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 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 complete.
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 RECAL.
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.
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