Re: Alignment again. . .

Chris Spratt

This method has been around for about a year. By now I think it could
have been included in the latest manuals and it would be nice to see
it posted on the AP web page.

-- In ap-gto@..., GrayFox65@a... wrote:
AP GTO'ers,

Yes, I know. Aligning the AP GTO mounts has been discussed
endlessly on this
group. By now, everyone who has owned their GTO for more than a
has long since mastered the (initially frustrating) procedure.
However, a
number of new GTO owners are now taking delivery of their mounts,
and since
alignment is a virtual necessity to use the mount, the simulated
procedure bears repeating for those who have not previously
encountered it.

This alignment method is a variation of Roland Christen's simulated
method. It is most appropriate for users who cannot (or do not
wish to) use
the Polaris-and-one-other-star method. Note, however, that I have
modified it
slightly based on experience. I think the variation works better
and is
faster than the original. Long time users are invited to comment.

The method can be performed in 15 minutes or less'a little
longer if
alignment in a high power eyepiece is desired. (Some are able to
this in 5 minutes; I have never been able to finish so quickly.)
aligned, and depending on care taken, the mount can reliably place
star or
deep sky object very near the center of a 3mm Radian eyepiece (365X
and 10
arc minutes FOV on a 155 EDF) for the rest of the evening. For
perfectionists (I count myself as such), an occasional RECAL will
cure any
remaining small errors, at least for a local sky area.

As others on this group have repeatedly noted, the limiting factor
achieving perfect alignment is not the altitude and azimuth
described below but rather the small errors introduced when
re-tightening the
azimuth bolts, and to a lesser degree the altitude bolts, once
alignment is

Simulated Star Drift Polar Alignment for Astro-Physics GTO Mount

1. If you do not use a polar alignment scope, rough align the
tripod to
North using a compass with magnetic anomaly adjustment calibrated
observing location. The tripod must be pre-aligned closely enough
to North
so that the mount's azimuth adjustment travel can remove the
residual azimuth
error. Rough level the tripod in N-S and E-W using a small level
calibrated eyeball method. Perfection is not required; the mount
does not
have to be level to be aligned. However, out-of-level errors will
result in
small altitude and/or azimuth errors that must be corrected during
alignment. Install mount head and telescope.

2. Select the two-star alignment method on the hand control and
center the
chosen first alignment star in a low power eyepiece. In this
variation, it
is not necessary to begin with an eastern star, and the same star
can be used
as both the first and second stars in the two-star process. Simply
chose the
same star when prompted by the hand control for a second star.
up the process but, needless to say, the mount is NOT yet properly

3. To set the altitude of the mount, use the STRS or TOUR
#5 or #8) to slew to a star that is near the meridian overhead.
Pick a star
that is a few degrees south of zenith (e.g. 10 degrees) to insure
that the
OTA does not hit the tripod legs when it is slewed to this overhead
Center the star in a low power eyepiece with the N-S-E-W buttons
a RECAL with button #9.

4. Change the clock time by 1 hour and choose the same star on the
control. (If the scope is on the East side of the mount pointing
West, set clock time one hour earlier; if on the West side pointing
East, set
clock time one hour later.) The telescope will slew to the other
side of the
mount. The star will be offset in declination by some amount. (It
can also
be offset in R.A. but this is not due to polar alignment error,
rather it is
due to non-orthogonality of the telescope vs. the mount.) Loosen
altitude bolts and move the star half way to the center of the
eyepiece field
in DEC with the altitude adjuster only. Move the star the rest of
the way to
the center of the eyepiece field with the N-S buttons only.

5. Reset the clock time to the original hour and slew back to the
same star
on the original side of the mounting. Using a high power eyepiece,
adjust DEC half way with the altitude adjuster and the rest of the
way with
the N-S buttons. Perform a RECAL.

6. Repeat steps 4 and 5 with higher power eyepieces as many times
as needed.
Retighten the altitude bolts but not so tight that the altitude
cannot move the mount. The altitude adjustment may change slightly
tightening. Make a final, small altitude adjustment without
loosening the
altitude bolts. Unless these bolts have been tightened very
securely, the
mount will move through the small displacement needed to finalize
altitude calibration.

Ignore any small displacement in R.A, as this represents
orthogonality error.
Adjust the orthogonality of the mounting by shimming up one of the
rings until the star is centered in R.A. also.

7. To set azimuth on the mount, pick a star near zenith and near
meridian. This can be the same overhead star that was used to
altitude. Center the star in R.A. using the E-W buttons only. Do
not use
the azimuth adjusters. Ignore any small displacement in DEC.
Perform a

8. Using STRS or TOUR, slew to a star in the south and at a
i.e. near the meridian. Loosen the azimuth bolts. Bring the star
all the
way to the center of the eyepiece with the azimuth adjuster. Do
use the
E-W buttons. The choice of a star directly overhead (or nearly so)
in step 7
creates a celestial "pivot point" for azimuth alignment, and it
insures that
virtually the entire error exhibited by the second azimuth
the one in the south and on the meridian, is due to azimuth
alignment error.
Perform a RECAL. Ignore any small displacement in DEC.

9. Slew back to the original star overhead and repeat steps 7 and
higher power eyepieces as many times as needed. Retighten the
azimuth bolts.
The mount may go off slightly in DEC; readjust altitude one last
time with
the altitude adjuster without loosening the mount's altitude bolts.

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