polar alignement,help (long response)


GrayFox65@...
 

Vincent,

Hello from Virginia. Trying to align the AP GTO mounts can be an intensely
frustrating experience until you get the hang of it. In answer to your first
question, no the mount does not have to be horizontal to successfully align
the mount. I made that mistake when I first got started, but "it just ain't
so!" I'm not sure why you aren't aligning successfully, I'll give some
thought to the steps you describe to see if I see anything wrong with them.
Meanwhile, I'm sure others in the group will do so as well.

One thought to keep in mind. Stars located on or very near the meridian (due
South) are very valuable for polar alignment. The reason is that when the
mount is pointed to the meridian, turning the mount's altitude (latitude)
adjuster translates into a purely vertical alignment star motion in the
eyepiece. As the mount is moved to the east or west, a horizontal component
is introduced into the movement of the alignment star when the altitude
adjuster is turned. By the time the mount is laid all the way over to the
east or west, turning the mount's altitude adjuster results in a nearly
horizontal movement of the alignment star in the eyepiece field. Since the
mount's azimuth adjuster always produces a horizontal movement no matter
where the mount is pointed, this can lead to confusion and a seeming
inability to bring the mount to proper alignment.

You may also be experiencing a problem with orthogonality due to the fact
that you are using an SCT on your mount. Others have suggested that it is
more difficult to maintain orthogonality with SCTs than with refractors. If
you use a star diagonal, lack of orthogonality there can be a contributor to
poor performance as well. The AP Maxbrite diagonal is machined to be very
close to true orthogonality.

Having said that, let me offer an alternate alignment method suggested by
Roland Christen, one that works superbly for me. Not only does it separate
altitude alignment and azimuth alignment into separate steps so you can more
easily see what is going on, but it also reveals any orthogonality errors
quite clearly. Furthermore, it doesn't require visibility of Polaris to
work, useful if your view to the North is blocked. I use this method all the
time now, and I can align my AP 155EDF and 600E GTO so well that objects fall
near the center of the field of view of a 3mm Radian, 10 arc minutes diameter
at 365X. Under those conditions, the GTO mount is truly a delight to use.

Here is Roland Christen's method. My amplifying comments are in [brackets].
Note that Roland says he can perform this method in 5 minutes. It take me
considerably longer, but the results are worth it.


GTO Mount Scope Alignment by Roland Christen

I use another method to achieve accurate results, a modification of
method 3 in the manual. [First, use either method 1 or method 2 to get close,
within one or two degrees.] I choose a star that is close to the meridian
overhead, and center the crosshairs on it. [Center the overhead star with the
N-S-E-W buttons and perform a Recal operation.] Then I change the clock time
by 1 hour and enter the same star on the hand control. The telescope now
slews to the other side of the mounting, and the star will be offset in
declination by some amount (the star can also be off the crosshair in R.A.
but this is not due to polar alignment, rather it is due to non-orthogonality
of the telescope vs. the mounting).

To polar align, the star is moved half way to the center of the crosshair
by turning only the altitude adjuster. The star is now brought the rest of
the way with the buttons, and a recal is done by pushing button #9. That in
effect sets the altitude of the mounting. If done properly, you don't need to
do it any more times, but you could do it again by resetting the clock time
to the original hour and slewing to the same star again on the other side of
the mounting. A final fine adjustment can be done the second time. [Ignore
any small displacement in R.A.] You can also adjust the orthogonality of
your mounting by shimming up one of the mounting rings until the star comes
in in R.A also.

The azimuth setting on the mount is done by picking a star in the north,
and then slewing to a star in the south at similar right ascensions. Bring
the star half way to the crosshair with the azimuth adjuster, and the rest of
the way with the buttons. [Ignore any small displacement in DEC] Then hit
#9 button to recal the position. This way, the two adjustment axes are done
totally independently. It is highly accurate and very fast. I can do it in
less than 5 minutes and be dead on all night long.

Sincerely
Roland Christen