Originally Posted by galexander
Brilliant, you explained it so well Astronut! This is why your video image of the ISS wanders around so erratically while your image of Saturn is so perfectly stationary even though of a similar angular diameter and viewed with the same computer tracking! You explained it so well, I couldn't have done better myself! Well done!
If you wanted to know why Saturn is relatively stable in the video while ISS is not, you should have asked that question. You claimed you're an amateur astronomer, so I would have expected you to know the reason why the two are not comparable. Saturn isn't noticeably moving against the background stars in that short of a timeframe, and it certainly isn't hauling ass at several arcminutes per second. ISS is moving so fast across the sky that even my telescope's drives can't keep up if I'm using a 12 volt power source (like the marine battery I sometimes use when I'm doing astronomy in the middle of nowhere). At its full 18 volt-powered slewing speed it can keep up, but any slight inaccuracies or errors in the gear or pointing system are amplified many times over because of the speeds involved - as you can see in the second video, ISS is there but slightly off-center. It can be corrected with joystick inputs, but as 400x it's very easy to over-correct because the command has to go through an external laptop and be processed before being reflected in the telescope's motion. Again, this isn't the case with Saturn. Also, the motion of the gears themselves vibrate the telescope quite badly, requiring the use of anti-vibration pads and even then the problem is still present. None of this is the case when all your telescope has to do is track at a sidereal speed, as is the case with Saturn. Trying to compare the two is intellectually dishonest to say the least.