alignment of the mount using the azimuth and elevation adjustment knobs until the object 

is again centered in the eyepiece.  Since the mount will approximate perfect polar 

alignment, all subsequent GoTo operations should result in target objects being near the 

center of the eyepiece.  If not (for instance, because the mount's initial misalignment 

exceeded several degrees), you may repeat the correction process.  Note that after using 

this function, the modeling parameters will be reset and you will need to rebuild the 

model by performing several alignments on different objects.   If you are within about 2 

degrees of being polar aligned and have done at least 5 Additional Aligns, you can expect 

this function to get you within 2 arc minutes of precise polar alignment.

3.4.3  Accuracy Required 

How well must Gemini be polar aligned?  The answer to this question really depends on 

what you are doing.  For visual observing, it does not need to be well aligned at all.  You 

can be as much as 6

 off, and the modeling parameters will still allow accurate GoTo 

performance.  In addition, you can use Closed Loop tracking to keep objects centered 

while observing.     

Imaging requires more accurate polar alignment due to the possibility of image rotation.  

While Gemini can GoTo and track objects when polar alignment is off, the field of view 

in the telescope will slowly rotate as tracking takes place.  The closer the mount is to 

being polar aligned, the less field rotation will occur.  Field rotation is a problem 

especially for longer exposures.  It is recommended that you use one of the polar 

alignment methods above to achieve the best possible alignment before imaging. 

3.5  Mount Modeling 

One of the features that distinguishes the Gemini control system from anything else 

available today is its built in sophisticated modeling that provides highly accurate 

pointing and tracking capabilities.  This modeling allows Gemini to compensate for 

inaccurate polar alignment and a large variety of mechanical misalignments in both the 

mount and the telescope. 

3.5.1  Definition 

Without any kind of modeling, a telescope control system must assume that a given 

number of pulses to the RA motor will always move the telescope a certain constant 

distance in RA with no movement in declination at all.  The same applies to pulses to the 

declination motor.  This would only be true if the mount were exactly polar aligned, the 

RA and Dec axes of the mount were exactly orthogonal, and there was no mechanical 

play of any sort in the system.  Unfortunately, this is never the case in even the best 

professional mounts. 

Modeling is a way to analyze the various imperfections in your system so that the system 

can compensate for them when pointing and guiding.  By pointing the telescope at 

various known objects (usually bright stars) and performing an   Additional Align   

function, you teach Gemini about the imperfections in your particular setup.  For each 

additional alignment, Gemini calculates the model parameters based on the best fit of all 

the alignment points so far and then synchronizes the model to the coordinates of the 

Gemini Users Manual 

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