Network of Small Telescopes Discovers Distant Planet Orbiting Another Star

http://www.lowell.edu/press_room/rel...rES-1_rls.html

For Immediate Release

Lowell Observatory
August 24, 2004

This is a joint announcement from the Astrophysical Institute of the
Canaries (IAC), National Center for Atmospheric Research (NCAR),
Harvard-Smithsonian Center for Astrophysics (CfA), Lowell Observatory,
and California Institute of Technology.

Note to Editors: High-resolution artwork and animation of the newly
discovered planet TrES-1 is posted online at
http://www.lowell.edu/press_room/TrES-1_images.html

Network of Small Telescopes Discovers Distant Planet

Flagstaff, AZ - Fifteen years ago, the largest telescopes in the world had
yet to locate a planet orbiting another star. Today telescopes no larger
than those available in department stores are proving capable of
spotting previously unknown worlds. A newfound planet detected by a
small, 4-inch-diameter telescope demonstrates that we are at the cusp of
a new age of planet discovery. Soon, new worlds may be located at an
accelerating pace, bringing the detection of the first Earth-sized world
one step closer.

"This discovery demonstrates that even humble telescopes can make huge
contributions to planet searches," says Guillermo Torres of the
Harvard-Smithsonian Center for Astrophysics (CfA), a co-author on the
study.

This is the first extrasolar planet discovery made by a dedicated survey
of many thousands of relatively bright stars in large regions of the
sky. It was made using the Trans-Atlantic Exoplanet Survey (TrES), a
network of small, relatively inexpensive telescopes designed to look
specifically for planets orbiting bright stars. A team of scientists
co-led by Edward Dunham of Lowell Observatory, Timothy Brown of NCAR,
and David Charbonneau (CfA), developed the TrES network. The network's
telescopes are located in Palomar Observatory (California, USA), Lowell
Observatory (Arizona, USA), and the Canary Islands (Spain).

"The advantage of working as a network is that we can 'stretch the
night' and monitor our fields for a longer time, increasing our chance
of discovering a planet," says Georgi Mandushev (Lowell Observatory), a
co-author of the paper.

This research study will be posted online at
http://arxiv.org/abs/astro-ph/0408421 and will appear in an upcoming
issue of The Astrophysical Journal Letters.

"It took several Ph.D. scientists working full-time to develop the data
analysis methods for this search program, but the equipment itself uses
simple, off-the-shelf components," says co-author David Charbonneau
(CfA/Caltech).

Although the small telescopes of the TrES network made the initial
discovery, follow-up observations at other facilities were required.
Observations at the W. M. Keck Observatory which operates the world's
two largest telescopes in Hawaii for the University of California,
Caltech, and NASA, were particularly crucial in confirming the planet's
existence.

Planet Shadows

The newfound planet is a Jupiter-sized gas giant orbiting a star located
about 500 light years from the Earth in the constellation Lyra. This
world circles its star every 3.03 days at a distance of only 4 million
miles (6 million kilometers), much closer and faster than the planet
Mercury in our solar system.

Although such planets are relatively common, astronomers used an
uncommon technique to discover it. This world was found by the "transit
method," which looks for a dip in a star's brightness when a planet
crosses directly in front of the star and casts a shadow. A
Jupiter-sized planet blocks only about 1/100th of the light from a
Sun-like star, but that is enough to make it detectable.

"This Jupiter-sized planet was observed doing the same thing that
happened in June when Venus moved across (or transited) the face of our
Sun," says Mandushev. "The difference is that this planet is outside our
solar system, roughly 500 light years away."

To be successful, transit searches must examine many stars because we
only see a transit if a planetary system is located nearly edge-on to
our line of sight. A number of different transit searches currently are
underway. Most examine limited areas of the sky and focus on fainter
stars because they are more common, thereby increasing the chances of
finding a transiting system. However the TrES network concentrates on
searching brighter stars in larger swaths of the sky because planets
orbiting bright stars are easier to study directly.

"All that we have to work with is the light that comes from the star,"
says Tim Brown (NCAR), a study co-author. "It's much harder to learn
anything when the stars are faint."

Most known extrasolar planets were found using the "Doppler method,"
which detects a planet's gravitational effect on its star by looking for
shifts in the star's spectrum, or rainbow of colors. However, the
information that can be gleaned about a planet using the Doppler method
is limited. For example, only a lower limit to the mass can be
determined because the angle at which we view the system is unknown. A
high-mass brown dwarf whose orbit is highly inclined to our line of
sight produces the same signal as a low-mass planet that is nearly edge-on.

