Astro-Physics 900 GTO mount
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I will never regret my choice..
This mount is a true marvel of technology. Its size and weight are very limited regarding its load capacity, thanks to the CNC machining and compact design. More, it comes apart in two light-weight elements without tools!
It is very rigid, which is really important in CCD imaging, when using a long instrument like a refractor, or when using a piggyback instrument.
It carries easily the 155 f7 StarFire EDF refractor, with its finderscope, in various configurations: with our without heavy guidescope, rings and guider, two mounting plates, heavy CCD camera, and field flattener or telecompressor.
I decided to mount it on a Losmandy G11 tripod, using the LT2APM adapter. Setup is rock solid, and levelling easy on uneven terrain.

Tracking and pointing accuracies are outstanding: with a well mounted orthogonal instrument like the refractor, the object is always centered on the CCD chip, even after a meridian flip. By the way, this is also the real advantage of this mount : its ability to continue to track well after  the meridian,without having to flip, with the instrument "below" the mount, with no  sign of degradation in tracking performance.
The periodic error is very small and low, between 4 to 5.5 arc seconds error, depending on tooth to tooth error of the worm wheel. Variations in the error curve are extremely smooth, so autoguiding using long guiding exposure times can be made without trouble: extremely useful with narrowband filtered imaging with dual chip CCD cameras.

The three stars polarscope is very accurate: once polar aligned, without additional effort, I can do more than 6 hours of imaging with my camera without noticeable field rotation, even with the large chip of the STL11000.

Mount's control doesn't need a computer , this is precious when a computer failure occurs in the field.
Several features like focusing, reticle, can be ajusted with the keypad.

I would definitely recommend this mount, technically as well as aesthetically, for the visual observer or demanding imager.








The Astro-Physics 900 GTO mount in action

"The single most important item for the imager is the mount. next comes the mount. Then comes the mount.... Get the picture?"  Roland Christen
Some unguided tracking performance examples:


Measurement of the periodic error on three worm cycles, PEM not activated










Measurement of the PE using Pempro, 7 worm cycles, after 3 years of use, no PEM: a mount which tracks!!!



Some tracking performance examples using autoguiding with MaximDL software:

Tracking performance, autoguiding using dithering. Guiding exposure 3 seconds, 10 minutes single exposure, ST8 XME camera.







Another screenshot, autoguiding using dithering. Guiding exposure 4 seconds, 15 minutes single exposure, STL11000 XM camera.








Autoguiding using dithering. Guiding exposure 6 seconds, 15 minutes single exposure, STL11000 XM camera.







Another autoguiding example using dithering. Guiding exposure 6 seconds, 10 minutes single exposure, ST10 XME camera.


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