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Re: (IAAC) Obj: M1 - Inst: MEADE ETX 3.5" MAK



At 10:45 PM 4/1/98 +0200, you wrote:
>This illustrates Roger CLARK's statement of the Optimum Magnification
>Visual Angle (described in his book "visual astronomy of the deep sky").
>The author says that the optimal size of detection for a faint object
>(using averted vision) is between 1 and 2 degrees. That's why the 8' of M1
>(and Helix is also around 8') are better seen with 12x than 50x or 100x.
>The formula giving the OMVA is
>OMVA=90/S
>where S is the size of the object in arcminutes and OMVA is in x.
>For S=8', OMVA=11,25 that is the typical magnification of a binocular.
Unfortunately, Dr. Clark makes an error in the functions leading to the
graph on page 13 of his book, which invalidates practically everything he
says about quantities for optimum magnification for detection (including
his method for calculating it for a given object). The error crept in at a
discreet point in the analysis of some very old research by a man named
Blackwell (which was not done for astronomical use, but for other
purposes). Corrections to Clark's material can be found at:
http://zebu.uoregon.edu/~mbartels/visual/nils/blackwel.html
which is a paper from Nils Olof Carlin. Nevertheless, though Clark's
quantities are out of whack, his idea is sound: there is indeed an optimum
detection magnification. It should be noted that although Clark's error
seems rather obvious to me and many others as a simple error of math, Clark
maintains he is correct, last I knew. Anyway, Carlin's page will let you
decide on that point yourself.
However, there is more recent research available on which to base
quantities for this optimum detection magnification. Though it is heavily
oriented to the sighting of faint stars in a crowded field, these
disadvantages are somewhat outweighed by the fact that the research is
based on visual astronomical observation (and the resulting theory is also
for visual observation), as Blackwell's research was not. You may find a
copy of Brad Schaefer's paper in the Publications of the Astronomical
Society of the Pacific, and on the web at (this is a massive link, you
might have to manually make it fit on one line in your web browser; sorry):
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1990PASP%2E%2E1
02%2E%2E212S&page=1&plate_select=NO&data_type=GIF&type=SCREEN_GIF
If that URL causes you trouble, I have the paper linked to from my deep
photometric sequences page at:
http://shutter.vet.ohio-state.edu/astronomy/mags/
Anyway, Brad Schaefer's article(s) in S&T's latest issues are very
watered-down versions of the PASP paper and the research leading up to it.
Nils Olof Carlin is working on an analysis of Schaefer's research in order
to compare it to the results of using the Blackwell research. It seems
extremely likely to me that a few things about the optimum detection
magnification and related issues will change:
1) The magnification will grow significantly higher than Carlin had it,
which was rather higher than Clark had it for most cases;
2) Wide angle eyepieces (>50d apparent fields) will be shown to be inferior
at the limits of detection to narrow angle (<50d apparent fields) ones.
3) The hoary myth about low magnifications, wide fields, and bright images
going hand in hand will be exploded again (as if Clark did not already
explode it once already).
4) It will be shown that there is a considerable personal equation to be
considered in determining the optimum detection magnification, which might
change the value by upwards of 20% from person to person.
At the moment, I tend to think that the rule of thumb stated in Skiff &
Luginbuhl's "Observing Handbook and Catalog of Deep Sky Objects" will not
go seriously challenged. To distill, they maintain that a magnification
that renders the object at two or three apparent degrees, or whatever
magnification makes the eyepiece field stop indistinguishable from the sky,
whichever is lower, is the best to use. It can be seen that this is already
some 50% or more larger than Clark's value and is quite consistent with
Schaefer's data - this from a book published nearly ten years ago.
Incidentally, I finally was able to find a copy of Luginbuhl&Skiff. The
person who suggested to me that I use amazon.com might be interested to
know that was a useless approach, even though it was a good idea; but I
found one at a local telescope shop not long ago. There are still things we
might learn from this book about observing disciplines and self-consistent
data sourcing; I certainly hope that it returns in the future, perhaps as a
paperbound reprint.
--
Jeff Medkeff          | Acting Assistant Coordinator
Rockland Observatory  | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section
On the web at http://shutter.vet.ohio-state.edu/