3.4.2  Techniques 

There are several techniques that you can use to polar align your mount.  Each method 

varies in the amount of time it takes and the accuracy it can achieve. 

3.4.2.1  Polar Alignment Scope 

Most of the German equatorial mounts supported by Gemini have the option of installing 

a polar alignment scope (PAS).  This is a small telescope that is attached to the mount 

parallel to the mount' s polar axis.  In most cases, it is actually installed inside the hollow 

RA axis.  Inside the alignment scope is a reticle with markings for certain alignment 

stars.  The idea is to adjust the mount in elevation and azimuth until the alignment stars 

match up with the marks on the reticule.  You should of course refer to the instructions 

for your particular polar scope for complete details. 

Using a polar alignment scope can be very fast.  It' s accuracy depends primarily on how 

well calibrated the reticle is and the degree of mechanical play between the polar scope 

and the mount.  For a well calibrated polar scope, you can expect to be within 2 or 3 

minutes of the actual pole. The accuracy is also limited because the stars around the pole 

are lifted up somewhat by refraction (most noticeable at lower latitudes).  The PAS will 

therefore align your mount to the "refracted pole.   

3.4.2.2  Drift Method 

The drift method of polar alignment is probably the most accurate and most time 

consuming method.  It involves centering a couple of well chosen stars in an eyepiece 

with some type of reticle, and watching for the star to drift north or south as the mount 

tracks in RA.  Depending on where the star is in the sky and whether it drifts north or 

south, you can determine which way to move the mount in elevation or azimuth.  You 

then repeat the process until you see no drift for 5 to 15 minutes.  The longer you can go 

without any north/south drift, the better polar aligned you are. 

Of course, stars used for drift aligning are also subject to refraction. Using a star near the 

eastern or western horizon aligns the telescope to a pole that is shifted slightly toward the 

zenith (as proposed in the King method.) Experienced astrophotographers recommend 

that a star you choose for drift alignment be near the object that is to be photographed.  

There are many good references on the web to give you detailed instructions on how to 

do drift alignment. 

3.4.2.3  Gemini Polar Alignment Tools 

Gemini provides two tools to help you achieve fairly accurate polar alignment.  They are 

  Polar Align Assist   and   Polar Axis Correction.    Both are available as submenus of 

  Align Telescope.    

You can use the Polar Align Assist to get close to polar alignment when you first start the 

system.  You can then use Polar Axis Correction to refine your polar alignment after you 

have built a good pointing model using several   Additional Align   commands. 

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