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Apologies if this is taking the thread too far from the original intention, but I'm wondering about this, from the webpage Bojan linked to:
"A very worth mentioning is the possibility of using Ronchi grating in front of the telescope focus (Ronchi "eyepiece"), using a bright star as the light source. With two intercepted lines (L=2), it will detect down to about 1/7 wave P-V of spherical aberration in the telescope objective at f/10. For faster objectives, the sensitivity can be easily increased with a good Barlow lens.
In conclusion, while the geometric Ronchi test certainly has its limitations, they do not prevent its use as a quick, simple and reliable - within given limitations - test for surface quality of both, single optical elements and telescopes (with the latter, Ronchi grating is placed in the infinity focus zone)"
Are both of these instances referring to a test with the grating over the corrector, i.e. between scope and star, or is the Ronchi "eyepiece" a grating between (primary) mirror and eyepiece?
Does the f/15 of my scope make this a less suitable test for spherical aberration?
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