This is an old revision of the document!

Not quite up-to-date anymore. Must be revised.

The accuracy of the tracking can be improved with a good alignment. This procedure can gather information on possible errors in the azimuth and pole-height settings (correction suggestions for the hardware), to reduce errors in the alignment of the pole axis.

Alignment with 2 stars

 MENU - Alignment - 2-Stars - ENTER

select one star from the presented list and press ENTER
select the second star from that list (it should have a large distance to the first in RA/DEC) and press ENTER
the telescope changes its position and points to the first star
the star should be in the center of the eyepiece, adjust the pointing with the N - S - E - W buttons as needed (use the eyepiece with the crosshair to easily estimate the center)
once the star is centered, confirm with ENTER
repeat the previous two steps with the second star

If the alignment has been successful, the hand terminal will show

 Alignment complete

It is advisable to refine the alignment with further stars. Select this option in the hand terminal

 MENU - Alignment - Refine 2-Stars - ENTER

as before, the hand terminal will display a list of stars
select one star to which the telescope will move
center the star in the eyepiece and confirm with ENTER
the hand terminal will then show:

 Alignment refined

repeat this procedure until an alignment with 10 stars has been completed
to improve the accuracy all stars should be spread across the whole sky

There are further methods to change/improve the alignment that are documented in the manual of the GM 4000 QCI, which can be found in room 2.009.

Once a good alignment has been reached, it is recommend to save it:

 MENU - Alignment - Aligne Database - Save Model

The telescope will then swing to the so called “Home Search” position and saves the alignment. This alignment can then be restored after an alignment loss with:

 MENU - Alignment - Aligne Database - Load Model

The telescope will then once again swing to the “Home Search” position and load the previously saved data.

It can be necessary to delete a (bad) alignment, e.g. if it only proves to be useless after it has been saved. An alignment can be deleted with:

 MENU - Alignment - Aligne Database - Delete Model

Alternatively one can reset the mount:

 MENU - Alignment - Reset at Home

In doing so, the alignment will be deleted and the mount restored to the factory settings. Only use this option with caution!

Information on the current alignment can be found at:

 MENU - Alignment - Align info

With this option, the mount among others gives information on how the orientation of the pole axis can be improved. The terminal will display something like:

 To centre pol ax
 move .07 Rt 0.05 Up

The Periodic Error Correction (PEC) can help to improve the tracking accuracy of the mounting by accounting for periodic errors of the worm gear. This error usually has a period of approx. 3 min 20 sec (3.3 minutes), while the PEC can correct for shorter periods, too. The error of the worm gear is usually irrelevant as long as the observations are only carried out with an eyepiece – in case longer observations are taken with CCDs, the corrections of this error are getting important.

The PEC has to be calibrated before it can operate properly. For this purpose, the mount needs detailed information on the deviations from the ideal tracking behavior. One possibility to achieve this is by centering a bright star in an eyepiece and following its motions with the hand terminal (the N - S - E - W buttons) for a while. A more sophisticated option is based on the autoguide functionality of our CCD cameras, as the precision is higher this way. Start the PEC calibration on the hand terminal with:

Drive -> A-PEC control -> A-PEC Training

In the next step choose the time period during which the PEC shall be “trained”: 15, 30, or 60 minutes. The longer the training is, the better is the result of the PEC. After the training has been completed the corrections are calculated and the PEC is automatically activated.

will follow…

Check

the telescope should have ambient temperature
drive to a bright star in zenith (Polaris can be used, too)
use the 25mm ocular (or the webcam)
defocus the star so it takes about one sixth of the FoV (field of view) and gets a doughnut shape
if the shadow of the secondary mirror (the inner part of the doughnut/defoussed star) is off-centered, the collimation of the telescope is erroneous

Improvement

move the star (using the N - E - S - W buttons) in the direction to which the shadow is off-centered until the edge of the FoV is reached
turn the three screws on the backside of the secondary mirror (see Figure below) until the star is again in the center of the FoV. To achieve that:
- tighten the screw(s) in the direction of the offset
- loosen the other(s)
repeat this procedure until the shadow is centered on the defocused star
repeat with eyepieces of higher magnification (note: in 6mm eyepieces the star is kept focused and the correction is carried out in relation to a (very) thin diffraction ring)