Alignment again. . .


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 few months
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 stardrift
procedure bears repeating for those who have not previously encountered it.

This alignment method is a variation of Roland Christen's simulated stardrift
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 precise
alignment in a high power eyepiece is desired. (Some are able to complete
this in 5 minutes; I have never been able to finish so quickly.) Once
aligned, and depending on care taken, the mount can reliably place a 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 alignment
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 in
achieving perfect alignment is not the altitude and azimuth adjustments
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 for
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 or
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 mount
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. This speeds
up the process but, needless to say, the mount is NOT yet properly aligned.

3. To set the altitude of the mount, use the STRS or TOUR selection (button
#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 star.
Center the star in a low power eyepiece with the N-S-E-W buttons and perform
a RECAL with button #9.

4. Change the clock time by 1 hour and choose the same star on the hand
control. (If the scope is on the East side of the mount pointing to the
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 the
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. Perform a RECAL.

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, again
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 adjuster
cannot move the mount. The altitude adjustment may change slightly during
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 the
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 mounting
rings until the star is centered in R.A. also.

7. To set azimuth on the mount, pick a star near zenith and near the
meridian. This can be the same overhead star that was used to adjust
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 similar R.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 not 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 alignment star,
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 8 with
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.

Join to automatically receive all group messages.