Polar Alignment and PE
The following method was described by Roland Christen recently
on the Yahoo Ap-Gto group.
This method does not require the constant slewing from meridian
to eastern horizon as for conventional drift alignment.
A star is chosen close to where one normally images. Towards
the south at 60degrees elevation or above for me.
After identifying the ccd orientation proceed as follows-
1. Work on one axis at a time.
2. Turn the azimuth knobs one direction, note how much drift
you have in a 2 minute exposure in the N-S direction. If the
drift is
worse, turn the azimuth the other way.
2. Then adjust the altitude axis the same way to eliminate the
E-W or RA drift.
3. Repeat until no drift is noticed in 2 minutes or more.
My set up is as follows. AP1200gto C11
and ST7E and CCDSoft 5.
(I received the mount in July 2002)
The ST7E TC211 guider chip at 2800mm yields a scale of close
to 1 arc sec per pixel.
The charts below show tracking records with guide corrections
disabled.
RA drift is indicated by the slope of the black lines.
Periodic error is superimosed on the RA drift ( curve between
the parallel black lines).
The RA is drifting at about 1.2 arc sec per minute. The underlying
PE is +- 3 arc sec with out PE correction.
Since there is drift in RA it requires an Altitude adjustment
to be made. The Dec drift is minimal so the azimuth is left alone.
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The chart below shows the alignment
after about 1 hour of making Alt-Az adjustments.( First time
newbie !)
It shows the RA axis is still drifting about 0.4 arc sec per
minute.
A further tweak of the altitude is still needed.
The PE correction is also enabled reducing the PE to almost below
the 'seeing' for that night.
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Even with my inexperience and inital
alignment and PE training the mount is showing impressive tracking
performance.
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The following information was posted on the Yahoo AP-gto group.
Using a CCD determine the drift rate
in arc seconds for a given time.
To convert drift error to mount error.
Error = (atan(b1/(15*b2))*360)/(2PI) .
Error in arc seconds. b1 drift in arc seconds. b2 drift time
in seconds
Or: 15.7 arc seconds per minute translates into a RA axis alignment
error of one degree (for alt or az).
To convert mount alignment error to screw turns:
On the 1200 mount, the azimuth threads are 24 TPI at a 4.5"
distance, which equals 0.53 degrees per turn of the adjuster
knob.
On the altitude axis, the threads are 20 TPI at about 5.7"
distance, which equals 0.5 degrees per turn.
So each axis moves at close to 1/2 degree for each full rotation
of the knob.
For example using the chart above:
The drift was found to be 5 arcsec in 2 minutes. The adjuster
has to be turned 115 degrees or 1/3 of a turn.
For final precice alignment, only 10-20 degrees of a turn is
needed on the adjuster knobs.
A larger version of the adjuster chart is HERE
Dave Rose