Mij Adyaw wrote:
Why do many serious planetary observers choose longer focal length APO
refractors over shorter length APO refractors for planetary observation?
Assuming a very well corrected APO like an AP or a TMB, what is to be gained
by the longer focal length assuming that you have a full compliment of
excellent eye pieces in all available focal lengths?
An F9 lens will have better color correction than an F6 one.
A longer focal length makes it easier to get optimum magnification
without using a barlow.
Many folks love barlows - but they add air to glass surfaces to your
light path and that costs a little contrast and adds light scatter.
Folks who like planets are also likely to use monocentric eyepieces -
these have a relatively small field of view, but the planets pop against
a nice black sky. Great for detail.