Martin, in case you didn't take in Dennis's comment on mirror cooling,
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and a thermally stable mirror (scope cooled to ambient)
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it's important for the mirror to be at or close to ambient telmperature. This can take a couple of hours, often in condions were the day has been hot and the temperature is dropping rapidly, the primary mirror will have difficulty in reaching equalibrium. If your OTA doesn't have cooling fan/s it may be worth fitting them. Also FWIW the best seeing conditions often occur in the wee small hours just before dawn twilight.