Smaller optical aberrations is one of the reasons too.
- If you compare a 80mm f/12 APO and a 80mm f/7.5 APO, both perfectly formed, the f/12 would have 4X less spherical aberration, 2,5X less coma and 1,6X less astigmatism, field curvature , distortion.
- Moreover a f/12 has 2,5X more focus depth than a f/7,5, which means 2,5 less vulnerable for a small distortion in the wave-front of light that reaches the objective, in other words less sensitive to seeing. ( less defocus aberration)
Furthermore the objective is higher up and further away from the bodyheat of observer and ground, which means less vulnerability to warmth radiation from these sources.
These are reasons, apart from less CA in an achromat, why long focal refractors still exist and are being made.
Source: Neil English, Choosing and using a refracting telescope, Springer, New York 2011. ISBN 978-1-4419- 6402-1
pp. 29, 30, 254, 266.
Telescopes in Schiedam : SW 150mm Achromat F/5, SW Evostar 120ED F/7.5, Vixen 102ED F/9, OMC140 Maksutov F/14.3, SW 102 Maksutov F/13 on Vixen GPDX.
Eyepieces: Kitakaru, Eudiascopic, Panoptic, Morpheus, Nagler, Leica ASPH zoom, CZJena, Brandon, Parks, Fujiyama, Pentax XO5 and XO2.5 .
Binoculars: Kasai 2.3X40, AusJena 10X50 Jenoptem, Swarovski Habicht 7X42, Celestron Skymaster 15X70, Swift Observation 20X80.
Rijswijk Observatory Foundation telescopes: Astro-Physics Starfire 130 f/8 on NEQ6, 6 inch Newton on GP, C8 on NEQ6, Meade 14 inch SCT on EQ8, Lunt.
Amateur since 1970.