"When astronomers find a transiting planet, we know that its orbit is
essentially edge-on, so we can calculate its exact mass. From the amount
of light it blocks, we learn its physical size. In one instance, we've
even been able to detect and study a giant planet's atmosphere," says
Charbonneau.

Sorting Suspects

The TrES survey examined approximately 12,000 stars in 36 square degrees
of the sky (about half of the size of the bowl of the Big Dipper) in the
constellation of Lyra. Roi Alonso (IAC), a graduate student of Brown's,
identified 16 possible candidates for planet transits. "The TrES survey
gave us our initial line-up of suspects. Then, we had to make a lot of
follow-up observations to eliminate the imposters," says co-author
Alessandro Sozzetti (University of Pittsburgh/CfA).

After compiling the list of candidates in late April, the researchers
used telescopes at CfA's Whipple Observatory in Arizona, Oak Ridge
Observatory in Massachusetts, and Lowell Observatory in Arizona to
obtain additional photometric (brightness) observations, as well as
spectroscopic observations that eliminated eclipsing binary stars.

In a matter of two month's time, the team had zeroed in on the most
promising candidate. High-resolution spectroscopic observations by
Torres and Sozzetti using time provided by NASA on the 10-meter-diameter
Keck I telescope in Hawaii clinched the case.

"Without this follow-up work the photometric surveys can't tell which of
their candidates are actually planets. The proof of the pudding is a
spectroscopic orbit for the parent star. That's why the Keck
observations of this star were so important in proving that we had found
a true planetary system," says co-author David Latham (CfA).

Remarkably Normal

The planet, called TrES-1, is much like Jupiter in mass and size. It is
likely to be a gas giant composed primarily of hydrogen and helium, the
most common elements in the Universe. But unlike Jupiter, it orbits very
close to its star, giving it a temperature of around 1500 degrees F.

Astronomers are particularly interested in TrES-1 because its structure
agrees so well with theory, in contrast to the first discovered
transiting planet, HD 209458b. The latter world contains about the same
mass as TrES-1, yet is around 30% larger in size. Even its proximity to
its star and the accompanying heat don't explain such a large size.

"Finding TrES-1 and seeing how normal it is makes us suspect that HD
209458b is an 'oddball' planet," says Charbonneau.

TrES-1 orbits its star every 72 hours, placing it among a group of
similar planets known as "hot Jupiters." Such worlds likely formed much
further away from their stars and then migrated inward, sweeping away
any other planets in the process. The many planetary systems found to
contain hot Jupiters indicate that our solar system may be unusual for
its relatively quiet history.

Both the close orbit of TrES-1 and its migration history make it
unlikely to possess any moons or rings. Nevertheless, astronomers will
continue to examine this system closely because precise photometric
observations may detect moons or rings if they exist. In addition,
detailed spectroscopic observations may give clues to the presence and
composition of the planet's atmosphere.

The paper, "TrES-1: The Transiting Planet of a Bright K0V Star,"
descibing these results is authored by: Roi Alonso (IAC); Timothy M.
Brown (NCAR); Guillermo Torres and David W. Latham (CfA); Alessandro
Sozzetti (University of Pittsburgh/CfA); Georgi Mandushev (Lowell
Observatory), Juan A. Belmonte (IAC); David Charbonneau (CfA/Caltech);
Hans J. Deeg (IAC); Edward W. Dunham (Lowell Observatory); Francis T.
O'Donovan (Caltech); and Robert Stefanik (CfA).

The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy, a scientific partnership of the California
Institute of Technology, the University of California, and the National
Aeronautics and Space Administration (NASA).

Funding for the research that led to this planet's discovery was
provided by NASA's Origins of Solar Systems Program.

Founded in 1894, Lowell Observatory pursues the study of astronomy,
conducts pure research in astronomical phenomena, and maintains quality
public education and outreach programs.

