Mag 17 to Mag 3 Comet

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

On Fri, 2 Nov 2007 18:56:04 -0700, "Bernard Isker"
wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

That's not a very useful way of thinking of it. It isn't like the comet
physically went from mag 17 to mag 3 - that would be quite impossible.
What happened is that the comet (as a small chunk of ice and rock)
produced something else (a huge cloud of dust and gas). In terms of
actual peak brightness, there's been little change in this comet after
the first few hours. Only the integrated brightness has increased,
because the coma has increased in size.

So the starting magnitude isn't the issue. The real question is how big
can a coma realistically get, and how much light can it reflect? Given
the rarified nature of a coma, I don't see that it could approach the
brightness of the Moon. And keep in mind, the magnitude of the Moon is
given for an extended object- that is, integrated brightness. In terms
of brightness per unit area, I'm pretty sure that some comets are of a
similar brightness to the Moon anyway.

_________________________________________________

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

Chris L Peterson wrote:
On Fri, 2 Nov 2007 18:56:04 -0700, "Bernard Isker"
wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

That's not a very useful way of thinking of it.

Quite right. But there's something else that's been annoying me just a
bit: this idea that it was 17th magnitude before the outburst. It is
true that it was reported to be magnitude 16.5 on CCD images obtained on
October 21. But it is well known that CCD photometry can often differ
by as much as three magnitudes from that of a visual observer,
particularly when they get that faint. And when they do differ it is
always in the sense that the visual estimate is brighter. It is nearly
impossible to do comparable CCD photometry once it becomes big and
bright. So in order to avoid these apples/oranges magnitude comparisons
it is best to go with the visual estimates. Although they weren't done
just a few days before hand, the most recent visual estimates were 14th
magnitude. So given the orbital geometry in the days before the
outburst it was still likely 14-15th magnitude. That is an increase in
brightness of something more like 12 magnitudes. Still incredibly
dramatic, but not quite the hyperbole that we've been fed.

Clear skies,
Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

On Fri, 02 Nov 2007 20:42:46 -0600, Greg Crinklaw
wrote:

Quite right. But there's something else that's been annoying me just a
bit: this idea that it was 17th magnitude before the outburst. It is
true that it was reported to be magnitude 16.5 on CCD images obtained on
October 21. But it is well known that CCD photometry can often differ
by as much as three magnitudes from that of a visual observer,
particularly when they get that faint. And when they do differ it is
always in the sense that the visual estimate is brighter. It is nearly
impossible to do comparable CCD photometry once it becomes big and
bright. So in order to avoid these apples/oranges magnitude comparisons
it is best to go with the visual estimates. Although they weren't done
just a few days before hand, the most recent visual estimates were 14th
magnitude. So given the orbital geometry in the days before the
outburst it was still likely 14-15th magnitude. That is an increase in
brightness of something more like 12 magnitudes. Still incredibly
dramatic, but not quite the hyperbole that we've been fed.

Personally, I place little confidence in visual estimates. There are
different ways of assessing brightness when discussing extended objects,
but regardless of which you prefer, I don't think you can beat an
instrumental measurement. I've been collecting photometric data since
the first night the comet brightened, and expect to be able to put it to
good use (I'm using it to test a diffusion model describing the comet's
growth). But it would definitely be an oversimplification to simply
assign a magnitude to the comet with no additional explanation.

_________________________________________________

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

Chris L Peterson wrote:

On Fri, 02 Nov 2007 20:42:46 -0600, Greg Crinklaw
wrote:

Quite right. But there's something else that's been annoying me just a
bit: this idea that it was 17th magnitude before the outburst. It is
true that it was reported to be magnitude 16.5 on CCD images obtained on
October 21. But it is well known that CCD photometry can often differ
by as much as three magnitudes from that of a visual observer,
particularly when they get that faint. And when they do differ it is
always in the sense that the visual estimate is brighter. It is nearly
impossible to do comparable CCD photometry once it becomes big and
bright. So in order to avoid these apples/oranges magnitude comparisons
it is best to go with the visual estimates. Although they weren't done
just a few days before hand, the most recent visual estimates were 14th
magnitude. So given the orbital geometry in the days before the
outburst it was still likely 14-15th magnitude. That is an increase in
brightness of something more like 12 magnitudes. Still incredibly
dramatic, but not quite the hyperbole that we've been fed.

Personally, I place little confidence in visual estimates.

Thank God somebody finally said it!



There are
different ways of assessing brightness when discussing extended objects,
but regardless of which you prefer, I don't think you can beat an
instrumental measurement. I've been collecting photometric data since
the first night the comet brightened, and expect to be able to put it to
good use (I'm using it to test a diffusion model describing the comet's
growth). But it would definitely be an oversimplification to simply
assign a magnitude to the comet with no additional explanation.

_________________________________________________

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

Bernard Isker wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

Ah but it didnt and there's the difference. Moreover .....

Chris L Peterson wrote:
On Fri, 02 Nov 2007 20:42:46 -0600, Greg Crinklaw
wrote:

Quite right. But there's something else that's been annoying me just a
bit: this idea that it was 17th magnitude before the outburst. It is
true that it was reported to be magnitude 16.5 on CCD images obtained on
October 21. But it is well known that CCD photometry can often differ
by as much as three magnitudes from that of a visual observer,
particularly when they get that faint. And when they do differ it is
always in the sense that the visual estimate is brighter. It is nearly
impossible to do comparable CCD photometry once it becomes big and
bright. So in order to avoid these apples/oranges magnitude comparisons
it is best to go with the visual estimates. Although they weren't done
just a few days before hand, the most recent visual estimates were 14th
magnitude. So given the orbital geometry in the days before the
outburst it was still likely 14-15th magnitude. That is an increase in
brightness of something more like 12 magnitudes. Still incredibly
dramatic, but not quite the hyperbole that we've been fed.

