drift method

Can anyone explain the drift method of polar alignment
in a clear concise way? How many here actually do it?
It seems to be a necessity to do long exposure deep sky photo work
irregarless of fancy goto's.

On Tue, 23 Feb 2010 19:59:38 GMT, "sunstuff" wrote:

Can anyone explain the drift method of polar alignment
in a clear concise way? How many here actually do it?
It seems to be a necessity to do long exposure deep sky photo work
irregarless of fancy goto's.

Google "drift method" and you'll find lots of good descriptions,
complete with images. Probably better than can be described here with
words alone.

Fair polar alignment is critical to imaging. The longer your
subexposures, the closer you need to be. For the length of subexposures
most imagers use, you only need to be within a degree or so of the pole,
which is easy to achieve. Many do better than this with just a polar
alignment scope. The drift method is only one approach. I personally
prefer semiautomated methods that utilize the imaging camera itself.
This includes tools like TPoint and PolarAlignMax.
_________________________________________________

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

What are "subexposures"..I don't see that terminology in polar align /
astrophoto descriptions.


"Chris L Peterson" wrote in message
...
On Tue, 23 Feb 2010 19:59:38 GMT, "sunstuff" wrote:

Can anyone explain the drift method of polar alignment
in a clear concise way? How many here actually do it?
It seems to be a necessity to do long exposure deep sky photo work
irregarless of fancy goto's.

Google "drift method" and you'll find lots of good descriptions,
complete with images. Probably better than can be described here with
words alone.

Fair polar alignment is critical to imaging. The longer your
subexposures, the closer you need to be. For the length of subexposures
most imagers use, you only need to be within a degree or so of the pole,
which is easy to achieve. Many do better than this with just a polar
alignment scope. The drift method is only one approach. I personally
prefer semiautomated methods that utilize the imaging camera itself.
This includes tools like TPoint and PolarAlignMax.
_________________________________________________

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

On Tue, 23 Feb 2010 23:36:42 GMT, "sunstuff" wrote:

What are "subexposures"..I don't see that terminology in polar align /
astrophoto descriptions.

To get good images of most astronomical objects requires very long
exposure times- several minutes to several hours. For a variety of
reasons, it is often impractical to make such a long image in a single
exposure. So it is common to break up the total exposure into
subexposures, and then add these together during post processing. For
example, if you need a 60-minute exposure, you might make 10 6-minute
exposures and then stack them all together into a single frame when you
are done.

The problem with poor polar alignment is that stars appear to rotate
around some common point in (or off of) and image. The longer you
expose, the greater the rotation (which causes streaked stars). If your
subexposures are short compared with the rotation rate, you can rotate
and align the individual images when you stack them, and the rotation
will not be apparent. If you use longer subexposures, however, you need
better polar alignment in order to reduce the rotation.
_________________________________________________

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

What ISO setting is good for say 8th magnitude and higher on a DSLR. 800 ?


"Chris L Peterson" wrote in message
...
On Tue, 23 Feb 2010 23:36:42 GMT, "sunstuff" wrote:

What are "subexposures"..I don't see that terminology in polar align /
astrophoto descriptions.

To get good images of most astronomical objects requires very long
exposure times- several minutes to several hours. For a variety of
reasons, it is often impractical to make such a long image in a single
exposure. So it is common to break up the total exposure into
subexposures, and then add these together during post processing. For
example, if you need a 60-minute exposure, you might make 10 6-minute
exposures and then stack them all together into a single frame when you
are done.

The problem with poor polar alignment is that stars appear to rotate
around some common point in (or off of) and image. The longer you
expose, the greater the rotation (which causes streaked stars). If your
subexposures are short compared with the rotation rate, you can rotate
and align the individual images when you stack them, and the rotation
will not be apparent. If you use longer subexposures, however, you need
better polar alignment in order to reduce the rotation.
_________________________________________________

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

On Wed, 24 Feb 2010 00:24:50 GMT, "6EQUJ5" wrote:

What ISO setting is good for say 8th magnitude and higher on a DSLR. 800 ?

It depends on the camera. Most DSLRs have their lowest noise between ISO
200 and 400. For most targets, there will be no advantage to going
faster- the images will appear brighter, but the S/N will be worse. When
I use a Canon DSLR I normally shoot DSOs at ISO 400.
_________________________________________________

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

"6EQUJ5" wrote in news:ml_gn.66566$Db2.54614@edtnps83:

What ISO setting is good for say 8th magnitude and higher on a DSLR.
800 ?


http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM
http://www.astropix.com/HTML/I_ASTROP/TRACKED/POLAR.HTM