Urban Richest-Field Telescope?

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

Well if 11th magnitude is deemed the ideal, then one can determine
which size scope is appropriate for each observer's location.

However an urban location will use a larger scope and hence smaller
field of view, so it will never match the richness at a dark site.

Alvan Clark

On Sep 28, 12:02 am, Margo Schulter wrote:
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.

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_.

While I can't comment on much of your post, as my own observing goals
for the present are of a different nature, I would tend to agree with
your conclusion.

After all, the dilation of the dark-adapted eye is 7mm (at least if
you're still only 30 years old) whether you are in the city or the
country, and that is what the definition of an RFT has been taken to
be.

The popular and inexpensive 7x50 binocular is another one that meets
that criterion.

John Savard

Quadibloc wrote:

After all, the dilation of the dark-adapted eye is 7mm (at least if
you're still only 30 years old) whether you are in the city or the
country, and that is what the definition of an RFT has been taken to
be.

Hi, John, and what you say raises an interesting question.

Based on a conversation with one experienced observer, I got the idea
that urban light pollution can limit dark adaption. Obviously direct
light trespass can do this at any site (thus star party etiquette!),
but somehow I drew the conclusion that urban skyglow, say magnitude 17
per square arcsecond, could prevent the pupil from reaching the dilation
it might attain at a dark site. Of course, I'd like to correct this
(mis)understanding if it's wrong.

If city skyglow doesn't substantially affect dark adaptation, then age
might be a main relevant factor: might we speak of 5mm exit pupil or so
as an "Elder's Richest Field Telescope"?

The popular and inexpensive 7x50 binocular is another one that meets
that criterion.

Yes, and curiously I'm still enjoying my pair from 21 years back,
although I realize that for someone my age, the effective aperture may
be a bit less than 50mm.

John Savard


Most appreciatively,

Margo Schulter

Lat. 38.566 Long. -121.430

Margo Schulter wrote:
Quadibloc wrote:

After all, the dilation of the dark-adapted eye is 7mm (at least if
you're still only 30 years old) whether you are in the city or the
country, and that is what the definition of an RFT has been taken to
be.

Hi, John, and what you say raises an interesting question.

Based on a conversation with one experienced observer, I got the idea
that urban light pollution can limit dark adaption. Obviously direct
light trespass can do this at any site (thus star party etiquette!),
but somehow I drew the conclusion that urban skyglow, say magnitude 17
per square arcsecond, could prevent the pupil from reaching the dilation
it might attain at a dark site. Of course, I'd like to correct this
(mis)understanding if it's wrong.

Not sure myself, but this other thought I had might provide a clue. I
thought that any local condition that permits you to see objects around
you will affect your night vision. Hence the moon, and particularly the
Full Moon, which often lights up a landscape brighter, it seems, than
scattered artificial lighting.

It amounts to photons entering the eye regardless of the source.


If city skyglow doesn't substantially affect dark adaptation, then age
might be a main relevant factor: might we speak of 5mm exit pupil or so
as an "Elder's Richest Field Telescope"?

That has to be another factor as we age. There is also the fact that
younger eyes are more sensitive to light than older ones, and can see
things sharper, in general.


The popular and inexpensive 7x50 binocular is another one that meets
that criterion.

Yes, and curiously I'm still enjoying my pair from 21 years back,
although I realize that for someone my age, the effective aperture may
be a bit less than 50mm.

John Savard


Most appreciatively,

Margo Schulter

Lat. 38.566 Long. -121.430

On 30 Sep 2007 07:05:33 GMT, Margo Schulter
wrote:

Based on a conversation with one experienced observer, I got the idea
that urban light pollution can limit dark adaption. Obviously direct
light trespass can do this at any site (thus star party etiquette!),
but somehow I drew the conclusion that urban skyglow, say magnitude 17
per square arcsecond, could prevent the pupil from reaching the dilation
it might attain at a dark site. Of course, I'd like to correct this
(mis)understanding if it's wrong.

Dark adaptation has nothing to do with pupil dilation; it is a matter of
the chemistry of the retina. Urban light pollution can easily affect
dark adaptation- it is often bright enough under urban skies to allow
for mesopic vision, which means it is bright enough that you will not
become fully dark adapted. But under those conditions, your pupil will
certainly reach the same degree of dilation as it would under the
darkest skies.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Sep 30, 11:15 am, Chris L Peterson wrote:
But under those conditions, your pupil will
certainly reach the same degree of dilation as it would under the
darkest skies.

Do you have a reference for this? That the pupil diameter would be the
same in the urban night vs. a dark country night. I googled around a
bit but could find no experiments on this.

Alvan Clark

On Sun, 30 Sep 2007 09:23:02 -0700, wrote:

Do you have a reference for this? That the pupil diameter would be the
same in the urban night vs. a dark country night. I googled around a
bit but could find no experiments on this.

No, but I've measured my own and other people's pupil diameters on many
occasions. The pupil becomes fully dilated under moderately dim
conditions, such as inside a room with the lights out and blinds drawn,
even with enough light to still support fully photopic vision.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:
On Sun, 30 Sep 2007 09:23:02 -0700, wrote:

Do you have a reference for this? That the pupil diameter would be the
same in the urban night vs. a dark country night. I googled around a
bit but could find no experiments on this.

No, but I've measured my own and other people's pupil diameters on many
occasions. The pupil becomes fully dilated under moderately dim
conditions, such as inside a room with the lights out and blinds drawn,
even with enough light to still support fully photopic vision.


Hi, Chris, and all.

Thank you for this very important information which will help me in getting
my facts straight when discussing exit pupil and light pollution. Maybe it
should be a FAQ, although I'm not sure how often it's asked.

Today, when I had a routine eye exam, I asked my doctor after explaining
that I'm an amateur astronomer if he could check my usual pupil dilation
in the dark. He turned down the lights, pretty much as you describe, and
said it was somewhere around 5-6mm. I noticed that the room still wasn't
that terribly dark, maybe somewhere around nautical twilight (very
unscientific guess!).

So I guess a summary might be about like this:

(1) Under urban light pollution, a person's pupils will still dilate
to the same size as under dark-sky conditions, so that the
specific issue of match between the observer's eye dilation and
the exit pupil for a given telescope-eyepiece combination is
not affected.

(2) However, urban light pollution definitely _can_ affect the
degree of dark adaptation, which is mostly a biochemical
process occurring over 30 minutes or more.

Again, this is a very helpful distinction.

Most appreciatively,

Margo Schulter

Lat. 38.566 Long. -121.430

Today, when I had a routine eye exam, I asked my doctor after explaining
that I'm an amateur astronomer if he could check my usual pupil dilation
in the dark. He turned down the lights, pretty much as you describe, and
said it was somewhere around 5-6mm. I noticed that the room still wasn't
that terribly dark, maybe somewhere around nautical twilight (very
unscientific guess!).

The pupil also tends to constrict when the eye accommodates for nearby
objects, and tends to dilate for more distant objects. Perhaps if your
telescope or binoculars are focused "too close" your pupils might
constrict more than they would otherwise?