Equinox Systems GEMINI Manuel d'utilisateur télécharger pdf (Page 39)

Gemini Users Manual 34

To solve this problem, Gemini now allows you to set a “GoTo Limit” to define a point

past which Gemini will always GoTo the object so that the telescope tube is on the east

side of the mount. Gemini will allow the mount to track past this point, but will not

GoTo any object west of this point without a meridian flip, if needed, to put the telescope

tube on the east side of the mount. Refer to section 5.3.10.2.6 for a description of the

“Set GoTo” limit command.

3.4 Polar Alignment

3.4.1 Definition

Polar alignment is the process of aligning the mount’s polar (RA) axis with the Earth’s

rotational axis. It involves physically adjusting the elevation and azimuth of the mount,

usually accomplished by turning adjustment knobs. Although the mount does not need

to be level to be polar aligned, a level mount can make the alignment process much easier

since adjusting the elevation knob on a level mount just moves the mount in elevation

(and the same for the azimuth adjustment). Moving the elevation knob on a mount that is

not level introduces some azimuth component as well. While leveling is an important

time saver for the Drift Alignment method, it is not necessary at all if you intend to use

Gemini's Polar Axis Correction feature. As mentioned earlier, an accurately polar aligned

mount can track the motions of the sky by driving just the RA motor.

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. Its 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 refracted 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

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Equinox Systems GEMINI Manuel d'utilisateur Page 39