#END#

contact: Steele Wotkyns
Public Relations Manager
(928) 233-3232

www.lowell.edu

For additional information:
This research study, "TrES-1: The Transiting Planet of a Bright K0V
Star," will be posted online at http://arxiv.org/abs/astro-ph/0408421
and will appear in an
upcoming issue of The Astrophysical Journal Letters.
High-resolution artwork and animation of the newly discovered planet
TrES-1 is online at
http://www.lowell.edu/press_room/TrES-1_images.html

Harvard-Smithsonian Center for Astrophysics press release
http://cfa-www.harvard.edu/ep/pressrel.html

National Center for Atmospheric Research press release
http://www.ucar.edu/news/releases/

Astrophysical Institute of the Canaries press release
http://www.iac.es/gabinete/noticias/noticias.htm

Thanks!
Very interesting information.
---Mac
**************************
On 24 Aug 2004 09:57:19 -0700, (Ron) wrote:
http://www.lowell.edu/press_room/rel...rES-1_rls.html
For Immediate Release
Lowell Observatory
August 24, 2004

This is a joint announcement from the Astrophysical Institute of the
Canaries (IAC), National Center for Atmospheric Research (NCAR),
Harvard-Smithsonian Center for Astrophysics (CfA), Lowell Observatory,
and California Institute of Technology.

Note to Editors: High-resolution artwork and animation of the newly
discovered planet TrES-1 is posted online at
http://www.lowell.edu/press_room/TrES-1_images.html

Network of Small Telescopes Discovers Distant Planet

Flagstaff, AZ - Fifteen years ago, the largest telescopes in the world had
yet to locate a planet orbiting another star. Today telescopes no larger
than those available in department stores are proving capable of
spotting previously unknown worlds. A newfound planet detected by a
small, 4-inch-diameter telescope demonstrates that we are at the cusp of
a new age of planet discovery. Soon, new worlds may be located at an
accelerating pace, bringing the detection of the first Earth-sized world
one step closer.
SNIP SNIP
******************************

It is certainly inspiring to learn that one can discover extrasolar
planets using a telescope with a roughly 4 inch aperture. I think I
understand why three different telescopes at three different sites were
needed to discover it, since they had to sift through so many candidates
and this helped with the process of weeding out false alarms. However,
now that it has been discovered and its discovery confirmed, what are
the difficulties one would face in using a beginner's telescope, say
one of the $200 computer controlled models from Mead, to look at the
star in question and confirm the observations oneself? That seems like
a more tractable project than discovering it or proving beyond a shadow
of a doubt that it is correct.

I looked at the article of Torres et al and didn't find as much detail
as I hoped for about the light gathering equipment and analytical techniques.
I think the basic reference for the equipment was Latham 1992. Is there
some kind of standard attachment one can add to the, say, Mead mentioned
above that is adequate to collect the light and send the information to one's
laptop for analysis?

It is nice to know it was done with a small telescope, but it would be nicer
to know that all the equipment one needs to duplicate the observation and
analysis could be equally humble.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.

Allan Adler wrote in message ...
It is certainly inspiring to learn that one can discover extrasolar
planets using a telescope with a roughly 4 inch aperture. I think I
understand why three different telescopes at three different sites were
needed to discover it, since they had to sift through so many candidates
and this helped with the process of weeding out false alarms. However,
now that it has been discovered and its discovery confirmed, what are
the difficulties one would face in using a beginner's telescope, say
one of the $200 computer controlled models from Mead, to look at the
star in question and confirm the observations oneself? That seems like
a more tractable project than discovering it or proving beyond a shadow
of a doubt that it is correct.

I looked at the article of Torres et al and didn't find as much detail
as I hoped for about the light gathering equipment and analytical techniques.
I think the basic reference for the equipment was Latham 1992. Is there
some kind of standard attachment one can add to the, say, Mead mentioned
above that is adequate to collect the light and send the information to one's
laptop for analysis?

It is nice to know it was done with a small telescope, but it would be nicer
to know that all the equipment one needs to duplicate the observation and
analysis could be equally humble.

Hi, Allan,

I'm not much more of an expert on this subject than you are, but what
the heck. Sci.astro desperately needs an increase in its signal:noise
ratio.

There are amateurs observing known extrasolar planetary occulations.
You can find out more about them and their work at the American
Association of Variable Star Observers (http://www.aavso.org). If you
want to look for a *known* exoplanet, you stand a decent chance of
finding it.

I have a friend who is a member of this organization. He owns a Meade
8" Schmidt-Cassegrain reflector, and a hand-made CCD camera which saw
its first light a few months ago. I work with microscopes more than
telescopes. Still, many of the issues surrounding getting a good
quantitative image are the same.

