MARS PARALLAX NIGHT

Apologies for shouting - there's not much lead time on this one...

A group is forming around the world to attempt to measure the parallax
of Mars (and from there its distance) on the night of October 22/23.
If you're interested in contributing to this rapidly assembled project
please visit

http://www.digitalsky.org.uk/Marsnig...005-10-23.html


--
Pete
http://www.digitalsky.org.uk

On Thu, 20 Oct 2005 23:33:29 +0100, Pete Lawrence
wrote:

Apologies for shouting - there's not much lead time on this one...

A group is forming around the world to attempt to measure the parallax
of Mars (and from there its distance) on the night of October 22/23.
If you're interested in contributing to this rapidly assembled project
please visit

http://www.digitalsky.org.uk/Marsnig...005-10-23.html

This seems like a complicated way to do something simple. Instead of
trying to estimate the limb position of Mars, I'd just shoot a
moderately wide field- say 15 arcmin or so- and just do standard
astrometry on the image. Mars will be a bit larger than the stars, but
can still have a centroid calculated. The key is to make sure Mars
doesn't cover too many pixels.

_________________________________________________

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

On Thu, 20 Oct 2005 22:44:54 GMT, Chris L Peterson
wrote:

On Thu, 20 Oct 2005 23:33:29 +0100, Pete Lawrence
wrote:

Apologies for shouting - there's not much lead time on this one...

A group is forming around the world to attempt to measure the parallax
of Mars (and from there its distance) on the night of October 22/23.
If you're interested in contributing to this rapidly assembled project
please visit

http://www.digitalsky.org.uk/Marsnig...005-10-23.html

This seems like a complicated way to do something simple. Instead of
trying to estimate the limb position of Mars, I'd just shoot a
moderately wide field- say 15 arcmin or so- and just do standard
astrometry on the image. Mars will be a bit larger than the stars, but
can still have a centroid calculated. The key is to make sure Mars
doesn't cover too many pixels.

Chris, do what you want - I'm not imposing any rules here, I'd just be
grateful of any results. Bear in mind though that the parallax shift
will be small. Over 10,000km we reckon on a 28 arcseconds shift. Mars
is about 20 arcseconds in apparent diameter at the moment.

If you make sure that Mars doesn't cover too many pixels - you're not
going to get much of a measurable shift.

--
Pete
http://www.digitalsky.org.uk

"Chris L Peterson" wrote in message
...
On Thu, 20 Oct 2005 23:33:29 +0100, Pete Lawrence
wrote:

Apologies for shouting - there's not much lead time on this one...

A group is forming around the world to attempt to measure the parallax
of Mars (and from there its distance) on the night of October 22/23.
If you're interested in contributing to this rapidly assembled project
please visit

http://www.digitalsky.org.uk/Marsnig...005-10-23.html

This seems like a complicated way to do something simple. Instead of
trying to estimate the limb position of Mars, I'd just shoot a
moderately wide field- say 15 arcmin or so- and just do standard
astrometry on the image. Mars will be a bit larger than the stars, but
can still have a centroid calculated. The key is to make sure Mars
doesn't cover too many pixels.


Hi Chris,

I would say that in an image exposed for mag 13 stars, Mars is going to be a
lot bigger than the stars given average optics. eg here is a shot of Mars
and Deimos at Mag 12.4 I took last opposition.
http://www.leadbeaterhome.fsnet.co.u...o_image_49.htm
As long as the bloated image of Mars is perfectly symmetrical a centroid
estimate should not be a problem - but who can guarantee that when you are
looking for arcsec precision?

Robin

On Fri, 21 Oct 2005 00:27:51 +0100, Pete Lawrence
wrote:

Chris, do what you want - I'm not imposing any rules here, I'd just be
grateful of any results. Bear in mind though that the parallax shift
will be small. Over 10,000km we reckon on a 28 arcseconds shift. Mars
is about 20 arcseconds in apparent diameter at the moment.

That's huge. Typical astrometry accuracy if you have a handful of stars
should be well under an arcsecond.

BTW, I'm not criticizing the project at all- just questioning whether
the methodology described is really the best.

If you make sure that Mars doesn't cover too many pixels - you're not
going to get much of a measurable shift.

I don't understand that. To get an astrometric position, an object need
be no more than a point source. In fact, an extended object may be more
difficult to work with.

_________________________________________________

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

On Fri, 21 Oct 2005 00:29:25 +0100, "Robin Leadbeater"
wrote:

I would say that in an image exposed for mag 13 stars, Mars is going to be a
lot bigger than the stars given average optics. eg here is a shot of Mars
and Deimos at Mag 12.4 I took last opposition.
http://www.leadbeaterhome.fsnet.co.u...o_image_49.htm
As long as the bloated image of Mars is perfectly symmetrical a centroid
estimate should not be a problem - but who can guarantee that when you are
looking for arcsec precision?

