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Takahashi FSQ-106ED (530mm F/5) is the premium instrument in the 4" range. There is something very special about the flat field modified petzval design (quadruplet). It’s good for visual and excellent for astrophotography. If money is an issue, you could always go with its cheaper sibling (from a design perspective) - Pentax 100 SDUF II (400mm F/4). It has a similar objective composition to the FSQ - 2 x 2 widely space element sets. Both are fast astrographs that will deliver pinpoint stars to the edge of most demanding large format chips - they each have 88mm image circle sizes.
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