You won't see these exoplanet transits by eye. Only a few extrasolar
planets have been observed by occultation so far. When these planets
pass in front of their parent stars, the light loss is pretty small,
peaking at around 2%. So you need to make really accurate
measurements of the intensity. Twinkling and other atmospheric
variations are a problem. The pixels on a CCD are not perfectly
uniform, either. How sharp is your focus? Is the light of your star
falling exactly on one pixel, or on several? What if the voltage that
you supply to the CCD varies a bit from time to time? Then,
successive images of the star would not be directly comparable. Have
you saturated any pixels? Is your CCD response linear? Is your
analog to digital conversion 8-bit or 12-bit?

To compensate for all of these possible problems, you would probably
want to image a star field that includes at least a few reference
stars that you do not expect to vary. You would want to take many
images, at a few different exposure times. Then you would need to do
a fair amount of math to tease out the variations as a function of
time.

I suspect that the use of three observing sites in the TReS study
improved the observations in at least three ways. First, one site
would often be able to observe when another was clouded out. Second,
the Canary Islands site and the Western U.S. sites were several time
zones apart, allowing almost 24-hour observations. Third, there would
be times of overlap, when light curves from multiple observing sites
could be compared.

So, can you go hunting for NEW expolanets yourself? Maybe. But
having a friend on another continent or two would help. And the
software to analyze the images is critical.

(Proposal for an amateur exoplanet hunting network: observers in
California, Chile, Canary Islands or Spain, South Africa, Japan, and
Australia.)

--
Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chainsaw.
John J. Ladasky Jr., Ph.D.

(John Ladasky) writes:

There are amateurs observing known extrasolar planetary occulations.
You can find out more about them and their work at the American
Association of Variable Star Observers (http://www.aavso.org). If you
want to look for a *known* exoplanet, you stand a decent chance of
finding it.

Thanks for the pointer. It looks very interesting.

I have a friend who is a member of this organization. He owns a Meade
8" Schmidt-Cassegrain reflector, and a hand-made CCD camera which saw
its first light a few months ago. I work with microscopes more than
telescopes. Still, many of the issues surrounding getting a good
quantitative image are the same.

I didn't know one could make one's own CCD camera. Is that more expensive
than buying one?

When these planets
pass in front of their parent stars, the light loss is pretty small,
peaking at around 2%. So you need to make really accurate
measurements of the intensity. Twinkling and other atmospheric
variations are a problem. The pixels on a CCD are not perfectly
uniform, either. How sharp is your focus? Is the light of your star
falling exactly on one pixel, or on several? What if the voltage that
you supply to the CCD varies a bit from time to time? Then,
successive images of the star would not be directly comparable. Have
you saturated any pixels? Is your CCD response linear? Is your
analog to digital conversion 8-bit or 12-bit?

Presumably one also uses suitable software to analyze the light
falling on the CCD. Apart from spectral analysis of the light, it
seems that the software would be designed to deal with these issues.
At any rate, Torres et al used CfA Digital Speedometers (whatever they
are) and compared their "observed spectra with synthetic spectra calculated
by J. Morse using Kurucz models (Morse & Kurucz, private communication)"
(whatever that means). I'm just referring to stuff done with the little
scope. Their photometric and radial velocity data (on a big scope?)
are supposed to be at:
http://www.hao.ucar.edu/public/resea...data/TrES1.asc
They didn't say anything about pixels or CCD cameras.

I just did a google search for CfA Digital Speedometers. CfA apparently
stands for "Center for Astrophysics". Then I went to
http://adsabs.harvard.edu
and searched for digital speedometer in the abstracts. The
earlilests reference so far involving the CfA is in the Bulletin of the
American Astronomical Society, vol.14, p.82, and I'm now downloading it.
Since it is so specialized to the CfA, I gather that one can't simply order
the equivalent from a catalogue.

I suspect that the use of three observing sites in the TReS study
improved the observations in at least three ways. First, one site
would often be able to observe when another was clouded out. Second,
the Canary Islands site and the Western U.S. sites were several time
zones apart, allowing almost 24-hour observations. Third, there would
be times of overlap, when light curves from multiple observing sites
could be compared.

One of the special features of this observation, according go the article,
is the fact that the exosolar planet takes 3.03 days to go around the star.
Apparently, the fact this is so close to an integral number of days
placed severe constraints on the places where one could observe the transits.

