Someone might try to use this test, described in the last S&T issue, to determine spherical aberration present in her/his scope. The method given is supposed to give approximate amount of longitudinal spherical aberration, indicating correction error of the optical set. However, even assuming pure form of spherical aberration, the method described will only give approximate separation between best foci of the aperture area inside and outside the 70% zone. This would make about half of the actual longitudinal aberration.
But even if corrected for this (doubling the measured focal disparity), the figure, as given in the accompanying table, would indicate much greater aberration than there really is.
For instance, a 6" f/8 sphere has longitudinal spherical aberration (given by D/32F) of 0.0234", or nearly 0.6mm. The method described would give ~0.3mm figure. According to the table, that would indicate spherical aberration in excess of 1 wave. The actual aberration here is 1/3.8 wave (from 22.6D/F^3).
There is also a problem with the assumption of pure spherical aberration. It probably exist only in textbooks; real telescopes have it more or less "modified", which changes the geometry of focal zone in a manner that can't be determined without diffraction analysis of the particular "modification". For instance, refractors often have spherical aberration purposely modified so that paraxial and marginal rays have identical focus, while the longitudinal spherical aberration is largest for rays from the 70% zone.
In this case, best focus of the aperture area outside the 70% zone will nearly coincide with the best focus of the area inside the 70% zone, regardless of the size of aberration. In other words, the test will indicate very low spherical aberration, even if it is, in fact, significant.
There is also a test for chromatism, which is not more reliable than that for spherical aberration. According to test results, a 5" f/15 achromat (which has as much chromatism as a 4" f/12) has less chromatism than an AP StarFire. If nothing else, that should have been something to ring the bell. For some reason, it didn't happen...
But even if corrected for this (doubling the measured focal disparity), the figure, as given in the accompanying table, would indicate much greater aberration than there really is.
For instance, a 6" f/8 sphere has longitudinal spherical aberration (given by D/32F) of 0.0234", or nearly 0.6mm. The method described would give ~0.3mm figure. According to the table, that would indicate spherical aberration in excess of 1 wave. The actual aberration here is 1/3.8 wave (from 22.6D/F^3).
There is also a problem with the assumption of pure spherical aberration. It probably exist only in textbooks; real telescopes have it more or less "modified", which changes the geometry of focal zone in a manner that can't be determined without diffraction analysis of the particular "modification". For instance, refractors often have spherical aberration purposely modified so that paraxial and marginal rays have identical focus, while the longitudinal spherical aberration is largest for rays from the 70% zone.
In this case, best focus of the aperture area outside the 70% zone will nearly coincide with the best focus of the area inside the 70% zone, regardless of the size of aberration. In other words, the test will indicate very low spherical aberration, even if it is, in fact, significant.
There is also a test for chromatism, which is not more reliable than that for spherical aberration. According to test results, a 5" f/15 achromat (which has as much chromatism as a 4" f/12) has less chromatism than an AP StarFire. If nothing else, that should have been something to ring the bell. For some reason, it didn't happen...
