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Antares secondaries are based on sizes that have been in use since the beginning of time. These are typically mounted in secondary holders that have a lip around the edge of the secondary serving three purposes: first to hold the secondary in place, second to mask any edge surface errors on the secondary and third to allow the secondary to float in its housing with no thermal stress from any directly applied glue. Antares uses what used to be industry standard sizes and are recommended if you have the correct holder.
The secondaries used today in most mass produced telescopes are based on metric dimensions likely sized to rounded numbers in mm but not in line with the previous industry standard sizes. Most telescopes too these days have their secondaries glued with no lip around the edge. I’m not sure that I’d want to glue a 1/20 or 1/30 secondary (for fear of glue not holding) and whether I’d even still get 1/20 or 1/30 performance from it should there be any thermal stress between the back of the secondary, the glue and the plate/stalk the glue adheres to. I’d be changing the secondary holder too which has its own challenges, one being the diameter of the centre bolt needs to fit the spider.
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