So, can you go hunting for NEW expolanets yourself? Maybe. But
having a friend on another continent or two would help. And the
software to analyze the images is critical.

I have no budget for astronomy and don't even own a scope. I have
an old pair of 10x50 binoculars and no mount for them. I rely on
friends who have telescopes to do any observing, by looking through
their scopes when they have them set up. However, I try to inform myself
about what things cost and at what point they become feasible, just
so that if I ever have any kind of budget for astronomy, I'll know what
is and what is not within that budget.

The CfA digital speedometers sound like they wouldn't be. So it's
good to know about the viability of CCD cameras for planet hunting.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.

Allan Adler wrote in message ...
(John Ladasky) writes:

There are amateurs observing known extrasolar planetary occulations.
You can find out more about them and their work at the American
Association of Variable Star Observers (http://www.aavso.org). If you
want to look for a *known* exoplanet, you stand a decent chance of
finding it.

Thanks for the pointer. It looks very interesting.

I have a friend who is a member of this organization. He owns a Meade
8" Schmidt-Cassegrain reflector, and a hand-made CCD camera which saw
its first light a few months ago. I work with microscopes more than
telescopes. Still, many of the issues surrounding getting a good
quantitative image are the same.

I didn't know one could make one's own CCD camera. Is that more expensive
than buying one?

Perhaps, but you won't be able to do much stellar photometry with an
off-the-shelf digital camera. The OTS digicams use decent CCD chips,
but there are others out there that are larger, and can gather more
light, if you are willing to pay. Also, the digicam CCD chips have
patterned RGB color masks in front of the pixels. What this means is
that in any one color range, only 1/3 of the chip is actually
receiving light. For some photometry work, you want to capture every
photon. The RGB chips throw 2/3 of them away. Finally, there's the
issue of thermal noise. A cold camera generates less background
signal. Consumer digicams aren't actively cooled.

My friend's custom rig uses a high-sensitivity CCD chip from Kodak,
one that doesn't have the color masks. He added an external color
filter wheel, for those rare times when he actually might want to
exclude certain colors, and a Peltier cooling device. Can you buy a
camera like this? It's similar in many ways to the cameras we use for
microscopes. We certainly buy those. But they'll cost a lot more
than your 10 X 50 binocs.

When these planets
pass in front of their parent stars, the light loss is pretty small,
peaking at around 2%. So you need to make really accurate
measurements of the intensity. Twinkling and other atmospheric
variations are a problem. The pixels on a CCD are not perfectly
uniform, either. How sharp is your focus? Is the light of your star
falling exactly on one pixel, or on several? What if the voltage that
you supply to the CCD varies a bit from time to time? Then,
successive images of the star would not be directly comparable. Have
you saturated any pixels? Is your CCD response linear? Is your
analog to digital conversion 8-bit or 12-bit?

Presumably one also uses suitable software to analyze the light
falling on the CCD. Apart from spectral analysis of the light, it
seems that the software would be designed to deal with these issues.
At any rate, Torres et al used CfA Digital Speedometers (whatever they
are) and compared their "observed spectra with synthetic spectra calculated
by J. Morse using Kurucz models (Morse & Kurucz, private communication)"
(whatever that means). I'm just referring to stuff done with the little
scope. Their photometric and radial velocity data (on a big scope?)
are supposed to be at:
http://www.hao.ucar.edu/public/resea...data/TrES1.asc
They didn't say anything about pixels or CCD cameras.

O.K., you're jumping to the second part of the TrES project -- looking
at Doppler velocity changes. Once you see a periodic, small change in
a star's light curve, you can't be SURE that it's due to a planet.
Suppose that you have two stars of almost equal intensity eclipsing
each other? Or a periodic, variable star? How can you distinguish
these possibilities from a planet?

This is what the radial velocity study will tell you. You can tell
whether a star is moving towards you or away from you by looking at
the blue-shifting and red-shifting of the star's light. A solitary,
variable star is not expected to move back and forth. Two stars
orbiting each other will fling each other back and forth hard -- the
velocity can change by tens of km/sec over the orbital period. A
planet will tug on its parent star fairly gently, resulting in
velocity changes which generally won't exceed 1 km/sec.

Velocity measurements are taken with spectrographs, rather than
imaging cameras. That's why you aren't seeing references to CCD's and
pixels in that part of the report.

