4  Considerations for Imaging 

The Gemini system has many features that make it especially suitable as an imaging 

platform.  These include polar alignment tools, multiple tracking rates, guiding modes, an 

autoguider input, and a very sophisticated set of Periodic Error Control (PEC) functions.  

This chapter describes how to take full advantage of your Gemini when imaging. 

4.1  Accuracy of Polar Alignment 

As mentioned previously, accurate polar alignment is extremely important when imaging.  

Even through Gemini' s pointing model and closed loop tracking can compensate for 

inaccurate polar alignment, photographs and CCD images can be ruined by field rotation.  

Gemini' s Polar Axis Correction function helps you achieve sufficiently accurate polar 

alignment for all but the longest wide field images, for which only the technique of drift 

alignment is likely to suffice.   

4.2  Guiding 

Theoretically, good polar alignment and the appropriate tracking rate (such as the 

adaptive King rate) should be enough to keep the telescope pointing at an object 

throughout a long exposure.   In reality, however, mechanical imperfections, alignment 

errors and other factors will cause the telescope to stray slightly from perfect tracking. 

This can result in stars being trailed or bloated, or otherwise distorted in an image.  To 

overcome this problem, most astrophotographers guide their mounts for exposures over a 

minute or two.  This involves making occasional slight adjustments to the RA and Dec. 

so that a star (or the object being photographed) stays in exactly the same spot in the 

telescope' s field of view throughout the exposure.  Guiding corrections can be made 

manually using buttons on a Hand Controller, or automatically using a CCD autoguider.  

In both cases, these corrections are fed into the Gemini system, which actually moves the 

mount. 

You can set the speed at which guiding corrections are made using the   Setup

Mount 

Parameters

Moving Speeds

Guiding   menu item.  You can select a guiding rate 

between 0.2x sidereal and 0.8x sidereal.  While the guiding occurs at a constant speed 

north or south in Dec., it is added to or subtracted from the tracking speed (1x sidereal) in 

RA.  The RA drive always tracks the telescope to the west; it just speeds up or slows 

down when it receives a guiding correction.  The standard Guiding Speed is 0.5x, giving 

0.5x and 1.5x sidereal speed in RA. The highest guiding speed is 0.8x, giving 

approximately 0.8x tracking speed in Dec., and 0.2x and 1.8x in RA. 

4.2.1  The Autoguider Port 

IMPORTANT:

 There are two different versions of the Gemini Autoguider port 

hardware.  The versions can be distinguished by the type of connectors used for the motor 

cables.  The older Gemini version uses RJ 45 modular connectors, and the newer version 

uses 6 pin DIN connectors.  The primary difference between the versions is that the older 

Gemini Users Manual 

3  

4