Hi Matthew,
The problem with we ground based observers is that we like to breathe oxygen
and it is this surrounding atmosphere that distorts the picture somewhat.
As I mentioned in my talk at the South Pacific Star Party last Friday, one
of the common misunderstandings with some amateur observers is that there is
some magical point in the sky, namely the NCP or SCP, that it is widely
believed if one aligns the polar axis of the scope to, will result in the optimal
point for astrophotography, Likewise, another common misunderstanding is that
if the tracking rate is exactly at the sidereal rate, this will also be the optimal value
for astrophotography.
Owing to the effects of atmospheric refraction, star trails do not result in perfect
circles in the sky and the tracking rate is not constant in RA.
The ideal polar axis setting depends on what declination and hour
angle you plan on observing and taking into account your latitude and barometric
pressure or at least an approximation based on your height above MSL.
Never forget that the ideal track rate varies continually with zenith distance.
Typically the optimal point to align the polar axis for any given part of the
sky will be somewhere between the true pole and refracted pole.
As a compromise, if one aligns the polar axis of the scope to your local refracted
pole, then the choice of the standard sidereal rate turns out to be pretty good
up near the zenith.
The cited web page is in error when the author states -
"The most accurate polar alignment method, the Drift Method ..."
Though the drift method can provide a good first order approximation of sufficient
accuracy for most imaging scenarios, it is not the most accurate polar alignment
method. The drift method does not take into account the other geometric
and flexure errors within the mount and is based typically only on a couple
of stars. A better analysis is to perform a star pointing test based on a much
larger number of stars and taking into account the geometric and flexure errors
within the mount/OTA.
Rather than look at the average tracking rate over several hours, a more useful
analysis is to determine the standard deviation of a given fixed tracking rate.
In other words, you want to make sure that when the motor is commanded
to track at some set speed when moving the weight of the OTA and moving parts
of the mount that it stays constant (i.e. that its speed is constant). Ideally the
motor can be externally commanded to speed up or slow down to match the optimal
tracking rate for the part of the sky you are imaging.
If the software that one uses to command the motor to speed up or slow down
is not closed loop, in other words, there is no velocity feedback for it to
make a correction, then one will need to characterize the average speed of the
motor (nominally near the sidereal rate) and set that as a constant in the
controlling program, if the controlling program allows for that. In other words,
some software packages will assume that the motor has a base rate that is the nominal
sidereal rate and may be applying speed offsets around that constant.
Reading between the lines I believe you suspect your RA motor is turning
at some speed considerably different to the nominal sidereal rate?
When making measurements and recoding them, make sure the scope
has reached ambient and record the temperature in your results to see if
there is any correlation with it. Come summer, you may get different results
depending upon the time source for your motor controller.
As you know, auto guiding mechanisms can also help close the loop independently
of velocity control.
Though it does not answer your query, I hope the above is of some help.
Best regards
Gary Kopff
Mt. Kuring-Gai