There is a VERY dedicated group of amateurs trying to do Doppler
velocimetry:

http://www.spectrashift.com/

But take a look at their work... thirty years ago, this project would
have been worthy of an NSF grant!

I just did a google search for CfA Digital Speedometers. CfA apparently
stands for "Center for Astrophysics". Then I went to
http://adsabs.harvard.edu
and searched for digital speedometer in the abstracts. The
earlilests reference so far involving the CfA is in the Bulletin of the
American Astronomical Society, vol.14, p.82, and I'm now downloading it.
Since it is so specialized to the CfA, I gather that one can't simply order
the equivalent from a catalogue.

I haven't followed your link, but I'm guessing that the "digital
speedometer" is probably the spectrograph that they use to reference
atomic absorption lines in the star's spectrum against a laboratory
spectrum reference (like an arc lamp).

I suspect that the use of three observing sites in the TReS study
improved the observations in at least three ways. First, one site
would often be able to observe when another was clouded out. Second,
the Canary Islands site and the Western U.S. sites were several time
zones apart, allowing almost 24-hour observations. Third, there would
be times of overlap, when light curves from multiple observing sites
could be compared.

One of the special features of this observation, according go the article,
is the fact that the exosolar planet takes 3.03 days to go around the star.
Apparently, the fact this is so close to an integral number of days
placed severe constraints on the places where one could observe the transits.

Some other planets will eventually be found that have more
accomodating periods, and thus can be seen more readily from all the
sites.

So, can you go hunting for NEW exoplanets yourself? Maybe. But
having a friend on another continent or two would help. And the
software to analyze the images is critical.

I have no budget for astronomy and don't even own a scope. I have
an old pair of 10x50 binoculars and no mount for them. I rely on
friends who have telescopes to do any observing, by looking through
their scopes when they have them set up. However, I try to inform myself
about what things cost and at what point they become feasible, just
so that if I ever have any kind of budget for astronomy, I'll know what
is and what is not within that budget.

The CfA digital speedometers sound like they wouldn't be. So it's
good to know about the viability of CCD cameras for planet hunting.

And now you also know that variation in the light intensity of a star
isn't enough, by itself, to be sure that you have seen a planetary
transit.

Have fun. Astronomy is addictive!

--
Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chainsaw.
John J. Ladasky Jr., Ph.D.

Thanks for answering my questions. I looked up the website of the
amateur spectroscopers and I'll read more of it later. Regarding the
home made CCD camera, where would one read detailed instructions on
how to do that? I like to read detailed instructions on how to do things,
even if I lack the skill or resources to actually do them.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.

In article ,
Allan Adler writes:
I didn't know one could make one's own CCD camera. Is that more expensive
than buying one?

(I'm following up to this message because it's convenient, but I'll
also be commenting on earlier messages in the thread.)

There is at least one company (Santa Barbara Instrument Group) that
claims to sell near-research-grade CCD cameras. There may be others;
check the ads in Sky and Telescope. I have never used any of these
products, so don't take this as a recommendation, but on paper the
cameras look promising. I have no idea of prices, but in general
building one's own electronics will be more expensive than buying
off-the-shelf products.

When these planets
pass in front of their parent stars, the light loss is pretty small,
peaking at around 2%. So you need to make really accurate
measurements of the intensity. Twinkling and other atmospheric
variations are a problem.

Also transparency variations from very think clouds, guiding errors,
and probably other things. I don't think it is possible to measure
at the sub-1% level with a single detector, either PIN diode or
photomultiplier. It is possible with a CCD because most of the
variations affect all the stars on the frame the same way.

The pixels on a CCD are not perfectly
uniform, either. How sharp is your focus? Is the light of your star
falling exactly on one pixel, or on several?

For accurate photometry, one generally wants to make sure the light
is spread over at least four pixels.

What if the voltage that
you supply to the CCD varies a bit from time to time?

I think CCD's are relatively insensitive to bias voltage variations,
but the temperature has to be stabilized. In any case, gain
variations should affect all stars on the frame the same way, and
thus _relative_ photometry should be unaffected.

Then,
successive images of the star would not be directly comparable. Have
you saturated any pixels? Is your CCD response linear? Is your
analog to digital conversion 8-bit or 12-bit?

I would expect 16-bit conversion to be needed. Otherwise there
simply isn't enough dynamic range.

Presumably one also uses suitable software to analyze the light
falling on the CCD.

