ATM what aperture, what magnification for visual detection

[Mel Bartels <mbartels@efn.org>]

I agree with Nils and others who point out that Clark has made an
interpretation error of his graph on pg 13 in his Visual Astronomy book.

I took the data presented earlier, recompiled it into a table, and wrote
a program that determines the object detectability over a range of
magnfications given aperture, sky background, and object magnitude and
size.  See my visual astronomy webpage off of my homepage.

Nils maintains that Clark has not presented Blackwell's data properly.
Nils has sent me a package of material but I have not gotten around to
looking it at, what with all the other more pressing projects in front
of me.

Even if Nils is correct, I am glad that I went ahead with my version of
'optimum detection magnification' across a range of magnifications
because it has affirmed what I have learned from experience.  Namely
that medium to high to very high magnifications can be, and should be,
employed to maximize object or detail detectability.  In addition, large
apertures do a much better job of preserving object detectability across
a wide range of magnifications.  Finally, the terrible effects of light
pollution are clear.

Dobson, Nagler, and other very experienced amateurs all advocate higher
magnifications than often used by amateurs.  I suspect that this too low
of power magnifications are popular because so many amateurs are using
large aperture on non-tracking mounts.  The problem of finding objects
and tracking them at very high power is not as trivial as many long time
large dob users would have the rest of us believe.  Ask yourself if it
is the exception or the rule to use 500x to 1000x on the Ring Nebula
with large dobs.  Using a tracking dob at high power has opened up a new
window to deep sky objects.  Objects are more visible, there is more
detail, and it is much more fun to view them at leisure.

For me, it is not so much that large aperture makes objects more
visible, instead it is that large aperture enables the use of a wide
range of magnfications.  An object might peter out at medium power in a
8", but in a 20", is still going strong at high powers.  While it is
amazing how much can really be seen in a 4" scope at low to medium
power, and that extremely high magnifications with small apertures with
very faint extended objects is the wrong approach, this is not the case
with large aperture.  However, you must get to dark skies and good
seeing.  Clark's data shows that while dark skies are worth incredible
gains in aperture, if you must observe in light polluted skies, a large
aperture scope, if used at appropriate magnifications, is always better
than a small aperture scope.

-- 
Clear skies, Mel Bartels    Programmer/Analyst, amateur astronomer
Eugene, Oregon, USA         homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org     atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html