Resolution and Magnifcation their relationship?

actually power is power independent of aperture. 100X with a 4" is the same
as in a 6" (same size image). It (typically) takes a different eyepiece to
achieve it but it is the same size. Resolution is somewhat independent of
power as well - it is a function of aperture. Where a bigger scope comes in
is not (at the same power now) in increaed size but increased brightness
allowing detection of difference in areas due to their differences in
contrast. It will however allow an increase in power above the smaller scope
and this is where its increased resolving capability comes into play.

One note: an interesting fact.. for the same focal length any given eyepiece
will product the SAME power/inch on all scopes. for example on a 10" f5 a
25mm eyepiece produces 50 power or 5X/". On a 20" F5 the same 25mm eyepieve
will prosuce 100 power or again 5X/"!

g.
#

...

OK I stand to be corrected, as written this is a bit ambiguous. But to make
it less so: psychophysical experiments on the human eye give it a best max
resolution at about 1 arcminute which is when the eye pupil is ~2-2.5mm
diameter. Obviously if your eye tested at a max 3-4 arcminutes resolution
in daylight, you'd take that into account.

When last tested, my own eyesight was actually a tad under 1 arcminute under
ideal conditions. Incidently, I can remember distinguishing (j-u-s-t) the
first Airy ring from the Airy disc with an exit pupil of ~1.7mm, i.e. at 41X
with a 70mm Tele Vue Pronto using an artifical star. I never had any
eyepieces to go slightly lower than this at the time.

You may be right. It's well known for resolving stars that the resolution
will be less than say that determined for high contrast detail on extended
objects. I can easily split the main components of the double-double naked
eye; but no way would I even consider trying to split stars separated by 1
arcminute! But remember here too, you are talking about looking at actual
stars. The eye-pupil has opened up from its optimal 2-2.5mm with a
reduction in its effective resolution even though the eye-pupil has a larger
diameter. At 5mm-7mm or whatever, the eye's inherrent abberations begin to
kick in. If you use a 2.5mm exit pupil (or less) provided by a scope, this
obviously no longer applies.

I should have emphasied that this is the lowest theoretical magnification -
I agree that the lowest theoretical magnification is not necessarily the
best or sensible practical option.

Again assuming the above, for "fine" (= high spatial frequenicies) detail.

Noted and understood...

I no way would I dissagree with Texereau; indeed my own experience seems to
tally with this figure; though if anything I go for higher magnifications
when not trying to experiment with these things, seeing how low I can go and
still detect/resolve things...

Which was precisely my point of not taking these things to be cast in
stone...

Regards,
Dave Randell

I very much doubt that is typical, although I have certainly heard
confirmed cases of people with resolution that good. I figure my
resolution at about 3-4 arcminutes, and there are plenty of people
who see worse than I do. And I am sure that only a small minority
of people can resolve an equal-magnitude double with a 2' separation.
In fact, I bet that at least 50% of all people can't resolve the
Double-Double (3.5') into two stars.

I doubt that you could get many serious planetary observers to
agree with that statement. Not for nothing does Texerau recommend
an 0.8mm exit pupil! And most people prefer to view the Moon at
even smaller exit pupils when seeking maximum detail.

In any case, optimal magnifications vary greatly from one person
to the next, so the best idea is to experiment for yourself.

- Tony Flanders

scope

Well, yes if you are gleaning planetary detail. And the highest power
you can "get away with" will be different every night. With my 7"
refractor I'm limited to 200 power or less on most nights. On the
rarest of nights It is so clear at 330x that I feel it could go
higher.

Would you get a bette picture on a larger scope at

Yes. The colors will be deeper, the contrast will be higher, the
image will be constructed of more "dots per inch". The dots being all
the over-lapping Arey disks, which are much smaller on a larger scope,
that make up the image.

Steve O.

Could you re-phrase all of that? I have no idea what you are trying
to ask. Yes, there is an optimal magnification for any given feature
through any given telescope, and yes, a bigger telescope shows more
than a smaller one at identical magnifications, assuming good seeing.
Is that all you are asking?

- Tony Flanders

When eyeballing through a scope the last filter in the equation is the eye
itself and its max native resolution, which is about 1 arcminute typical.
This cashes out with an optimal pupillary aperture of around 2-2.5mm. At
larger pupillary apertures aberrations kick in so the resolution does not
increase, even thought the aperture has increased.

So when you use a scope, you can think of the exit pupil - clear
aperture(mm) / magnification - matching these figures. If the exit pupil is
equal to 2-2.5mm you will be using the lowest magnification to get the most
detail possible for whatever scope you use. A larger exit pupil (higher
magnification) and you will not see more fine detail; while a smaller one it
will not resolve more than that falling on the retina.

I say "general" here, because there are a few assumptions being made here.
For example, I have assumed the native max resolution of the eye is in fact
1 arcminute - it may be less than this for a particular person. Also even
though theoretically you can match the exit-pupil to the resoultion of the
eye in this way, often cranking up the power can help in the easier
detection of well contrasted detail. Obviously there is a trade-off on the
perceived contrast as the magnification is increased, low contrast features
will eventually get washed out if you crank the power up too much.

The bottom line is to use this stuff as a guideline, but do not see it as
cast in stone. Do what you are doing and experiment and hone up your
observing skills, and see what suits you.

Here's a general guide on matching exit pupils to targets derived from
Rutten and van Venrooij (Telescope Optics):

Observational Task Exit Pupil (mm)

wide fields under dark skies 5 to 7
(large galaxies, Milky Way, diffuse nebulae, open clusters)

general viewing 3 to 4
(nebulae, clusters)

Best match to eye's resolution 2
(Moon, globular clusters)

Max planetary detail 0.8
(Texerau)

Close double satrs under best skies 0.5

but even this even this is not absolute, e.g. 173X on a 7cm TV Pronto (0.4mm
EP - 62X/inch or 24.7D(cm)) resulted in a clearer detection of central hole
in M57 than with lower powers.

For low power viewing their is no lower limit for an unobstructed scope,
e.g. a refractor. Just frame what you want to see in the eyepiece. In the
case of a centrally obstructed scope, though, the shadow of the secondary
will eventually dominate. You will soon notice this as on-axis the central
part of the image will appear to blank out. Drop the power enough and you
will literally see nothing as more and more the the exit pupil circle of
light fills with the image of the secondary's shadow. So their is a
practical limit here.

Regards,
Dave Randell

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