Naked-Eye Visual Magnitude Limit

In a recent thread about naked-eye visual star splitting, David
Knisely and Chris Peterson discussed visual magnitude limit for
unaided vision. It really does vary quite a bit between individuals.
Here's are excerpts from a posting I made on this subject to another
list in 1999.


"...We used the equilateral triangle of stars formed by eta, beta, and
mu Pegasi to guide our way to the the test stars.

....Very near my limit were HIP 113005 and 113063, both listed as
magnitude 7.4. I could hold these stars with averted vision most of
the time. I hit the wall at HIP 113092, which lies 10' west of a
slightly brighter star. It has a Hipparcos V magnitude of 7.7.

....While I stared at the screen, I had [Bernie Sanden] describe the
positions of these stars. He was able to see HIP 114833 and 114832,
magnitudes 8.4 and 8.3, respectively. Then he went on to mention HIP
113481 and 113486, which are 8.0- and 8.5-magnitude stars."

These observations were made at a dark site in southeast Arizona. I
can't say that I have ever tried to assess the effect of scintillation
on naked-eye magnitude limit, but I bet the effect is negligible.
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

Tom

On 30 Jun 2004 09:04:23 -0700, (Tom Polakis) wrote:

These observations were made at a dark site in southeast Arizona. I
can't say that I have ever tried to assess the effect of scintillation
on naked-eye magnitude limit, but I bet the effect is negligible.
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

I would say rather that scintillation and bad seeing are different but related
phenomena. In my location, they are rather well correlated (in the sense that
the stellar FWHM of both short (.1 sec) and long (1 min) exposures increases
with scintillation about 75% of the time. I occasionally have bad seeing without
scintillation. I virtually never have good seeing with scintillation.

My reference to scintillation with respect to visual limiting magnitude was
based on the [possible faulty] assumption that scintillation is the primary
seeing effect at visual resolutions. My own experience is that stars at my
borderline detection level (about mag 6.7) are seen when the sky is steady, but
not when there is high scintillation.

_________________________________________________

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

On 30 Jun 2004 09:04:23 -0700, (Tom Polakis) wrote:

These observations were made at a dark site in southeast Arizona. I
can't say that I have ever tried to assess the effect of scintillation
on naked-eye magnitude limit, but I bet the effect is negligible.
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

I would say rather that scintillation and bad seeing are different but related
phenomena. In my location, they are rather well correlated (in the sense that
the stellar FWHM of both short (.1 sec) and long (1 min) exposures increases
with scintillation about 75% of the time. I occasionally have bad seeing without
scintillation. I virtually never have good seeing with scintillation.

My reference to scintillation with respect to visual limiting magnitude was
based on the [possible faulty] assumption that scintillation is the primary
seeing effect at visual resolutions. My own experience is that stars at my
borderline detection level (about mag 6.7) are seen when the sky is steady, but
not when there is high scintillation.

_________________________________________________

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

"Chris L Peterson" wrote in message
...

My reference to scintillation with respect to visual limiting magnitude
was
based on the [possible faulty] assumption that scintillation is the
primary
seeing effect at visual resolutions. My own experience is that stars at my
borderline detection level (about mag 6.7) are seen when the sky is
steady, but
not when there is high scintillation.

I concur.

Instability in the atmosphere seems almost the equal to some upper threshold
on degraded transparency, given a "cloudless" night of good seeing.

I wish I could quanitify that a little better, but I can't. My experience
simply being that as the seeing degrades, my mag 5.7 NELM "detection
threshold" star between eta and zeta UMi becomes only momentarily
detectable, in a similar fashion as when the transparency degrades. At some
point, either degraded seeing or degraded transparency exceed a threshold
and the star becomes invisible, but long before the skies are what I would
consider poor for astronomy.

