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Hi Steve
I think you are confusing spider spikes with the obstruction caused by the secondary itself.
People argue about how much secondary obstruction causes a noticeable impact on performance. Most would agree that an obstruction of less than 20% - eg 40 mm secondary for a 200mm primary causes very little impact.
You’ll notice Texereau suggests an optical window for a specialist planetary cassegrain with a small secondary. The focal ratio that he suggests for a classical cassegrain is F20 or higher. Part of his reason for the window is to remove the spikes that are always visible independent of focal ratio.
Yes affordable ronchi gratings are hard to get now. You could make a rough one by printing one on an overhead transparency. I know there is a file on Stellafane you can use to do that. There may be other sites with files too. I might have something you can use. I’ll have a look and get back to you.
You can do most of the testing with just a ronchi screen and a suitable light source. Much more convenient than a star test. I think most people use the star test for final figuring and confirmation of bench test results.
Rod
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