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ST-7 Used as a Guider

Normal operation of the ST-7 uses its large 765 by 510 pixel chip for imaging and a small 192 by 164 guider chip for guiding the imager. It is possible to use the large chip in the ST-7 as a guider as well. This might be done in a system which uses a separate guide telescope. The ST-7 can then be used on the guide telescope to control the pointing of the main telescope. This function is useful if the main telescope is to be used for film photography or a piggy back camera is to be used. In fact both piggy back and prime focus photography can be done at the same time using the guider telescope with the ST-7 to control the guiding operation.

One might even have a situation where the ST-7 is used for guiding and a larger imager is used on the main telescope or with a piggy back mounted lens. Whatever the particular system setup, The ST-7 makes a guider with a large surface, small pixels for accurate guiding and a large field of view compared to many guider chips in current use.

The ST-7 is shown on the C-5 guide scope in the figure on the left and the par focal, illuminated eyepiece is shown on the right. Notice the use of a very short 2" tube to connect the ST-7 to the C-5. This insures great rigidity of the guider structure. Additionally, the C-5 is very firmly mounted to the main telescope tube with a custom mount that allows for several degrees of adjustment with respect to the main optical tube. The par focal eyepiece is a convenience but almost unnecessary with the large field of view provided by the ST-7. The large focussing knob has been made to replace the small C-5 focussing knob and is provided with index marks to facilitate focussing. For final use, the mirror will be locked into place and the focus set permanently at infinity. (There is provision for a small fine focus adjustment. But guiding does not require the precision focus that imaging does in any case.)

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There are immense advantages to using a large chip in the ST-7 and a medium focal length guider. A principle one is that the field of view is so large that it is easy to find a good guide star within the large field of view provided. This feature alone is worth the cost when doing remote controlled setups. The pointing accuracy is still determined by the pixel size, the focal length of the guider telescope and the resolution accuracy of the program used to run the guider. The ST-7 is not a self guider, but is run by the CCDOPS software through its parallel port.

The following setup procedure is as close to accurate as I can get it. References to the various manuals may vary with the particular manual you are using. The manual for CCDOPS 3.5 is not quite correct for CCDOPS 3.6 and the operating manual for the ST-7/8 is not quite correct either. So you have to hunt around a bit to find the pertinent reference. It goes something like the following.

In order to use the main chip in the ST-7 for guiding, it must be selected in the Camera Menu under ST 7/8 Camera Setup Command select the imaging CCD. Or in the Track Menu under Calibrate Track Command select the imaging CCD. This will enable the imaging CCD to be used as the Calibrate CCd as well as the guiding CCD. Follow the instructions in the operations manual to calibrate your CCD and telescope. (p62 and 63 in either manual) Once the imager and the telescope have been calibrated the setting should not have to be changed unless you change declination by a very large amount. For example guiding near the pole is a bit strange. You can fine tune the guiding behavior using the Tracking Parameters Command.

The imager is now ready to guide the telescope. Select the Autoguide Command under the track Menu. Enter the approximate declination of the object you are guiding on, set the exposure time and follow the directions that appear. After choosing a star for guiding, the guider will start guiding and report on the screen its progress. Typically there will be a series of small plus and minus corrections. If these remain stable, the guiding is working properly.

You can then start imaging with any of the optical systems you have mounted such as the main telescope or any piggyback cameras.

I have tested the accuracy of the ST-7 used as a guider and found the guider servo control loop to be very tight. Guiding accuracy should easily be to better than 1 pixel. This will depend on your telescope, the brightness of the guide star and the frequency of the guider correction function. This is the best performing guider of several that I have tested.

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