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This lesson explains how to use the auto-guider for long exposures. It also explains how to bring the auto-guider to focus and a little theory on how auto-guiding works.
With the target centred in the field of view the mount will attempt to track its sidereal motion. However, due to numerous physical flaws and imperfections, no mount can track reliably on its own. Auto-guiders are auxiliary cameras which are able to watch a star in the field, measure how its position relative to the field changes with time and issues tracking corrections directly to the mount to keep its target in the centre of the field.
Finding a guide star can be hard with an off-axis guider. This is because the field of view with the primary telescope is quite small (approx. 43x28 arcminutes) and a sufficiently bright enough star may not be in the field. With the addition of a camera rotator, more guide stars may be brought into the field. This is a possible future procurement of the Observatory.
To begin looking for a suitable guide star, once the target has been centred, go to the ''Guide'' tab in the ''Camera Control Window''. Select an exposure time; about 5 seconds is a good place to start. Now click the ''Start'' button.
The camera will expose and the computer will then download the image. Currently, guider images are not calibrated in any way but this can be changed to somewhat improve the performance of the guider if necessary. When the image is downloaded it will be displayed. At this point it is possible that the auto-guider will need to be focused, judging by the image it produces here. See [[Lesson5#Focusingthe_Guider | Focusing the Guider]] below.
If a suitable guide star is not found, consider the following options:
Increase the exposure time. This can improve the quality of the guider image, however, it will less frequently correct the position of the mount meaning it must make larger corrections. This can cause the mount to 'overshoot the mark' on occasion.
Focus the auto-guider. The focus of the guide camera does not need to be perfect by any means and it is possible to track an out of focus star. However, the auto-guider works by measuring the dis placement of the ''centroid'' of the guide star in the current image from the last so an auto-guider cannot reliably track a star which appears as a donut.
If a suitable guide star is found the following steps must be completed. The location of the guide star in the frame is shown at the centre of the ''Guide'' tab window.
The guide camera is located in front of the filter wheel and is mounted on an off-axis guider assembly. There are two adjustments that can be made; the position of the camera, which changes the focus; and the position of the pick-off prism.
The position of the camera is changed by loosening two small grubb screws on the barrel of the guide camera. They can be adjusted with the ''smallest metric Allen key''. The metric Allen keys are kept in the dome in a blue holder. The screws use the 'right hand' rule. A torch may be needed to be able to see the tiny screws. Gently loosen each screw, move the barrel a small distance in or out along the assembly and then tighten the screws again. There is a great deal of trial and error involved in this focusing process. To aid the observer he can take images with the guide camera after each adjustment to see how the movement of the guider affects its focus.
When focus is sharp across the field, tighten the screws a final time. Hopefully, the guider will not lose focus for a long time.
If no guide stars can be found in the field then the position of the pick-off prism may need to be changed. Since the guider is an off-axis guider, it takes a small amount of light out of the side of the main camera's field. It does this by way of a prism in the optical path of the telescope which reflects light onto the guide CCD. The guide CCD only has a small view of the whole field and it may not see any stars sufficient for guiding. It is more likely that better guide stars are in the centre of the field so to gain more potential guide stars the prism should be moved closer to the centre of the field.
To move the prism, loosen the large thumbscrew on the side of the off-axis assembly. Gently move the off-axis assembly inwards slightly. Tighten the thumbscrew. Take another guide star image to see if some better guide stars have moved into the field.
Be warned. Moving the prism too far into the field will cast a shadow on the main camera's CCD. This will ruin images if the observer is not careful. A test image should be taken after moving the prism to make sure that the main camera image is not affected.
The next step is to 'calibrate' the guider. Here, much like when the joystick is used to centre the target, the guider gets used to how various mount commands alter the position of the guide star. To do this select ''Calibrate'' and click on the ''Start'' button. The guider image will be manipulated and the change in the position of the centroid of the star will be recorded with a red line on the image. The star is moved in the two axes so eventually an 'L' shape will be traced on the guider image. One of the guiding error messages is 'does not produce 'L' shape'. This can be because the guide star is too near the edge of the frame.
Once this calibration process has been completed then tracking can begin.
Next, click the ''Track'' option. Settings such as ''Watch Star'' should be checked so that the observer can always see the condition of the guide star in a little window in the corner of the !MaxIm window. The ''Aggressiveness'' settings should be changed according to the seeing conditions. Since seeing from Armagh is generally quite poor it is currently set at 5 (out of 10) in both X and Y. This reduces the possibility of the guider 'chasing the seeing'. There is no reason to change the ''Aggressiveness'' setting to anything higher than 8 in either axis.
Once all the settings are modified the ''Start'' button should be clicked. The mount will begin to receive corrections from the auto-guider and these corrections produce a clicking sound in the mount. !MaxIm logs the status of the guide star and the corrections being made. It will also attempt to relocate the guide star if it fades.
To stop auto-guiding, merely click the ''Stop'' button. Do this before slewing to a new target.
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