LX200 to photograph in ALTAZ....

Hi all
With a LX200 12" (305mm) and 6 meters of focal in posting ALTAZ. how many
seconds can I photograph
before underlining the rotation of field??

thanks in advance...


Giorgio Mengoli
e-mail:
astrophotography: http://digilander.libero.it/gm2
mirror web: http://astrosurf.com/alpha

From "Astrophotography for the Amateur" by Covington,
1) Field rotation is independent of focal length.
A degree is a degree.
2) The direction of the alignment error matters.
More rotation on meridian than rising / setting.
3) "The maximum possible field rotation, for object between -60 deg
and +60 deg, is:
Field rotation = 0.01 deg * exposure time (minutes) * alignment error
(degrees)
4) 0.1 deg is generally not visible in a photograph.

So for worst case scenario, the north celestial pole where no ALT-AZ
tracking is taking place and 100% of the error is field rotation...
I figure 24 seconds for 0.1 deg field rotation.

Of course I think that supposes no tracking error and guiding on the
center of the field. Guiding off to the side of the field, e.g. self
guiding cameras or other off axis guiders probably would shorten that.
Also guiding errors are multiplied because you are now tracking in two
axis instead of one. Adding a field de-rotator means tracking in
three axis.

Leonard


On Mon, 14 Jun 2004 14:12:40 GMT, "GiMen" wrote:

Hi all
With a LX200 12" (305mm) and 6 meters of focal in posting ALTAZ. how many
seconds can I photograph
before underlining the rotation of field??

thanks in advance...


Giorgio Mengoli
e-mail:
astrophotography: http://digilander.libero.it/gm2
mirror web: http://astrosurf.com/alpha

Leonard wrote:

From "Astrophotography for the Amateur" by Covington,
1) Field rotation is independent of focal length.


Such is true only when the guide star is *exactly* coincident with the
center of the camera's field. And that's rarely the case in actual
practice.

This issue was hashed out on sci.astro.amateur some 20 months ago. To
read all the gory details, go to...

http://groups.google.com/

After entering the search terms...

Covington Gingrich "field rotation"

the relevant thread will appear "automagically."


--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich San Leandro, California

Hi Mark,
Thanks for the information... !!

Regards

G.Mengoli



Mark Gingrich wrote in message ...
Leonard wrote:

From "Astrophotography for the Amateur" by Covington,
1) Field rotation is independent of focal length.


Such is true only when the guide star is *exactly* coincident with the
center of the camera's field. And that's rarely the case in actual
practice.

This issue was hashed out on sci.astro.amateur some 20 months ago. To
read all the gory details, go to...

http://groups.google.com/

After entering the search terms...

Covington Gingrich "field rotation"

the relevant thread will appear "automagically."

On Tue, 15 Jun 2004 05:16:52 +0000 (UTC), Mark Gingrich
wrote:

Leonard wrote:

From "Astrophotography for the Amateur" by Covington,
1) Field rotation is independent of focal length.


Such is true only when the guide star is *exactly* coincident with the
center of the camera's field. And that's rarely the case in actual
practice.

That would only make sense. I did mention "Of course I think that
supposes no tracking error and guiding on the center of the field.
Guiding off to the side of the field, e.g. self guiding cameras or
other off axis guiders probably would shorten that."

This issue was hashed out on sci.astro.amateur some 20 months ago. To
read all the gory details, go to...

http://groups.google.com/

After entering the search terms...

Covington Gingrich "field rotation"

the relevant thread will appear "automagically."

I understand. The arc described by the star at the focal plane is
directly proportional to the distance from the guide center. It would
be double in size if you guide on the edge of the photographic field
instead of the center as the stars on the opposite side of the field
would describe an arc twice as large. That would make any "field
rotation" twice as obvious and detectable. Guiding outside the
photographic field would magnify the affect even further.

Leonard

Wow! Thanks for that explanation Leonard. I never thought of it that way;
reducing the problem to it's worst case, then asking what happens.
--
Sincerely,
--- Dave

----------------------------------------------------------------------
A man is a god in ruins.
--- Duke Ellington
----------------------------------------------------------------------

"Leonard" wrote in message
...
From "Astrophotography for the Amateur" by Covington,
1) Field rotation is independent of focal length.
A degree is a degree.
2) The direction of the alignment error matters.
More rotation on meridian than rising / setting.
3) "The maximum possible field rotation, for object between -60 deg
and +60 deg, is:
Field rotation = 0.01 deg * exposure time (minutes) * alignment error
(degrees)
4) 0.1 deg is generally not visible in a photograph.

So for worst case scenario, the north celestial pole where no ALT-AZ
tracking is taking place and 100% of the error is field rotation...
I figure 24 seconds for 0.1 deg field rotation.

Of course I think that supposes no tracking error and guiding on the
center of the field. Guiding off to the side of the field, e.g. self
guiding cameras or other off axis guiders probably would shorten that.
Also guiding errors are multiplied because you are now tracking in two
axis instead of one. Adding a field de-rotator means tracking in
three axis.

Leonard


On Mon, 14 Jun 2004 14:12:40 GMT, "GiMen" wrote:

Hi all
With a LX200 12" (305mm) and 6 meters of focal in posting ALTAZ. how many
seconds can I photograph
before underlining the rotation of field??

thanks in advance...


Giorgio Mengoli
e-mail:
astrophotography: http://digilander.libero.it/gm2
mirror web: http://astrosurf.com/alpha