Equinox Systems GEMINI Manuel d'utilisateur télécharger pdf (Page 45)

Gemini Users Manual 40

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 though 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 quickly 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, 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, automatically using a CCD autoguider or by sending

guiding commands over the serial ports. In all 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

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Equinox Systems GEMINI Manuel d'utilisateur Page 45