Re: SV: ATM barlows, magnification

["Alan Levin" <ailevin@ix.netcom.com>]

> Bob,
> there are 2 things that limit the possible resoultion you can get from your
> telescope - you, and your telescope :-) The retina of the eye has finite
> sized cones to pick up detail, and you need magnification to see small
> things. The telescope has an aperture that will limit the resolution due to
> the wavelength of light (if the "seeing" due to air currents is good
> enough).
< much good analysis snipped>
 
> Nils Olof
> 
> No sensible person could disagree with this, but go ahead anyway.  
> 

Nils,

I couldn't resist after that last line <G>.

I like your analysis although I would give even more emphasis to the
fact that atmosphere, and the quality/collimation of his optics are
most likely to limit Bob's planetary viewing.  In my experience 150x
most commonly seems to be the best compromise between magnification
and seeing for deep sky at my observing site.  However, seeing
changes from moment to moment and you must integrate the views from
those few moments when the planet "snaps" into observability.  At 
other sites and with patient viewing, I found magnification up to 
about 250x useful in an 8 inch SCT.

I think Bob needs a lower power eyepiece more that a Barlow.  Either
a 32mm Plossl or a slightly shorter widefield type.  A 2x Barlow
would give just under 1mm exit pupil with the 12mm eyepiece which is
all the magnification he is likely to find useful except for
critical optical testing or for for 1 in 100 nights of steadiness, or
for that move to Chiefland, FL with the rock steady sub arcsecond
seeing.  BTW the Kellner should be fine on axis for high
magnification viewing with a barlow as long as the eye relief is
sufficient.

The place I disagree with you is in your implicit treatment of the
retina as an photographic device rather than a neural device.  The
detectablility of objects depends on a rather subtle  balance of the
contrast difference between the object and the background, and the
size of the object.  Thus, while contrast may decrease with more
magnification, detectability may increase.  The rods not only
respond nonlinearly, but they have a cooperative behavior so that a
larger number of contiguous cones may respond at a light level that
is below threshhold for a smaller number of cones.

I believe that Mel has written some software that is based on (?)
the physiological measurement data in Roger Clark's book "The Visual
Astronomy Of The Deep Sky."  This software allows you to calculate
the optimal magnification for detection based on the magnitude of
the object, the size of the object, the magnitude of the sky
background, and the aperture of the telescope.

Good Observing,
Alan