There are free (but hard to use) packages and (I think) commercial
packages, which I have never used. I would expect camera sellers to
offer software packages, but my expectations are often wrong.

At any rate, Torres et al used CfA Digital Speedometers (whatever they
are) and compared their "observed spectra with synthetic spectra calculated
by J. Morse using Kurucz models (Morse & Kurucz, private communication)"

This sounds like spectrographs that measure radial velocity. The
analysis technique is to cross-correlate a theoretical ("synthetic")
spectrum of the star with the measured spectrum. The
cross-correlation is maximum when the velocity exactly matches the
stellar velocity. This bit is custom software, but it isn't what you
would use for measuring magnitudes. "Kurucz models" are the output
of a widely-used stellar atmosphere code (written by Robert Kurucz,
as it happens); I believe the code is public.

About converting magnitudes to photon rates: you don't want a
bolometric magnitude, you want an apparent magnitude at whatever
color you plan to measure (maybe B, V, R, or I). Then you need to
know the flux density in physical units for zero magnitude at that
wavelength. Then just use the usual magnitude formula, and convert
watts to photons per second via Planck's constant. I have a table of
flux density for zero mag at home but not with me. A quick ADS
search finds Gray (1998 AJ 116, 482) for Stromgren uvby, Fukugita et
al. (1996 AJ 111, 1748) for the Sloan filters (but check their web
site sdss.org for updates), but nothing recent for B and V. The
classic reference for these is Schild & Oke (1970 ApJ 162, 361). The
HST web site probably has zero points for the HST filters.

I expect all the stars with known extrasolar planets will be pretty
bright -- they had to be bright enough to get good spectra.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
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Steve Willner wrote:
In article ,
Allan Adler writes:
I didn't know one could make one's own CCD camera. Is that more expensive
than buying one?

(I'm following up to this message because it's convenient, but I'll
also be commenting on earlier messages in the thread.)

There is at least one company (Santa Barbara Instrument Group) that
claims to sell near-research-grade CCD cameras. There may be others;
check the ads in Sky and Telescope. I have never used any of these
products, so don't take this as a recommendation, but on paper the
cameras look promising. I have no idea of prices, but in general
building one's own electronics will be more expensive than buying
off-the-shelf products.

We've just ordered one of SBIG's high-end cameras, so should know
in a few months how robust they are in student hands.

About converting magnitudes to photon rates: you don't want a
bolometric magnitude, you want an apparent magnitude at whatever
color you plan to measure (maybe B, V, R, or I). Then you need to
know the flux density in physical units for zero magnitude at that
wavelength. Then just use the usual magnitude formula, and convert
watts to photons per second via Planck's constant. I have a table of
flux density for zero mag at home but not with me. A quick ADS
search finds Gray (1998 AJ 116, 482) for Stromgren uvby, Fukugita et
al. (1996 AJ 111, 1748) for the Sloan filters (but check their web
site sdss.org for updates), but nothing recent for B and V. The
classic reference for these is Schild & Oke (1970 ApJ 162, 361). The
HST web site probably has zero points for the HST filters.

A table I've pieced together for zero-magnitude flux includes:


Band Eff lambda Zero point: F-lambda F-nu
U 0.36 mu 4.35(-9) 1.88(-23)
B 0.44 7.20(-9) 4.44(-23)
V 0.55 3.92(-9) 3.81(-23)
R 0.70 1.76(-9) 3.01(-23)
I 0.90 8.3(-10) 2.43(-23)
J 1.25 3.4(-10) 1.77(-23)
K 2.2 3.9(-11) 6.3(-24)
L 3.4 8.1(-12) 3.1(-24)
M 5.0 2.2(-12) 1.8(-24)
N 10.2 1.23(-13) 4.3(-25)
for F-lambda in erg/cm**2 s A
F-nu in W/m**2 Hz

For ISO/ESO system:
Band Eff lambda Delta lambda F0(Jy)
J 1.24mu 0.2 1587
H 1.64 0.3 1074
K 2.18 0.4 653
L' 3.76 0.7 253
M 4.69 0.5 150
N 10.3 5.2 29.4
N1 8.38 0.8 48.7
N2 9.67 1.6 34.9
N3 12.9 3.7 19.7
Q 18.6 5.6 9.5

And from the Fukugita et al. paper:


