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UNGUIDED IMAGING WITH THE GTOCP4 AND GTOCP5
Unguided imaging is the holy grail for many imagers. It requires the right combination of mount capabilities, modeling
software and attention to your equipment setup.
All of the Astro-Physics mounts are capable of unguided imaging if they have the GTOCP4 or GTOCP5 servo control
boxes. This includes the non-encoder mounts and as well as the AE or AEL mounts that have the Renishaw absolute
encoders. Unguided imaging requires good polar alignment, very low or zero periodic error and some degree of modeling
using either APCC-Pro or version 5.xx of the keypad software.
Non-encoder mounts will have inherent periodic error which can be compensated with a good clean PE compensation
curve that is stored in the GTOCP4 memory. PE curves can be generated using PEMPro software, a program that
produces the best possible result and lets the user also analyze the tracking performance of the mount. A typical tracking
error with PEM turned on can be 1 arc sec or less over a 6.4 minute worm period. Modern astro cameras can produce
excellent results with exposures between 3 to 6 minutes, and so it may not be necessary to guide a short-to-medium focal
length imaging setup.
The Astro-Physics 1100 or 1600 AE or AEL mounts and the Mach2GTO have high-resolution and high-accuracy Absolute
Encoders that produce excellent tracking over long time periods. With these mounts, there is no periodic error and the
tracking errors are very small, well below 1 arc second over 30 minutes or more.
Good tracking by itself will not produce round stars because the stars will drift slightly minute by minute due to many
factors (polar misalignment, atmospheric refraction, differential flexure and other effects). Modeling does two things - it can
result in good pointing, but more importantly, it can be used to compensate for drift in both the R.A, and Dec axes.
We have developed several methods for modeling that the user can implement for unguided imaging, depending on the
equipment used for imaging. APCC-Pro can be used for partial or full-sky modeling, and the results are stored on the
laptop. We now also have a keypad program (v5.x) which can be used to produce a partial or full-sky model by measuring
the position of bright reference stars on both sides of the meridian.
A second method in the Keypad does not model the sky. Rather, it measures the actual path used by the object to be
imaged over a multi-hour time period and compensates automatically for any drift. This method is fast and easy to do with
a cross-hair eyepiece or a cross-hair image.
The keypad models do not need a laptop computer, and they are stored in the GTOCP4 or GTOCP5 control boxes for
future use. This makes it possible to do astro-photography with minimal equipment, no guider or guide scope and just a
stand-alone camera on the main telescope. Refer to the Keypad manual for a full description of these two methods.
