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I asked Testar a few questions about these mounts - mainly prompted by a very good review on YouTube, and received the answers quoted below. Very helpful people there:
1. The quoted 0.5 to 0.8 arc seconds figure is the typical guided tracking accuracy.
2. There is no ( power failure) brake on the DEC axis (like there is on RA.) According to ZWO this is not necessary, but I have reached out to them for more information.
3. The mount is suitable for long exposures, depending on your focal length, guiding configuration, seeing and environmental conditions, etc. Generally I wouldn’t go longer than 10 minute exposures with mounts in this class. If you need greater accuracy, there is always Avalon or 10 Micron (although of course these are a step up in price).
4. The 900mm figure is based on ZWO’s recommendation to keep your pixel scale above 1 arcsecond/pixel, given the common pixel size of around 4um. Lower than that, and you may hit the limits of the guided tracking performance.
I’m a bit puzzled by item 4. I fail to make the leap from the pixel scale of the imaging camera and guiding performance. Still, I admit to spending a lot of time head-scratching these days.
I’m still chewing this over.
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