This was also asked back here
http://www.iceinspace.com.au/forum/s...ad.php?t=89225
Does the 11kg include any extra counterweight above the standard issue ?
Total Payload is usually inclusive of standard counterweights only - if the OTA payload is increased then you usually have to add more counterweight and you might come unstuck.
It doesnt sound like your payload is excessive, but mirrors can be like sails in the wind.
Is yours in an Obs or out in the open ? - the breeze might be a limitation.
With a longer focal length any instability will be more likely to show up, plus you have the extra load on the bearings, motors and gears trying to move all that mass.