Jim Beam wrote in message ...
I think 0.8mm exit pupil was on the table, which holds to the 31x per inch
rule. Beyond that, you are just making the image larger, which has the
double edged sword effect of also making the image dimmer (as you
suggested). So the questions are, and this is subjective, at what exit pupil
are images too dim for you, and at what exit pupil do your floaters (if any)
become a serious bother? These values will hold in any scope. You then
simply scale up the aperture, to increase image scale.

Stephen Paul


Hi Stephen, thanks for your response. I don't understand. You mention
0.8mm as if it has almost the authority of a speed limit. Is there a reason
why 0.8mm would be the point where you go from seeing more detail to
just making things bigger? Does it have to do with 0.8mm making the
Airy disc visible to the eye?

In fact it might not be the case. It depends on the visual acuity of
the observer. Most observers, expecially so if no longer young, are
comfortable with seeing details when they cover about 3 arcminutes but
they can still do see them, albeit less comfortably, when the apparent
size is 1 arcminute if the relative contrast is very high. Returing to
the hypothetical 7" this means anything between 94x to 282x for
details at the instrumental limit (in green light, where the peak
visual acuity is) of 0.64". The pupil exit size therefore varies from
1.8mm to just 0.6mm, or from 13x to 40x. I would consider 75x as the
normal upper limit in very good seeing for any quality scope,
regardless whether it's an APO or whatever else. With my 6" mak 282x
is my normal planetary magnification, regardless of seeing.

Andrea T.