Amateur astronomy used to be about making and using telescopes. There was a camaraderie amongst ATM's and a healthy diversity of home built telescopes. Nowadays it seems completely preoccupied with Dobo's, commercial SCT's & CCD's. And according to the, "Serious amateurs do real science," brigade the only "science" worth doing is astronomical discovery.
Commercial SCT's are a hybrid, bastardised optical design. For a compact f/10 or f/6.3 SCT to give a wide (1º) coma free, diffraction limited field, it must be aplanatic. This requires a hyperbolic primary and an ellipsoidal secondary as well as the fourth order corrector plate. The corrector plate must also be thick enough to support the secondary without sensible flexure (more than 1/2 wave [P-V]).
NO commercial SCT is aplanatic in the true sense. They also all have thin corrector plates that introduce astigmatism.
"But I need the convenience of a compact Cassegrain," I hear you protest. Very well, why not opt for a Schmidt-Newtonian or a Maksutov-Cassegrain?
What is the point of buying an LX200, fitting a 2-inch focuser, and kitting it out with Nagler or TeleVue eyepieces, when the diffraction limited field is less than 3/8" diameter? If you want a wide field telescope for CCD work, then buy a wide field 'scope. If you want a compact hi-res 'scope, then buy a Maksutov?
For true diffraction limited performance the optical system must have a Strehl ratio of at least 0.8. "What is the Strehl ratio?" You may well ask. The fact that you need to speaks volumes about the inadequacies of books dealing with telescope optics and resolution over the past half century.
The field of theoretical optics has, as far as amateur astronomy is concerned, been in stasis. Resolution is still thought of in terms of the separation of double stars, either using the Rayleigh limit or the empirical Dawes' limit. Despite attempts by BAA stalwarts like R.A. Marriott to get Dawes' name canonised, his empirical formula does not tell the full picture because the simplistic relationship ignores the effect image contrast, particularly contrast in an extended image, plays in optical resolving power. And yet the mathematics of this difficult area was thrashed out 50 years ago. It should however come as no surprise that BAA "experts" who are proud to espouse the irrelevance of mathematics in amateur astronomy, should display a congruent wholesale ignorance of this fact.
The resolution of any optical system can be directly measured using a target phase plate. The resolution is then expressed as a function of the amount of detail, at all contrast levels, transferred from the target to the image plane. This is termed the Optical Transfer Function. If the image is virtual, as it is in a telescopic optical system, then it is referred to as the Modulation Transfer Function.
Seidel errors (such as spherical aberration) in an optical imaging system degrade the MTF. So does the introduction of a large central obstruction. The Strehl ratio corresponds to the ratio of the central light in the Airy diffraction disc produced by a point source, compared to the theoretical value of an ideal objective. The Strehl ratio at the Rayleigh 1/4wave wavefront tolerance is 0.8. An ideal telescope would have a Strehl ratio of 1.0.
A typical commercial SCT or a thin mirror Dobo has a Strehl ratio less than 0.7. As a comparison, my 6"f/15 Maksutov-Cassegrain has a Strehl ratio of 0.88, and my 10"f/12.8 Calver (with the new Duran 50 primary) 0.9. An unobstructed reflector or an achromatic refractor can be expected to have a Strehl ratio between 0.92 & 0.97.
Prior to Tom Johnson marketing that hideous orange Celestron 8 back in 1972, no one would have dreamt of intentionally designing a telescope whose optical system, even if made perfectly, could not give a Strehl ratio higher than 0.7. The fact that they now appear to be what amateur's most desire just goes to show how far down hill this hobby has slid.
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