From: "Richard Klappal" 
To: "Scott Ewart" , 
Reply-To: "Richard Klappal" 


I knew I had it here somewhere ...

        J.B. Sidgwick's "Amateur Astronomer's Handbook" lists
observations of
dark
spots against light backgrounds of Dawes/3.  Dark line against light
background out to Dawes/15. (Pickering was able to see a dark line that was
0.03 arc-seconds thru a 10-inch scope.  Most of these kinds of observations
were lunar craters or rills.

        Dawes' work was separating light objects against dark backgrounds, not
the
DETECTION of something.


Rich


-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Scott
Ewart
Sent: Friday, January 10, 2003 04:25 To: atm{at}shore.net
Subject: Re: ATM Ultimate Optical Capability



|
| "However, small extended object details are often detected in the image but
| not resolved. For example, the presence of a narrow but finite bright or
| dark line on a contrasting background may be detected, but it's width may
| not be resolved. Thus in an aerial photograph, a road may be resolved,
but | the painted line down the middle may only be detected. Unresolved
details | appear relatively larger than they really are due to image
spreading. Be | vigilant against this detection versus resolution source of
error." |
| >So what's the REAL limit?
|
| Maybe someone with time will look it up for you. |

Here's what I found.  Look in the Nov. 2000 S&T, p.117

|
| Furthermore, be careful what you say about "Encke's" Division.
Many people | who think they have seen it have actually seen the difference
in albedo of | various parts of the A-ring. And ask yourself, did Encke
ever see the | division in the A-ring? Isn't it really called Keeler's
Division?? |
    I've seen the actual gap at the Winter Star party and nowhere else.
According to the above article, Encke saw the "minimum", but
ironically, it was Dawes who first reported the actual gap in 1850 through
a 6.3" scope, and Keeler who later confirmed it in 1888 in the Lick's
36" Clark's FIRST LIGHT!  Hellofa first day.  Somehow it still ended
up credited to Encke. The Keeler gap is another even smaller gap even
closer to the outside edge of the A ring, but even the Hubble can't see
this one.  It was found by Voyager.  Lord knows why it was named so.  Maybe
someone figured he had it coming after losing the other one.
    So there you have it.  Cassini really did discover Cassini's division
(in 1675 with a 2.5" scope), Encke never saw the Encke gap, and Keeler
never saw the Keeler gap.
    The Cassini division is 4700km. wide, or .7" at mean opposition, the
Encke gap is 14 times smaller at 325km., or .05" and the Keeler gap 10
times smaller that that at 35km. wide, or .005"
    Also in that article, it says a Harvard astronomer found he could detect
a human hair at nearly 1/4 mile with an 11"scope.  That's .03",
1/14 Dawes limit.
    What I saw was only .01" plus maybe a little extra from limb darkening
(how much I don't know) with what I'm now finding out is a superb 8"
telescope.  And it wasn't smeared to anywhere near .5"(Dawes limit). 
I posted my observing report to my local club (DVAA) and it turns out a
fellow member logged the same thing independently with his 14".  Hell,
the Dec. '02 S&T article has an amateur IMAGE of the crescent
terminator and dark limb of Io against the Red Spot.  This stuff is not
impossible to see.  I don't know why it is.  It shouldn't be.
    So why am I bothering with an observing report on a telescope making
forum?  If you're making a mirror and you're thinking of quitting at 1/4
wave because it's "diffraction limited", you may reach
"Dawes limit", but that's about all.  If you ever want to use
more 150 power on a planet, don't stop at 1/4 wave, don't stop at 1/8 wave.
 If you have to settle for 1/16 wave, you probably won't be too unhappy. 
So take the time, whatever it takes, to make it absolutely the best you
can, because eventually, you'll be glad you did.  It DOES make a
difference.

Scott

--- BBBS/NT v4.00 MP