Deciding between two mounts for AP
Posted 30 December 2012 - 03:40 AM
Going back to the op's question I wouldn't agonize over which mount to buy. The reality of this hobby is that there is no lifetime purchase. Whichever mount you end up with chances are you will be upgrading again and again and again....
Posted 30 December 2012 - 12:01 PM
Posted 08 January 2013 - 05:03 AM
Since working out that there's 3 secs of DEC backlash (slop in the geared motor) I've got outstanding DEC tracking at around 0.5 arcsec.
The RA tracking is much more interesting. The three motors I've tested all exhibit a perfect sinusoidal periodic error of 4-7 arcsec every 6.5 secs. To see this properly you need a high resolution encoder sampling many times a second. This corresponds perfectly to the geared motor's 64:1 ratio. I've built a high resolution RA encoder to autoguide the mount using the fast ST4 interface, but found that at my high focal length coped poorly with motor gear periodic error, leading to stars that are more elliptical than round. This is because a 6.5 sec period is too fast to correct.
Over the last few (cloudy) days I've managed to run wires from the motor encoder to my own RA encoder micro-controller, and add predictive slow down/speed up commands to adjust for motor encoder periodic error. The results look like excellent RA tracking to +/- 1 arcsec (not yet field tested with stars).
This info is long-winded evidence to back up the warning I'm giving you that you could be in for elliptical stars if aiming for maximum quality at high focal lengths. The more rough your polar alignment and DEC tracking is, the less you'll notice RA problems where stars aren't round
Here's an image to explain the periodic error (worm + geared motor) - http://www.flickr.co.../in/photostream
For visual use I find the mount to be awesome, and if you're not too demanding in the areas I described, you should also find it good for AP.
Posted 08 January 2013 - 01:40 PM
I find that my mount has acceptable tracking for my 750 mm fl reflector.But... I wouldn't push it above that .