Conversely, if that star is visible directly, then the skies are excellent
(or very close to "really good"), which I think is the significance of the
exercise in detecting stars at the limit. The ultimate limit being defined
by the dimmest star an individual can see under the best conditions the
local atmosphere can offer.

The fact that someone half my age could see to magnitude 6.2 in UMi under
the same conditions, is meaningless. It only defines _their_ indicator of
good sky conditions. Mine is simply a half magnitude less because of
physiology. Thankfully, binoculars and telescopes allow us to see much
deeper than our eyes can without aid. :-)

-Steve Paul

"Chris L Peterson" wrote in message
...

My reference to scintillation with respect to visual limiting magnitude
was
based on the [possible faulty] assumption that scintillation is the
primary
seeing effect at visual resolutions. My own experience is that stars at my
borderline detection level (about mag 6.7) are seen when the sky is
steady, but
not when there is high scintillation.

I concur.

Instability in the atmosphere seems almost the equal to some upper threshold
on degraded transparency, given a "cloudless" night of good seeing.

I wish I could quanitify that a little better, but I can't. My experience
simply being that as the seeing degrades, my mag 5.7 NELM "detection
threshold" star between eta and zeta UMi becomes only momentarily
detectable, in a similar fashion as when the transparency degrades. At some
point, either degraded seeing or degraded transparency exceed a threshold
and the star becomes invisible, but long before the skies are what I would
consider poor for astronomy.

Conversely, if that star is visible directly, then the skies are excellent
(or very close to "really good"), which I think is the significance of the
exercise in detecting stars at the limit. The ultimate limit being defined
by the dimmest star an individual can see under the best conditions the
local atmosphere can offer.

The fact that someone half my age could see to magnitude 6.2 in UMi under
the same conditions, is meaningless. It only defines _their_ indicator of
good sky conditions. Mine is simply a half magnitude less because of
physiology. Thankfully, binoculars and telescopes allow us to see much
deeper than our eyes can without aid. :-)

-Steve Paul

These observations were made at a dark site in southeast Arizona.

Curious what area you site might be in, probably a secret.... :-)

A while back, we camped at the Cochise Stronghold for several nights, quite
dark but ringed by mountains so the view was limited. I think the altitude was
about 5000 feet IMMSMW.

jon

These observations were made at a dark site in southeast Arizona.

Curious what area you site might be in, probably a secret.... :-)

A while back, we camped at the Cochise Stronghold for several nights, quite
dark but ringed by mountains so the view was limited. I think the altitude was
about 5000 feet IMMSMW.

jon

Tom Polakis wrote, in part:
[snip]
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

In its "50 & 25 years ago" column, the August 2004 "Sky &Telescope" references
an August 1954 article about research showing that seeing and scintillation are
independent phenomena.

So to all the budding backyard observers out there, don't assume the seeing
will be bad just because the stars are twinkling. You might miss an excellent
seeing night.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

Tom Polakis wrote, in part:
[snip]
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

In its "50 & 25 years ago" column, the August 2004 "Sky &Telescope" references
an August 1954 article about research showing that seeing and scintillation are
independent phenomena.

So to all the budding backyard observers out there, don't assume the seeing
will be bad just because the stars are twinkling. You might miss an excellent
seeing night.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

c (Bill Ferris) wrote in message ...
Tom Polakis wrote, in part:
[snip]
This site at which we saw so deep is known for both bad seeing and
scintillation (which are two different phenomena).

In its "50 & 25 years ago" column, the August 2004 "Sky &Telescope" references
an August 1954 article about research showing that seeing and scintillation are
independent phenomena.

So to all the budding backyard observers out there, don't assume the seeing
will be bad just because the stars are twinkling. You might miss an excellent
seeing night.


I have never seen in quite a number of years good seeing with high
scintillation. Conversely I have seen very bad seeing with virtually
no or little scintillation. Few odd times I had decent seeing (5-6/10)
with noticeable scintillation. This spanning three continents and tens
of different locations.

Andrea T.