Band W(eff) FWHM Weff(Vega) Flam(Vega) Fnu(Vega) mag AB
Johnson U 3652 526 3709 4.28e-9 1.89e-20 -0.181 0.710
B 4448 1008 4393 6.18 4.02 -0.342 -0.110
V 5505 827 5439 3.60 3.59 0.083 0.011

Cousins R 6588 1568 6410 2.15 3.02 0.399 0.199
I 8060 1542 7977 1.11 2.38 0.752 0.456

Johnson R 6930 2096 6688 1.87 2.89 0.473 0.249
I 8785 1706 8571 0.912 2.28 0.805 0.504

Sandage/Smith u 3647 595 3710 4.30 1.89
b 4466 1028 4407 6.10 3.97
v 5423 823 5368 3.75 3.64
r 6712 969 6628 1.96 2.90

Stromgren u 3465 363 3496 3.24 1.31
v 4109 197 4119 7.21 4.12
b 4668 176 4666 5.68 4.15
y 5459 244 5455 3.62 3.60

Kron Uk 3656 566 3737 4.32 1.93 -0.195 0.689
Jk 4625 1550 4537 5.54 3.82 -0.256 -0.056
Fk 6168 1330 5978 2.64 3.25 0.271 0.120
Nk 7953 1786 7838 1.17 2.44 0.723 0.434

Couch/Newell Bj 4604 1490 4515 5.73 3.95 -0.281 -0.091
Rf 6694 517 6679 1.92 2.86 0.481 0.259

Thuan/Gunn u 3536 412 3542 3.33 1.38 0.000 1.049
v 3992 469 4013 6.62 3.50 -0.440 0.041
g 4927 709 4888 4.84 3.89 -0.126 -0.075
r 6538 893 6496 2.09 2.96 0.429 0.221

Schneider g4 5147 913 5083 4.34 3.78 -0.047 -0.043
r4 6659 1028 6600 1.99 2.92 0.455 0.236
i4 8056 1604 7942 1.13 2.41 0.739 0.445
z4 9141 1472 9071 0.797 2.20 0.851 0.545

PFUEI g 5238 882 5166 4.14 3.74 -0.016 -0.031
r 6677 916 6602 1.98 2.91 0.458 0.241
i 7973 1353 7876 1.16 2.43 0.730 0.437
z 9133 984 9054 0.798 2.19 0.853 0.547

Tyson Bj 4614 1215 4562 5.46 3.80
R 6585 1373 6503 2.08 2.97
I 8668 1725 8532 0.928 2.28

WFPC2 F555W 5536 1480 5387 3.62 3.60 0.069 0.009
F606W 6102 2050 5901 2.73 3.28 0.250 0.111
F702W 6979 1957 6826 1.77 2.82 0.511 0.275
F814W 8092 1653 7906 1.14 2.43 0.726 0.434

POSS II g 5154 942 5121 4.25 3.74 -0.035 -0.034
r 6696 1050 6632 1.96 2.90 0.459 0.244
i 7837 1469 7756 1.14 2.46 0.709 0.424

SDSS u 3585 556 3594 3.67 1.54 -0.077 0.928
g 4858 1297 4765 5.11 3.93 -0.178 -0.087
r 6290 1358 6205 2.40 3.12 0.342 0.163
i 7706 1547 7617 1.28 2.51 0.687 0.401
z 9222 1530 9123 0.783 2.19 0.855 0.549



Bill Keel

"Steve Willner" wrote in message
...
...
About converting magnitudes to photon rates: you don't want a
bolometric magnitude, you want an apparent magnitude at whatever
color you plan to measure (maybe B, V, R, or I).

The rate will depend both on the filter and the colour of
the star so for a general guide I thought I would start
with bolometric to get an upper limit on the rate as a
guide to bandwidth if photon counting was to be tried.

Then you need to
know the flux density in physical units for zero magnitude at that
wavelength. Then just use the usual magnitude formula, and convert
watts to photons per second via Planck's constant.

That's my self-imposed homework question, I learn best by
working through things myself.

I have a table of
flux density for zero mag at home but not with me. A quick ADS
search finds Gray (1998 AJ 116, 482) for Stromgren uvby, Fukugita et
al. (1996 AJ 111, 1748) for the Sloan filters (but check their web
site sdss.org for updates), but nothing recent for B and V. The
classic reference for these is Schild & Oke (1970 ApJ 162, 361). The
HST web site probably has zero points for the HST filters.

Thanks to you and Bill Keel for the pointers, they will
be bookmarked!

best regards
George