I'm not sure what you mean. It doesn't seem like a tautology to me unless I slept through my entire mathematical logic course.
I already agreed with you. Good scopes are good and mediocre scopes are mediocre. That is a tautology, in the sense it says that good scopes will do good and mediocre scopes will do - well not so good. This is a case of grammatical tautology not mathematical. Sorry if I offended you in some way, I dont claim to be a stylistic expert either.
I dont generally agree with Daniel on everything, but I think it is true that a good planetary image by itself, says virtually nothing about the quality of the optic. It says more about the quality of technique, the aperture and even more importantly on how good the seeing was. As I mentioned earlier a scope of sufficient aperture even compromised by aberrations giving a Strehl of 0.5 can produce stunning images.
Many dont appreciate that under normal backyard seeing a larger scope is normally running under effective Strehls in the low 20's, sometimes single digits!
Coma is a really nasty aberration. It is difficult to correct for it in IP as it is not constant for the field, but it is possible. You can have fairly significant spherical errors and correct for them quite easily. The original HST had an absurd level of spherical, yet with IP it could produce really good images of Saturn and R Aquarii. You can duplicate some of the early HST deconvolution today quite easily.
Dan (maadscientist) wrote:
We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger.
The Fried coherence length r0 is rarely as good as 8" or 20cm. Normal backyard seeing is more likely given by a 2 arcsec seeing disk. The seeing disk is the time averaged psf of a point source (so called long exposure).
The full width half max of the seeing disk in radians as a function of r0 is given approximately by:
FWHM = 0.976*(lambda/r0)
where lambda is the operating wavelength. Hence for lambda 550nm and doing a little algebra (and recalling that 1 radian is ~ 206264.8 arcsec) we find that for 2" seeing r0 is ~ 5.5cm and for 1" seeing it is ~ 11cm. Now 1" seeing is pretty good and does happen often enough to keep us productive. In lucky imaging 1" seeing will achieve diffraction limited imaging with larger apertures.
For brief moments the effective r0 may improve to yield nearly diffraction limited frames in lucky imaging. However as the aperture goes up the frequency of those frames drops exponentially. This exponential drop is not appreciated by many, unless they have a collection of scopes small and large and pursue high resolution, visual or imaging.
And finally, back to the OP. I have a very good IM715 mak. But honestly I can tell you I have seen plenty of SCT8's (both C & M !!!) which will easily match if not eclipse it. Mechanically this Mak is superior, it includes a fan, and has a decent focusing mechanism. I like the scope very much, but there is nothing magic about it.
Edited by DesertRat, 03 November 2015 - 09:22 PM.