Personally, I place little confidence in visual estimates. There are
different ways of assessing brightness when discussing extended objects,
but regardless of which you prefer, I don't think you can beat an
instrumental measurement. I've been collecting photometric data since
the first night the comet brightened, and expect to be able to put it to
good use (I'm using it to test a diffusion model describing the comet's
growth). But it would definitely be an oversimplification to simply
assign a magnitude to the comet with no additional explanation.

Of course visual estimates are problematic (I never said they weren't).
What I did say was that the visual estimates are consistent with each
other. That is what is important in this particular case.

Those with experience with comet photometry know that the problems posed
are very difficult to overcome. So far I am unaware of any amateurs who
have been able to do photometry on comets in a consistent repeatable way
over the wide range of size and magnitudes presented. This is why the
visual estimates are still considered useful and still done routinely.
The most powerful shortcut to accurate photometry--differential
photometry--does not typically apply. It doesn't help that even most
professionals are sloppy about their photometry (not taking enough
standards, not making enough observations to properly characterize the
extinction, or not making the effort to accurately tie their
observations to a standard system), let alone most amateurs. The
magnitudes for small faint comets aren't too bad, but the large bright
ones become very problematic. Without consistency over this range the
photometry isn't any better than the visual estimates.

I am sure that accurate consistent photometry of comets of all sizes and
shapes can in theory be done routinely by amateurs, but I have seen no
evidence that it has yet happened. How does one remove the background
stars? How does one remove the background skyglow? How does one handle
the systematic effects of going from a tiny starlike object to one that
requires a mosaic to capture of the entirety? How much of the tail
should be included in order to be consistent?

In my experience there are no trivial solutions to the comet magnitude
problem.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

On Fri, 2 Nov 2007 18:56:04 -0700, "Bernard Isker"
wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

If Hale-Bopp (it should be a a dash, not a slash, between "Hale" and
"Bopp") had appeared as a zero magnitude object without a coma, that
alone would have been a sensation! O.t.o.h. that would have required a
planet-sized nucleus of Hale-Bopp.

If the dwarf planet Pluto somehow would suddenly be brought into the
inner solar system, it could appear as a magnitude zero object before
the outgassing started. The outgassing would later produce a coma
probably many degrees large - and then you might have had a comet
(Pluto) outshining the full moon!

For real comets, which always have nuclei far smaller than planets,
the only chance for a comet to outshine the full moon is to either
pass very close to the Sun (and then it'll be visible only in
daytime), or very close to the Earth.

The s-f author H.G. Wells once wrote a novel, "In the days of the
comet", which included a comet outshining the full moon shortly before
it collided with the Earth.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

In article ,
Bernard Isker wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

If Hale-Bopp (it should be a a dash, not a slash, between "Hale" and
"Bopp") had appeared as a zero magnitude object without a coma, that
alone would have been a sensation! O.t.o.h. that would have required a
planet-sized nucleus of Hale-Bopp.

If the dwarf planet Pluto somehow would suddenly be brought into the
inner solar system, it could appear as a magnitude zero object before
the outgassing started. The outgassing would later produce a coma
probably many degrees large - and then you might have had a comet
(Pluto) outshining the full moon!

For real comets, which always have nuclei far smaller than planets,
the only chance for a comet to outshine the full moon is to either
pass very close to the Sun (and then it'll be visible only in
daytime), or very close to the Earth.

The s-f author H.G. Wells once wrote a novel, "In the days of the
comet", which included a comet outshining the full moon shortly before
it collided with the Earth.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

In article ,
Chris L Peterson wrote:

On Fri, 2 Nov 2007 18:56:04 -0700, "Bernard Isker"
wrote:

If Hale/Bopp had done what Comet Holmes did this week when it was at it's
best would it have been brighter than the moon??

Hale/Bopp was about mag zero or even brighter for quite awhile and going 14
magnitudes brighter would put it about 2X brighter than a full moon.

That's not a very useful way of thinking of it. It isn't like the comet
physically went from mag 17 to mag 3 - that would be quite impossible.
What happened is that the comet (as a small chunk of ice and rock)
produced something else (a huge cloud of dust and gas). In terms of
actual peak brightness, there's been little change in this comet after
the first few hours. Only the integrated brightness has increased,
because the coma has increased in size.

So the starting magnitude isn't the issue. The real question is how big
can a coma realistically get, and how much light can it reflect? Given
the rarified nature of a coma, I don't see that it could approach the
brightness of the Moon. And keep in mind, the magnitude of the Moon is
given for an extended object- that is, integrated brightness. In terms
of brightness per unit area, I'm pretty sure that some comets are of a
similar brightness to the Moon anyway.

Why not start using a better terminology here?

"integrated brightness" = irradiance
"brightness per unit area" = radiance

instead of using the ambiguous term "brightness" ? I mean, you say
"mass" and "diameter" rather than "massive size" and "geometric size",
don't you?

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/