Hi Robin-

I'd probably shoot something around 1 second, which should give quite a
lot of stars without resulting in Mars blooming. Alternatively, I'd
shoot an image for a few seconds and then another for 1/10 second. I'd
plate solve the first, and use the terms to determine the position of
the centroid in the second. This should easily give sub-arcsecond
accuracy even with poor seeing and less than perfect optics.

_________________________________________________

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

Pete Lawrence wrote:
snip A group is forming around the world to attempt to
measure the parallax of Mars (and from there its distance) on the
night of October 22/23. If you're interested in contributing to this rapidly assembled project please visit
http://www.digitalsky.org.uk/Marsnig...005-10-23.html

After thinking about this overnight, I decided the dinural method using
two measurements on the same latitude probably is the more difficult of
the two and the best results would be obtained by two observers taking
pictures around the same time at different latitudes (the
Cassini-Picard-Richer method).

With respect to the diurnal method, I ran my op (Salt Lake City, Utah)
through the MPC Epheremis generator for the night of opposition -
11/7/2005 at approx. 8:30 UTC. During that night, the MPC Ephemeris
predicts that Mars true position Mars will move about 30 arcsecs
between 7:00 UTC and 11:00 UTC.

Table 1 -
Mars Opposition Data for 11/7/2005 from SLC, UT
Date UTC RA/DEc Sun-Earth-Mars Angle
2005-Nov-07 07:00 025027.82+155222.3 179.5308
2005-Nov-07 08:30 025021.91+155211.3 179.5369 Opposition
2005-Nov-07 11:00 025012.21+155151.7 179.5110
From - http://ssd.jpl.nasa.gov/cgi-bin/eph

While part of this movement is parallax, part is also due to the
Earth's and Mars's movement around the Sun. Without some means of
teasing that movement out of photographs taken that night, how can a
small 3-5 arcsec parallax be measured?

Two observers at the same latitude suffer similar problems. To be
comparable, their photos must be taken at the same time.

Conversely, using two observing points at nearly the same longitude but
widely differing latitudes at the same time, it would be easier to
measure parallax accurately. In the two-observing-point two-latitude
method, a series of astrophotos are taken at let's say 15 or 30 minute
intervals for a couple of hours before and after opposition. The
photos are then aligned and overlaid. A line is drawn between the
central point of Mars for each group of observers' photographs. The
distance between the lines gives the parallax of Mars. For this method,
it does not matter if the astrophotos are taken at exactly the same
time.

The downside is the northern-hemisphere dominance of participants in
these newsgroups. For the previous Lunar Parallax Demonstration
project, observers were principally located in similar latitudes in the
northern hemisphere in Europe and the United States.

The Mars opposition presents an opporunity for a cooperative project
between amateurs and clubs located in the northern and southern
hemisphere, e.g. the U.S. and South America and Europe and Africa.

The Observatorio ARVAL maintains links for astronomy clubs in various
countries.
http://www.oarval.org/AstroLnk.htm#Countries
E.g. -
Buenos Aries - http://www.cielosur.com/
Costa Rica - http://www.liada.net/
Jordan - http://www.jas.org.jo/
South Africa - http://www.astronomysa.co.za/

- Canopus56

Kurt (canopus56) wrote:
While part of this movement is parallax, part is also due to the
Earth's and Mars's movement around the Sun. Without some means of
teasing that movement out of photographs taken that night, how can a
small 3-5 arcsec parallax be measured?

I've thought about this problem. I suggest taking the measurement
three times. Ideally, the first and third times would be on successive
evenings, at the same sidereal time. Using the same sidereal time would
ensure that the only difference between the two positions is the Earth's
translation in its orbit around the Sun.

The second measurement would ideally be half a sidereal day later; in
this case, you average the first and third measurements, and compare
them directly against the second.

These don't exactly *need* to be at those times, but if they're not,
then some interpolation needs to be done.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Brian Tung wrote:
snip
These don't exactly *need* to be at those times, but if they're not,
then some interpolation needs to be done.

Brian, given at opposition, Mars will be about 69,415,600 km
(43,132,800 miles) from the Earth, what do you predict the expected
parallax to be on a 6000 and 12000 km baseline. - Canopus56

Hi "Parallax" Pete,

No need to apologize. I have echoed your post to the Lunar
Observing group photographers.

Looks good!

Chuck Taylor
*********************************************
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