Equinox Systems GEMINI Uživatelský manuál stáhnout pdf (Strana 42)

Gemini Users Manual 37

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

latest alignment object. Because there are several parameters in the model, it won’t

converge on nominal values until you have performed at least 4 or 5 alignments on one

side of the meridian. As mentioned earlier, you will get the best results if you pick

alignment objects with substantially different hour angles. An additional source of

inaccuracy – gear play – is calculated with the first alignment on the opposite side of the

meridian.

Up to 10 different models can be stored and reestablished. This can be useful if you swap

your Gemini system between multiple mounts.

3.5.2 System Startup Modes

Because Gemini stores the modeling parameters in non-volatile memory, it may be

powered off at any time without losing its pointing model. In fact, the model can continue

to be refined from observing session to observing session by adding more alignment

points. However, the modeling parameters are only valid if the telescope and mount have

not been moved between observing sessions. Whenever you turn the Gemini unit on, you

need to tell it whether anything has been moved since it was turned off. This allows

Gemini to decide how much of the model is still valid. There are 3 startup modes that tell

Gemini what has been moved:

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Equinox Systems GEMINI Uživatelský manuál Strana 42