One more CGEM DX question
Posted 26 November 2012 - 10:51 AM
I went out recently with this mount and successfully (I think) aligned using 2-star + 3 calibration stars. After alignment the hand control said "CGEM Ready", so I chose M31 as my first test and the mount did successfully slew to it. I assumed at this point the mount was tracking the object (note that I did not have an auto-guider setup yet).
I took a series of 10 30-second widefield pictures of M31, and in each I could see star trails and the position of M31 was different in each frame. After the 10 pictures I again chose M31 using the control and the mount kicked in and moved once again to centre M31 in the frame. Given that it was already supposed to be tracking M31 at this point what could I have done wrong?
After choosing M45 as a test I again picked M31 and took another 30 second exposure:
No star trails in this picture which is good, but I have no idea what I did differently between my first attempt and second.
Once alignment is complete I should be able to choose any object and have the mount slew there and track, correct?
Any advice would be most appreciated.
Posted 26 November 2012 - 11:35 AM
(ASPA) and 'drift alignment'. I prefer the later but others claim good results with the former on a CGEM.
Posted 26 November 2012 - 12:18 PM
the nexstar software can compensate for polar mis-alignment when pointing (doing GoTo slews) but when tracking, only the RA motor is turning so if you have polar misalignment you would get trails.
and for the drift to be visible in wide-field images (that looks like a 50mm lens or thereabouts) and 30-second images means you must be grossly polar misaligned.
Posted 26 November 2012 - 04:11 PM
It's still not good enough for unguided... 30 seconds is about as good as you can expect depending on your focal length, camera pixel size, etc... The CGEM just isn't a beast in that regards. If you're interested in AP, I'd recommend getting a guider package of some type. They're cheap and will deal with any small mis-alignments you have.