diffraction limited field of view. This means off axis aberrations will be reduced and images better quality off axis. Notably coma and field curvature are reduced in the F7 scope.
Its also a lot easier to grind say a 1/10th wave F7 mirror than it is to grind a 1/10th wave F5 mirror. In addition to this, as the mirror gets larger it also gets harder to grind an outstanding one, due to the steeper radius of curvature and greater parabolisation required to correct for spherical aberration. Hence the likelihood of a great F5 mirror under mass production is slightly less than the likelihood of a great F7 mirror.
The above having been said, with the exceptional quality of fast mirrors from premium mirror makers like Mark Suchting, Carl Zambuto, Bob Royce etc, you would be hard pressed to tell the difference at the eyepiece, using premium eyepieces. Instruments certainly could detect the difference. This is particularly so with the F5 scope being a truss design with the Central Obstruction under 20%.
There is some worthwhile reading about the effects of F-Ratio and Central Obstruction on Bob Royce's website:-
http://www.rfroyce.com/mirror_performance.htm
http://www.rfroyce.com/optimum.htm
The main difference with mass produced scopes like the 12" GSO dobs is that the quality of the mirror can vary a lot more than it will from a premium mirror maker. I have used several samples of the 12" GSO dobs and they have all been good to very good, certainly no lemons. I am yet to see a 12" GSO dob that was outstanding optically.
I have seen a large number of 10" dobs and they have ranged in quality from good to very good to outstanding. A couple of the 8" scopes I have used have also been excellent optically. My own 10"/F5 GSO dob has an outstanding mirror in it. A fellow AS of NSW member and very experienced mirror maker (David Collis Bird) commented to me a few years ago how good the mirror in it was. He said, "I can't believe they can make a mirror this good for this amount of money, it is better than 1/8th wave and would have a strehl over .96" Several of the guys on the forum have viewed Jupiter and Saturn through it at high power as recently as last Saturday and could attest to its quality. The problem however is how many do they make this good ? Probably 1 in 20 at best, with a premium mirror maker they are all this good or a tad better. Given that all the GSO scopes are as a worst case scenario good and ranging to well above good, they represent excellent value for money.
Mark Suchting is a forum member under the name of Lambda20 and could probably explain all this a lot better than I have.
CS-John B