Over the last 12 months or so there has been an increasing amount of interest in testing one's own optics. The most often sited test here in the Refractor forum is the Double Pass Auto Collimation test as it gives you an excellent picture of surface quality and spherical correction of the lens. It can also give insight into how a lens behaves at different wavelengths - ie. "color correction" as well as spherochromatic correction and focus point as a function of wavelength. Its not a very sensitive test for astigmatism or coma. For these, the star test is preferable. And, actually, when testing a lens you should seek to have at least two corroborating test methods before determining the "goodness" of an optic. In other words DPAC and star test that agree with each other is far stronger then any single qualitative test method.
Below are tests of a 76mm ED doublet and a 9.25" SCT. The SCT (has the central obstruction) These telescopes both suffer from some minor spherical aberration of opposite sign - the 76mm telescope is undercorrected and the SCT is over corrected. You can see the correction is "opposite" as the curvature of the bands are opposite when comparing the resulting images of the different telescopes on the same side of focus. You can also see that 76mm scope is has a very smooth lens. The SCT has a somewhat rough corrector but not too bad - I've seen worse.
Inside of focus images are on the left; At focus images in the center; Outside of focus on the right.
Edited by peleuba, 12 March 2020 - 01:17 PM.