Hello, everyone.

Over the last seven decades, S. L. Walkden's famous article of 1936
about "The Richest-Field Telescope" (RFT) has inspired many
variations. Walkden celebrates the special charms of a small telescope
designed to show as many stars as possible in a single field of view
when sweeping the Milky Way: limiting magnitude around 11; aperture
about 3"; fast optics around f/5 or f/6; and magnification near 10X.
The exit pupil should match the full dilation of the observer's eyes.

http://adsabs.harvard.edu/abs/1936PA.....44..146W

Today a good pair of 11X80 binoculars might neatly realize this
original RFT concept at a dark-sky site for a younger observer
with 7mm dilation -- or maybe 16X80 for an older observer with more
like a 5mm dilation. An 80mm f/6 refractor plus an ultra-wide 30mm
eyepiece, both of high quality, might carry the ideal yet further
while holding to Walkden's scenario of a small-aperture instrument:

-----------------------------------------------------------------
Aperture FL f-ratio EP mag EP FL AFOV TFOV Exit pupil
-----------------------------------------------------------------
80mm 480mm f/6 16X 30mm 82.0d 5d08' 5.0mm
-----------------------------------------------------------------

However, I would like to consider a different kind of observing
scenario here which some might deem a self-contradiction: a
realization of Walkden's RFT ideal at a light-polluted urban site.
Can one really speak of "the RFT ideal" in skies where the Milky Way
is invisible to the naked eye?

From my own observing experience in such urban conditions, however
modest, I would answer, "Yes." Even in a heavily light-polluted area,
one can still relish having a wide-field view of the Milky Way in
Sagittarius, say, filled as richly with stars _as the circumstances
permit_.

One of my favorite objects, M24, may illustrate how Walkden's ideal of
richest Milky Way viewing may overlap with the recent RFT ethos of
optimized viewing of large deep-sky objects (DSO's). Happily, M24 is
at once a fine example of a large DSO spread out over about 2 degrees,
and an integral part of the Milky Way, the Small Sagittarius Star
Cloud.

Here is the configuration for my "Urban Richest-Field Telescope," or
possibly URFT for short, a Sky-Watcher 200mm f/6 Dobsonian:

-----------------------------------------------------------------
Aperture FL f-ratio EP mag EP FL AFOV TFOV Exit pupil
-----------------------------------------------------------------
200mm 1200mm f/6.0 40X 30mm 82.0d 2d03' 5.0mm
-----------------------------------------------------------------

Increasing the aperture and magnification as a measure to alleviate
(very partially!) the effects of light pollution results in a rather
narrower field than for a classic RFT at around 3-5 degrees -- but
still relatively ample at 2 degrees. For a not-so-young urban viewer
like myself, even one enjoying the benefits of the Orion observing
canopy, I suspect that the 5mm exit pupil may be quite adequate to
match the viewer's likely maximal eye dilation.

Under urban conditions, such a medium-size Newtonian reflector may
actually best realize another aspect of Walkden's RFT concept: a
limiting magnitude somewhere around 11. At sites like mine where stars
in the densely populated regions of Scorpius or Sagittarius may have a
naked eye limiting magnitude (NELM) somewhere around 3.0-3.5, a 200mm
aperture for an observer with a maximal pupil dilation of 5-7mm should
produce a gain of about 7-8 magnitudes, taking us to the desired
vicinity of magnitude 11.

Most appreciatively,

Margo Schulter

Lat. 38.566 Long. -121.430