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.